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Recognizing Blazars Using Radio Morphology from the VLA Sky Survey: Blazars are radio-loud Active Galactic Nuclei (AGN) whose jets have a very
small angle to our line of sight. Observationally, the radio emission are
mostly compact or a compact-core with a 1-sided jet. With 2.5$^{\prime\prime}$
resolution at 3 GHz, the Very Large Array Sky Survey (VLASS) enables us to
resolve the structure of some blazar candidates in the sky north of Decl. $-40$
deg. We introduce an algorithm to classify radio sources as either blazar-like
or non-blazar-like based on their morphology in the VLASS images. We apply our
algorithm to three existing catalogs, including one of known blazars
(Roma-BzCAT) and two of blazar candidates identified by WISE colors and radio
emission (WIBRaLS, KDEBLLACS). We show that in all three catalogs, there are
objects with morphology inconsistent with being blazars. Considering all the
catalogs, more than 12% of the candidates are unlikely to be blazars, based on
this analysis. Notably, we show that 3% of the Roma-BzCAT "confirmed" blazars
could be a misclassification based on their VLASS morphology. The resulting
table with all sources and their radio morphological classification is
available online. | astro-ph_GA |
Probing the initial conditions of high-mass star formation -- IV. Gas
dynamics and NH$_2$D chemistry in high-mass precluster and protocluster
clumps: The initial stage of star formation is a complex area study because of its
high density and low temperature. Under such conditions, many molecules become
depleted from the gas phase by freezing out onto dust grains. However, the
deuterated species could remain gaseous and are thus ideal tracers. We
investigate the gas dynamics and NH$_2$D chemistry in eight massive
pre/protocluster clumps. We present NH$_2$D 1$_{11}$-1$_{01}$ (at 85.926 GHz),
NH$_3$ (1, 1) and (2, 2) observations in the eight clumps using the PdBI and
the VLA, respectively. We find that the distribution between deuterium
fractionation and kinetic temperature shows a number density peak at around
$T_{\rm kin}=16.1$ K, and the NH$_2$D cores are mainly located at a temperature
range of 13.0 to 22.0 K. We detect seven instances of extremely high deuterium
fractionation of $1.0 \leqslant D_{\rm frac} \leqslant 1.41$. We find that the
NH$_2$D emission does not appear to coincide exactly with either dust continuum
or NH$_3$ peak positions, but often surrounds the star-formation active
regions. This suggests that the NH$_{2}$D has been destroyed by the central
young stellar object (YSO) due to its heating. The detected NH$_2$D lines are
very narrow with a median width of $\rm 0.98\pm0.02 km/s$. The extracted
NH$_2$D cores are gravitationally bound ($\alpha_{\rm vir} < 1$), are likely
prestellar or starless, and can potentially form intermediate-mass or high-mass
stars. Using NH$_3$ (1, 1) as a dynamical tracer, we find very complicated
dynamical movement, which can be explained by a combined process with outflow,
rotation, convergent flow, collision, large velocity gradient, and rotating
toroids. High deuterium fractionation strongly depends on the temperature
condition. NH$_2$D is a poor evolutionary indicator of high-mass star formation
in evolved stages, but a useful tracer in the starless and prestellar cores. | astro-ph_GA |
The numbers of z~2 star-forming and passive galaxies in 2.5 square
degrees of deep CFHT imaging: We use an adaptation of the BzKs technique to select ~40,000 z~2 galaxies (to
K(AB) = 24), including ~5,000 passively evolving (PE) objects (to K(AB) = 23),
from 2.5 deg^2 of deep CFTH imaging. The passive galaxy luminosity function
exhibits a clear peak at R = 22 and a declining faint-end slope ({\alpha} = -
0.12 [+0.16 -0.14]),while that of star-forming galaxies is characterized by a
steep faint-end slope ({\alpha} = -1.43 +- [0.02] (systematic) [+0.05 -0.04]
(random)). The details of the LFs are somewhat sensitive (at <25% level) to
cosmic variance even in these large(~0.5 deg^2) fields, with the D2 field
(located in the COSMOS field) most discrepant from the mean. The shape of the z
~ 2 stellar mass function of passive galaxies is remarkably similar to that at
z ~ 0.9, save for a factor of ~4 lower number density. This similarity suggests
that the same mechanism may be responsible for the formation of passive
galaxies seen at both these epochs. This same formation mechanism may also
operate down to z ~ 0 if the local PE galaxy mass function, known to be
two-component, contains two distinct galaxy populations. This scenario is
qualitatively in agreement with recent phenomenological mass-quenching models
and extends them to span more than three quarters of the history of the
Universe. | astro-ph_GA |
The dragonfly nearby galaxies survey. Iv. A giant stellar disk in ngc
2841: Neutral gas is commonly believed to dominate over stars in the outskirts of
galaxies, and investigations of the disk-halo interface are generally
considered to be in the domain of radio astronomy. This may simply be a
consequence of the fact that deep HI observations typically probe to a lower
mass surface density than visible wavelength data. This paper presents low
surface brightness optimized visible wavelength observations of the extreme
outskirts of the nearby spiral galaxy NGC 2841. We report the discovery of an
enormous low-surface brightness stellar disk in this object. When azimuthally
averaged, the stellar disk can be traced out to a radius of $\sim$70 kpc (5
$R_{25}$ or 23 inner disk scale lengths). The structure in the stellar disk
traces the morphology of HI emission and extended UV emission. Contrary to
expectations, the stellar mass surface density does not fall below that of the
gas mass surface density at any radius. In fact, at all radii greater than
$\sim$20 kpc, the ratio of the stellar to gas mass surface density is a
constant 3:1. Beyond $\sim$30 kpc, the low surface brightness stellar disk
begins to warp, which may be an indication of a physical connection between the
outskirts of the galaxy and infall from the circumgalactic medium. A
combination of stellar migration, accretion and in-situ star formation might be
responsible for building up the outer stellar disk, but whatever mechanisms
formed the outer disk must also explain the constant ratio between stellar and
gas mass in the outskirts of this galaxy. | astro-ph_GA |
Toward an Empirical Theory of Pulsar Emission. X. On the Precursor and
Postcursor Emission: Precursors and postcursors (PPCs) are rare emission components detected in a
handful of pulsars that appear beyond the main pulse emission, in some cases
far away from it. In this paper we attempt to characterize the PPC emission in
relation to the pulsar main pulse geometry. In our analysis we find that PPC
components have properties very different from that of outer conal emission.
The separation of the PPC components from the main pulse center remains
constant with frequency. In addition the beam opening angles corresponding to
the separation of PPC components from the pulsar center are much larger than
the largest encountered in conal emission. Pulsar radio emission is believed to
originate within the magnetic polar flux tubes due to the growth of
instabilities in the outflowing relativistic plasma. Observationally, there is
strong evidence that the main pulse emission originates at altitudes of about
50 neutron star radii for a canonical pulsar. Currently, the most plausible
radio emission model that can explain main pulse emission is the coherent
curvature radiation mechanism, wherein relativistic charged solitons are formed
in a non-stationary electron-positron-pair plasma. The wider beam opening
angles of PPC require the emission to emanate from larger altitudes as compared
to the main pulse, if both these components originate by the same emission
mechanism. We explore this possibility and find that this emission mechanism is
probably inapplicable at the height of the PPC emission. We propose that the
PPC emission represents a new type of radiation from pulsars with a mechanism
different from that of the main pulse. | astro-ph_GA |
ALMA unveils a triple merger and gas exchange in a hyper-luminous radio
galaxy at z=2: the Dragonfly Galaxy (II): The Dragonfly Galaxy (MRC0152-209), at redshift z~2, is one of the most
vigorously star-forming radio galaxies in the Universe. What triggered its
activity? We present ALMA Cycle 2 observations of cold molecular CO(6-5) gas
and dust, which reveal that this is likely a gas-rich triple merger. It
consists of a close double nucleus (separation ~4 kpc) and a weak CO-emitter at
~10 kpc distance, all of which have counterparts in HST/NICMOS imagery. The
hyper-luminous starburst and powerful radio-AGN were triggered at this
precoalescent stage of the merger. The CO(6-5) traces dense molecular gas in
the central region, and complements existing CO(1-0) data, which revealed more
widespread tidal debris of cold gas. We also find ~10$^{10}$ M(sun) of
molecular gas with enhanced excitation at the highest velocities. At least
20-50% of this high-excitation, high-velocity gas shows kinematics that
suggests it is being displaced and redistributed within the merger, although
with line-of-sight velocities of |v| < 500 km/s, this gas will probably not
escape the system. The processes that drive the redistribution of cold gas are
likely related to either the gravitational interaction between two kpc-scale
discs, or starburst/AGN-driven outflows. We estimate that the rate at which the
molecular gas is redistributed is at least ~1200 +- 500 M(sun)/yr, and could
perhaps even approach the star formation rate of ~3000 +- 800 M(sun)/yr. The
fact that the gas depletion and gas redistribution timescales are similar
implies that dynamical processes can be important in the evolution of massive
high-z galaxies. | astro-ph_GA |
ATLASGAL --- properties of compact HII regions and their natal clumps: We present a complete sample of molecular clumps containing compact and
ultra-compact (UC) HII regions between \ell=10\degr and 60\degr\ and
$|b|<1\degr, identified by combining the the ATLASGAL submm and CORNISH radio
continuum surveys with visual examination of archival infrared data. Our sample
is complete to optically thin, compact and UCHII regions driven by a zero age
main sequence star of spectral type B0 or earlier embedded within a 1,000 Msun
clump. In total we identify 213 compact and UCHII regions, associated with 170
clumps. Unambiguous kinematic distances are derived for these clumps and used
to estimate their masses and physical sizes, as well as the Lyman continuum
fluxes and sizes of their embedded HII regions. We find a clear lower envelope
for the surface density of molecular clumps hosting massive star formation of
0.05 g cm^{-2}, which is consistent with a similar sample of clumps associated
with 6.7 GHz masers. The mass of the most massive embedded stars is closely
correlated with the mass of their natal clump. Young B stars appear to be
significantly more luminous in the ultraviolet than predicted by current
stellar atmosphere models. The properties of clumps associated with compact and
UCHII regions are very similar to those associated with 6.7 GHz methanol masers
and we speculate that there is little evolution in the structure of the
molecular clumps between these two phases. Finally, we identify a significant
peak in the surface density of compact and UCHII regions associated with the
W49A star-forming complex, noting that this complex is truly one of the most
massive and intense regions of star formation in the Galaxy. | astro-ph_GA |
A phase-space view of cold-gas properties of Virgo-cluster galaxies:
multiple quenching processes at work?: We investigate the cold-gas properties of massive Virgo galaxies ($>10^9$
M$_\odot$) at $<3R_{200}$ ($R_{200}$ is the radius where the mean interior
density is 200 times the critical density) on the projected phase-space diagram
(PSD) with the largest archival dataset to date to understand the environmental
effect on galaxy evolution in the Virgo cluster. We find: lower HI and H$_2$
mass fractions and higher star-formation efficiencies (SFEs) from HI and H$_2$
in the Virgo galaxies than the field galaxies for matched stellar masses; the
Virgo galaxies generally follow the field relationships between the offset from
the main sequence of the star-forming galaxies [$\Delta$(MS)] with gas
fractions and SFEs but slightly offset to lower gas fractions or higher SFEs
than field galaxies at $\Delta({\rm MS})< 0$; lower gas fractions in galaxies
with smaller clustocentric distance and velocity; lower gas fractions in the
galaxies in the W cloud, a substructure of the Virgo cluster. Our results
suggest the cold-gas properties of some Virgo galaxies are affected by their
environment at least at $3 R_{200}$ maybe via strangulation and/or
pre-processes and HI and H$_2$ in some galaxies are removed by ram pressure at
$<1.5 R_{200}$. Our data cannot rule the possibility of the other processes
such as strangulation and galaxy harassment accounting for the gas reduction in
some galaxies at $<1.5 R_{200}$. Future dedicated observations of a
mass-limited complete sample are required for definitive conclusions. | astro-ph_GA |
Unresolved z~8 point sources and their impact on the bright end of the
galaxy luminosity function: The distribution and properties of the first galaxies and quasars are
critical pieces of the puzzle in understanding galaxy evolution and cosmic
reionization. Previous studies have often excluded unresolved sources as
potential low redshift interlopers. We combine broadband color and photometric
redshift analysis with morphological selections to identify a robust sample of
candidates consistent with unresolved point sources at redshift $z\sim8$ using
deep Hubble Space Telescope images. We also examine G141 grism spectroscopic
data to identify and eliminate dwarf star contaminants. From these analyses, we
identify three, bright ($M_{UV}\lesssim-22$ ABmag) dropout point sources at
$7.5<z<8.1$. Spectral energy distribution analyses suggest that these sources
are either quasars or compact star-forming galaxies. The flux captured by the
IRAC 4.5 $\mu$m channel suggests that they have moderate $H\beta$+$[OIII]$
equivalent widths. We calculate the number density of point sources at
$z\sim7$-8, and find that a double powerlaw model well describes the point
source distribution. We then extend our analysis to estimate the combined point
source + galaxy luminosity function and find that the point sources have a
non-negligible contribution to the bright-end excess. The fact that the point
sources dominate only at $M_{UV}\lesssim-22$ suggests that their contribution
to cosmic reionization is likely limited. While spectroscopic follow-up is
needed to confirm the nature of these point sources, this work demonstrates
that the inclusion of Lyman dropout point sources is necessary for a complete
census of the early galaxies at the epoch of cosmic reionization. | astro-ph_GA |
Better Together: The Complex Interplay Between Radiative Cooling and
Magnetic Draping: Rapidly outflowing cold H-I gas is ubiquitously observed to be co-spatial
with a hot phase in galactic winds, yet the ablation time of cold gas by the
hot phase should be much shorter than the acceleration time. Previous work
showed efficient radiative cooling enables clouds to survive in hot galactic
winds under certain conditions, as can magnetic fields even in purely adiabatic
simulations for sufficiently small density contrasts between the wind and
cloud. In this work, we study the interplay between radiative cooling and
magnetic draping via three dimensional radiative magnetohydrodynamic
simulations with perpendicular ambient fields and tangled internal cloud
fields. We find magnetic fields decrease the critical cloud radius for survival
by two orders of magnitude (i.e., to sub-pc scales) in the strongly magnetized
($\beta_{\rm wind}=1$) case. Our results show magnetic fields (i) accelerate
cloud entrainment through magnetic draping, (ii) can cause faster cloud
destruction in cases of inefficient radiative cooling, (iii) do not
significantly suppress mass growth for efficiently cooling clouds, and,
crucially, in combination with radiative cooling (iv) reduce the average
overdensity by providing non-thermal pressure support of the cold gas. This
substantially reduces the acceleration time compared to the destruction time
(more than due to draping alone), enhancing cloud survival. Our results may
help to explain the cold, tiny, rapidly outflowing cold gas observed in
galactic winds and the subsequent high covering fraction of cold material in
galactic halos. | astro-ph_GA |
Variations of the initial mass function in semi-analytical models:
implications for the mass assembly and the chemical enrichment of galaxies in
the GAEA model: In this work, we investigate the implications of the Integrated Galaxy-wide
stellar Initial Mass Function (IGIMF) approach in the framework of the
semi-analytic model GAEA (GAlaxy Evolution and Assembly), which features a
detailed treatment of chemical enrichment and stellar feedback. The IGIMF
provides an analytic description of the dependence of the stellar IMF shape on
the rate of star formation in galaxies. We find that our model with a universal
IMF predicts a rather flat [$\alpha$/Fe]-stellar mass relation. The model
assuming the IGIMF, instead, is able to reproduce the observed increase of
$\alpha$-enhancement with stellar mass, in agreement with previous studies.
This is mainly due to the fact that massive galaxies are characterized by
larger star formation rates at high-redshift, leading to stronger
$\alpha$-enhancement with respect to low-mass galaxies. At the same time, the
IGIMF hypothesis does not affect significantly the trend for shorter star
formation timescales for more massive galaxies. We argue that in the IGIMF
scenario the [$\alpha$/Fe] ratios are good tracers of the highest star
formation events. The final stellar masses and mass-to-light-ratio of our model
massive galaxies are larger than those estimated from the synthetic photometry
assuming a universal IMF, providing a self-consistent interpretation of similar
recent results, based on dynamical analysis of local early type galaxies. | astro-ph_GA |
How does star formation proceed in the circumnuclear starburst ring of
NGC 6951?: Gas inflowing along stellar bars is often stalled at the location of
circumnuclear rings, that form an effective reservoir for massive star
formation and thus shape the central regions of galaxies. However, how exactly
star formation is proceeding within these circumnuclear starburst rings is
subject of debate. Two main scenarios for this process have been put forward:
In the first the onset of star formation is regulated by the total amount of
gas present in the ring with star forming starting once a mass threshold has
reached in a `random' position within the ring like `popcorn'. In the second
star formation preferentially takes place near the locations where the gas
enters the ring. This scenario has been dubbed `pearls-on-a-string'. Here we
combine new optical IFU data covering the full stellar bar with existing
multi-wavelength data to study in detail the 580 pc radius circumnuclear
starburst ring in the nearby spiral galaxy NGC 6951. Using HST archival data
together with Sauron and Oasis IFU data, we derive the ages and stellar masses
of star clusters as well as the total stellar content of the central region.
Adding information on the molecular gas distribution, stellar and gaseous
dynamics and extinction, we find that the circumnuclear ring in NGC 6951 is
~1-1.5 Gyr old and has been forming stars for most of that time. We see
evidence for preferred sites of star formation within the ring, consistent with
the `pearls-on-a-string' scenario, when focusing on the youngest stellar
populations. Due to the ring's longevity this signature is washed out when
older stellar populations are included in the analysis. | astro-ph_GA |
Galactic Chemical Evolution and the Oxygen Isotopic Composition of the
Solar System: We review current observational and theoretical constraints on the Galactic
chemical evolution (GCE) of oxygen isotopes in order to explore whether GCE
plays a role in explaining the lower 17O/18O ratio of the Sun, relative to the
present-day interstellar medium, or the existence of distinct 16O-rich and
16O-poor reservoirs in the Solar System. Although the production of both 17O
and 18O are related to the metallicity of progenitor stars, 17O is most likely
produced in stars that evolve on longer timescales than those that produce 18O.
Therefore the 17O/18O ratio need not have remained constant over time, contrary
to preconceptions and the simplest models of GCE. An apparent linear, slope-one
correlation between delta17O and delta18O in the ISM need not necessarily
reflect an O isotopic gradient, and any slope-one galactocentric gradient need
not correspond to evolution in time. Instead, increasing 17O/18O is consistent
both with observational data from molecular clouds and with modeling of the
compositions of presolar grains. Models in which the rate of star formation has
decelerated over the past few Gyr or in which an enhanced period of star
formation occurred shortly before solar birth ("starburst") can explain the
solar-ISM O-isotopic difference without requiring a local input of supernova
ejecta into the protosolar cloud. "Cosmic chemical memory" models in which
interstellar dust is on average older than interstellar gas predict that
primordial Solar System solids should be 16O-rich, relative to the Sun, in
conflict with observations. However, scenarios in which the 16O-rich
contribution of very massive stars could lead to 16O-poor solids and a 16O-rich
bulk Sun, if the Solar System formed shortly after a starburst, independent of
the popular scenario of photochemical self-shielding of CO. | astro-ph_GA |
Confirmation and refutation of very luminous galaxies in the early
universe: During the first 500 million years of cosmic history, the first stars and
galaxies formed, seeding the Universe with heavy elements and eventually
reionizing the intergalactic medium. Observations with JWST have uncovered a
surprisingly high abundance of candidates for early star-forming galaxies, with
distances (redshifts, $z$), estimated from multi-band photometry, as large as
$z\approx 16$, far beyond pre-JWST limits. While generally robust, such
photometric redshifts can suffer from degeneracies and occasionally
catastrophic errors. Spectroscopic measurement is required to validate these
sources and to reliably quantify physical properties that can constrain galaxy
formation models and cosmology. Here we present JWST spectroscopy that confirms
redshifts for two very luminous galaxies with $z > 11$, but also demonstrates
that another candidate with suggested $z\approx 16$ instead has $z = 4.9$, with
an unusual combination of nebular line emission and dust reddening that mimics
the colors expected for much more distant objects. These results reinforce
evidence for the early, rapid formation of remarkably luminous galaxies, while
also highlighting the necessity of spectroscopic verification. The large
abundance of bright, early galaxies may indicate shortcomings in current galaxy
formation models, or deviation from physical properties (such as the stellar
initial mass function) that are generally believed to hold at later times. | astro-ph_GA |
Spatially-resolved spectroscopy of narrow-line Seyfert 1 host galaxies: We present optical integral field spectroscopy for five $z<0.062$ narrow-line
Seyfert 1 galaxies (NLS1s) host galaxies, probing their host galaxies at
$\gtrsim 2-3$ kpc scales. Emission lines in the nuclear AGN spectra and the
large-scale host galaxy are analyzed separately, based on an AGN-host
decomposition technique. The host galaxy gas kinematics indicates large-scale
gas rotation in all five sources. At the probed scales of $\gtrsim 2-3$ kpc,
the host galaxy gas is found to be predominantly ionized by star formation
without any evidence of a strong AGN contribution. None of the five objects
shows specific star formation rates exceeding the main sequence of low-redshift
star forming galaxies. The specific star formation rates for MCG-05-01-013 and
WPVS 007 are roughly consistent with the main sequence, while ESO 399-IG20, MS
22549-3712, and TON S180 show lower specific star formation rates, intermediate
to the main sequence and red quiescent galaxies. The host galaxy metallicities,
derived for the two sources with sufficient data quality (ESO 399-IG20 and
MCG-05-01-013), indicate central oxygen abundances just below the low-redshift
mass-metallicity relation. Based on this initial case study, we outline a
comparison of AGN and host galaxy parameters as a starting point for future
extended NLS1 studies with similar methods. | astro-ph_GA |
An infrared view of AGN feedback in a type-2 quasar: the case of the
Teacup galaxy: We present near-infrared integral field spectroscopy data obtained with
VLT/SINFONI of "the Teacup galaxy". The nuclear K-band (1.95-2.45 micron)
spectrum of this radio-quiet type-2 quasar reveals a blueshifted broad
component of FWHM~1600-1800 km/s in the hydrogen recombination lines
(Pa$\alpha$, Br$\delta$, and Br$\gamma$) and also in the coronal line [Si
VI]$\lambda$1.963 micron. Thus the data confirm the presence of the nuclear
ionized outflow previously detected in the optical and reveal its coronal
counterpart. Both the ionized and coronal nuclear outflows are resolved, with
seeing-deconvolved full widths at half maximum of 1.1$\pm$0.1 and 0.9$\pm$0.1
kpc along PA$\sim$72-74 deg. This orientation is almost coincident with the
radio axis (PA=77 deg), suggesting that the radio jet could have triggered the
nuclear outflow. In the case of the H$_2$ lines we do not require a broad
component to reproduce the profiles, but the narrow lines are blueshifted by
~50 km/s on average from the galaxy systemic velocity. This could be an
indication of the presence of a nuclear molecular outflow, although the bulk of
the H$_2$ emission in the inner ~2 arcsec (~3 kpc) of the galaxy follows a
rotation pattern. We find evidence for kinematically disrupted gas (FWHM>250
km/s) at up to 5.6 kpc from the AGN, which can be naturally explained by the
action of the outflow. The narrow component of [Si VI] is redshifted with
respect to the systemic velocity, unlike any other emission line in the K-band
spectrum. This indicates that the region where the coronal lines are produced
is not co-spatial with the narrow line region. | astro-ph_GA |
Chemical Evolution of R-process Elements in Stars (CERES). I. Stellar
parameters and chemical abundances from Na to Zr: Aims. The Chemical Evolution of R-process Elements in Stars (CERES) project
aims to provide a homogeneous analysis of a sample of metal-poor stars
([Fe/H]<-1.5). We present the stellar parameters and the chemical abundances of
elements up to Zr for a sample of 52 giant stars.Methods. We relied on a sample
of high signal-to-noise UVES spectra. We determined stellar parameters from
Gaia photometry and parallaxes. Chemical abundances were derived using spectrum
synthesis and model atmospheres.Results. We determined chemical abundances of
26 species of 18 elements: Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni,
Cu, Zn, Sr, Y, and Zr. For several stars, we were able to measure both neutral
and ionised species, including Si, Sc, Mn, and Zr. We have roughly doubled the
number of measurements of Cu for stars at [Fe/H] <= -2.5. The homogeneity of
the sample made it possible to highlight the presence of two Zn-rich stars
([Zn/Fe]~+0.7), one r-rich and the other r-poor. We report the existence of two
branches in the [Zn/Fe] versus [Ni/Fe] plane and suggest that the high [Zn/Fe]
branch is the result of hypernova nucleosynthesis. We discovered two stars with
peculiar light neutron-capture abundance patterns: CES1237+1922 (also known as
BS 16085-0050), which is ~1 dex underabundant in Sr, Y, and Zr with respect to
the other stars in the sample, and CES2250-4057 (also known as HE 2247-4113),
which shows a ~1 dex overabundance of Sr with respect to Y and Zr.Conclusions.
The high quality of our dataset allowed us to measure hardly detectable ions.
This can provide guidance in the development of line formation computations
that take deviations from local thermodynamic equilibrium and hydrodynamical
effects into account. | astro-ph_GA |
Spectral scaling laws in MHD turbulence simulations and in the solar
wind: The question is addressed to what extent incompressible magnetohydrodynamics
(MHD) can describe random magnetic and velocity fluctuations measured in the
solar wind. It is demonstrated that distributions of spectral indices for the
velocity, magnetic field, and total energy obtained from high resolution
numerical simulations are qualitatively and quantitatively similar to solar
wind observations at 1 AU. Both simulations and observations show that in the
inertial range the magnetic field spectrum E_b is steeper than the velocity
spectrum E_v with E_b >~ E_v and that the residual energy E_R = E_b-E_v
decreases nearly following a k_perp^-2 scaling. | astro-ph_GA |
An uncertainty principle for star formation -- III. The characteristic
emission time-scales of star formation rate tracers: We recently presented a new statistical method to constrain the physics of
star formation and feedback on the cloud scale by reconstructing the underlying
evolutionary timeline. However, by itself this new method only recovers the
relative durations of different evolutionary phases. To enable observational
applications, it therefore requires knowledge of an absolute 'reference
time-scale' to convert relative time-scales into absolute values. The logical
choice for this reference time-scale is the duration over which the star
formation rate (SFR) tracer is visible because it can be characterised using
stellar population synthesis (SPS) models. In this paper, we calibrate this
reference time-scale using synthetic emission maps of several SFR tracers,
generated by combining the output from a hydrodynamical disc galaxy simulation
with the SPS model SLUG2. We apply our statistical method to obtain
self-consistent measurements of each tracer's reference time-scale. These
include H${\alpha}$ and 12 ultraviolet (UV) filters (from GALEX, Swift, and
HST), which cover a wavelength range 150-350 nm. At solar metallicity, the
measured reference time-scales of H${\alpha}$ are ${4.32^{+0.09}_{-0.23}}$ Myr
with continuum subtraction, and 6-16 Myr without, where the time-scale
increases with filter width. For the UV filters we find 17-33 Myr, nearly
monotonically increasing with wavelength. The characteristic time-scale
decreases towards higher metallicities, as well as to lower star formation rate
surface densities, owing to stellar initial mass function sampling effects. We
provide fitting functions for the reference time-scale as a function of
metallicity, filter width, or wavelength, to enable observational applications
of our statistical method across a wide variety of galaxies. | astro-ph_GA |
WALLABY Early Science - III. An HI Study of the Spiral Galaxy NGC 1566: This paper reports on the atomic hydrogen gas (HI) observations of the spiral
galaxy NGC 1566 using the newly commissioned Australian Square Kilometre Array
Pathfinder (ASKAP) radio telescope. We measure an integrated HI flux density of
$180.2$ Jy km s$^{-1}$ emanating from this galaxy, which translates to an HI
mass of $1.94\times10^{10}$M$_\circ$ at an assumed distance of $21.3$ Mpc. Our
observations show that NGC 1566 has an asymmetric and mildly warped HI disc.
The HI-to-stellar mass fraction of NGC 1566 is $0.29$, which is high in
comparison with galaxies that have the same stellar mass
($10^{10.8}$M$_\circ$). We also derive the rotation curve of this galaxy to a
radius of $50$ kpc and fit different mass models to it. The NFW, Burkert and
pseudo-isothermal dark matter halo profiles fit the observed rotation curve
reasonably well and recover dark matter fractions of $0.62$, $0.58$ and $0.66$,
respectively. Down to the column density sensitivity of our observations
($N_{HI} = 3.7\times10^{19}$ cm$^{-2}$), we detect no HI clouds connected to,
or in the nearby vicinity of, the HI disc of NGC 1566 nor nearby interacting
systems. We conclude that, based on a simple analytic model, ram pressure
interactions with the IGM can affect the HI disc of NGC 1566 and is possibly
the reason for the asymmetries seen in the HI morphology of NGC 1566. | astro-ph_GA |
Red Clump stars from LAMOST II: the outer disc of the Milky Way: We present stellar density maps of the Galactic outer disc with red clump
stars from the LAMOST data. These samples are separated into younger (mean age
~ 2.7 Gyr) and older (mean age ~ 4.6 Gyr) populations so that they can trace
the variation of the structures with ages in the range of the Galactocentric
radius R from 9 to 13.5 kpc. We show that both the scale heights for the two
populations increase with R and display radial gradients of 48 +/- 6 and 40 +/-
4 pc/kpc for the older and younger populations, respectively. This is evident
that the flaring occurs in the thin disc populations with a wide range of ages.
Moreover, the intensity of flaring seems not significantly related to the age
of the thin disc populations. On the other hand, the scale lengths of the
radial surface density profiles are 4.7 +/- 0.5 kpc for the younger and 3.4 +/-
0.2 kpc for the older population, meaning that the younger disc population is
more radially extended than the older one. Although the fraction of the younger
population mildly increases from 28% at R ~ 9 to about 35% at R ~ 13 kpc, the
older population is prominent with the fraction no less than 65% in the outer
disc. | astro-ph_GA |
A comparison of the distribution of satellite galaxies around Andromeda
and the results of $Λ$CDM simulations: Ibata et al. (2013) recently reported the existence of a vast thin plane of
dwarf galaxies (VTPD) orbiting around Andromeda. We investigate whether such a
configuration can be reproduced within the standard cosmological framework and
search for similar planes of co-rotating satellite galaxies around
Andromeda-like host haloes in data from the Millennium II simulation combined
with a semi-analytic galaxy formation model. We apply a baryonic mass cut of
$2.8\times 10^4 \text{M}_{\text{sun}}$ for the satellite haloes and restrict
the data to a PAndAS like field. If we include the so-called orphan galaxies in
our analysis, we find that planes with a rms lower than the VTPD are common in
Millennium II. This is partially due to the strongly radially concentrated
distribution of orphan galaxies. Excluding part of the orphan galaxies brings
the radial distributions of Millennium II satellites into better agreement with
the satellite distribution of Andromeda while still producing a significant
fraction of planes with a lower rms than the VTPD. We also find haloes in
Millennium II with an equal or higher number of co-rotating satellites than the
VTPD. This demonstrates that the VTPD is not in conflict with the standard
cosmological framework, although a definite answer of this question might
require higher resolution cosmological simulations that do not have to consider
orphan galaxies. Our results finally show that satellite planes in Millennium
II are not stable structures, hence the VTPD might only be a statistical
fluctuation of an underlying more spherical galaxy distribution. | astro-ph_GA |
High velocity stars from the interaction of a globular cluster and a
massive black hole binary: High velocity stars are stars moving at velocities so high to require an
acceleration mechanism involving binary systems or the presence of a massive
central black hole. In the frame of a galaxy hosting a supermassive black hole
binary (of total mass $10^8$ M$_\odot$), we investigated a mechanism for the
production of high velocity stars due to the close interaction between a
massive and orbitally decayed globular cluster and the super massive black hole
binary. Some stars of the cluster acquire high velocities by conversion of
gravitational energy into kinetic energy deriving from their interaction with
the black hole binary. After the interaction, few stars reach a velocity
sufficient to overcome the galactic gravitational well, while some of them are
just stripped from the globular cluster and start orbiting around the galactic
centre. | astro-ph_GA |
The Optical to Mid-Infrared Extinction Law Based on the APOGEE, Gaia
DR2, Pan-STARRS1, SDSS, APASS, 2MASS and WISE Surveys: A precise interstellar dust extinction law is critically important to
interpret observations. There are two indicators of extinction: the color
excess ratio (CER) and the relative extinction. Compared to the CER, the
wavelength-dependent relative extinction is more challenging to be determined.
In this work, we combine spectroscopic, astrometric, and photometric data to
derive high-precision CERs and relative extinction from optical to mid-infrared
(IR) bands. A group of 61,111 red clump (RC) stars are selected as tracers by
stellar parameters from APOGEE survey. The multiband photometric data are
collected from Gaia, APASS, SDSS, Pan-STARRS1, 2MASS, and WISE surveys. For the
first time, we calibrate the curvature of CERs in determining CERs
E(lambda-GRP)/E(GBP-GRP) from color excess--color excess diagrams. Through
elaborate uncertainty analysis, we conclude that the precision of our CERs is
significantly improved (sigma < 0.015). With parallaxes from Gaia DR2, we
calculate the relative extinction A_GBP/A_GRP for 5051 RC stars. By combining
the CERs with the A_GBP/A_GRP, the optical--mid-IR extinction A_lambda/A_GRP
has been determined in a total of 21 bands. Given no bias toward any specific
environment, our extinction law represents the average extinction law with the
total-to-selective extinction ratio Rv=3.16+-0.15. Our observed extinction law
supports an adjustment in parameters of the CCM Rv=3.1 curve, together with the
near-IR power-law index alpha=2.07+-0.03. The relative extinction values of HST
and JWST near-IR bandpasses are predicted in 2.5% precision. As the observed
reddening/extinction tracks are curved, the curvature correction needs to be
considered when applying extinction correction. | astro-ph_GA |
Dark Matter In Disk Galaxies II: Density Profiles as Constraints on
Feedback Scenarios: The disparity between the density profiles of galactic dark matter haloes
predicted by dark matter only cosmological simulations and those inferred from
rotation curve decomposition, the so-called cusp-core problem, suggests that
baryonic physics has an impact on dark matter density in the central regions of
galaxies. Feedback from black holes, supernovae and massive stars may each play
a role by removing matter from the centre of the galaxy on shorter timescales
than the dynamical time of the dark matter halo. Our goal in this paper is to
determine constraints on such feedback scenarios based on the observed
properties of a set of nearby galaxies.
Using a Markov Chain Monte Carlo (MCMC) analysis of galactic rotation curves,
via a method developed in a previous paper, we constrain density profiles and
an estimated minimum radius for baryon influence, $r_1$, which we couple with a
feedback model to give an estimate of the fraction of matter within that radius
that must be expelled to produce the presently observed halo profile. We show
that in the case of the gas rich dwarf irregular galaxy DDO 154, an outflow
from a central source (e.g. a black hole or star forming region) could produce
sufficient feedback on the halo without removing the disk gas.
We examine the rotation curves of 8 galaxies taken from the THINGS data set
and determine constraints on the radial density profiles of their dark matter
haloes. For some of the galaxies, both cored haloes and cosmological $\rho
\propto r^{-1}$ cusps are excluded. These intermediate central slopes require
baryonic feedback to be finely tuned. We also find for galaxies which exhibit
extended cores in their haloes (e.g. NGC 925), the use of a split power-law
halo profile yields models without the unphysical, sharp features seen in
models based on the Einasto profile. | astro-ph_GA |
Gaia FGK Benchmark Stars - Metallicity: To calibrate automatic pipelines that determine atmospheric parameters of
stars, one needs a sample of stars -- ``benchmark stars'' -- with well defined
parameters to be used as a reference We provide a detailed documentation of the
determination of the iron abundance of the 34 FGK-type benchmark stars selected
to be the pillars for calibration of the one billion Gaia stars. They cover a
wide range of temperatures, surface gravities and metallicities. Up to seven
different methods were used to analyze an observed spectral library of high
resolution and high signal-to-noise ratio. The metallicity was determined
assuming a value of effective temperature and surface gravity obtained from
fundamental relations, i.e. these parameters were known a priori independently
from the spectra. We present a set of metallicity values obtained in a
homogeneous way for our sample of Benchmark Stars. In addition to this value,
we provide a detailed documentation of the associated uncertainties. Finally,
we report for the first time a value of the metallicity of the cool giant psi
Phe. | astro-ph_GA |
A Bayesian Approach to the Vertical Structure of the Disk of the Milky
Way: This work investigates the vertical profile of the stars in the disk of the
Milky Way. The models investigated are of the form $sech^{2/n}(nz/(2H))$ where,
setting $\alpha = 2/n$, the three functions of the sequence $\alpha = 0,1,2$
correspond to exponential, $sech$, $sech^2$ functions. We consider symmetric
models and asymmetric models, above and below the plane. The study uses the
large sample of K and M stars of Ferguson et al. (2017) and applies the methods
of Bayesian model comparison to discriminate between the 6 models. Two
inconsistencies in Ferguson et al. (2017), concerning the vertical height cut
and the model continuity across the plane, are noted and addressed. We find
that (1) in the Milky Way the symmetric disc models are decisively ruled out,
with northern thin disc scale heights $\sim25\%$ larger than southern, (2)
there is moderate evidence for the exponential and $sech$ models over the
$sech^2$ model, though a sample extending further into the Galactic mid-plane
is needed to strengthen this result, (3) the photometric distances used by
Ferguson et al. underestimate the GAIA distances by a factor of roughly 1.16,
and (4) the increase of scale height with Galactic latitude observed by
Ferguson et al. is due to incorrect cuts to the data. | astro-ph_GA |
Denoising Diffusion Probabilistic Models to Predict the Density of
Molecular Clouds: We introduce the state-of-the-art deep learning Denoising Diffusion
Probabilistic Model (DDPM) as a method to infer the volume or number density of
giant molecular clouds (GMCs) from projected mass surface density maps. We
adopt magnetohydrodynamic simulations with different global magnetic field
strengths and large-scale dynamics, i.e., noncolliding and colliding GMCs. We
train a diffusion model on both mass surface density maps and their
corresponding mass-weighted number density maps from different viewing angles
for all the simulations. We compare the diffusion model performance with a more
traditional empirical two-component and three-component power-law fitting
method and with a more traditional neural network machine learning approach
(CASI-2D). We conclude that the diffusion model achieves an order of magnitude
improvement on the accuracy of predicting number density compared to that by
other methods. We apply the diffusion method to some example astronomical
column density maps of Taurus and the Infrared Dark Clouds (IRDCs) G28.37+0.07
and G35.39-0.33 to produce maps of their mean volume densities. | astro-ph_GA |
Resolved Star Formation on Sub-galactic Scales in a Merger at z=1.7: We present a detailed analysis of Hubble Space Telescope (HST), Wide Field
Camera 3 (WFC3) G141 grism spectroscopy for seven star-forming regions of the
highly magnified lensed starburst galaxy RCSGA 032727-132609 at z=1.704. We
measure the spatial variations of the extinction in RCS0327 through the
observed H$\gamma$/H$\beta$ emission line ratios, finding a constant average
extinction of $\mathrm{E(B-V)_{gas}}=0.40\pm0.07$. We infer that the star
formation is enhanced as a result of an ongoing interaction, with measured star
formation rates derived from demagnified, extinction-corrected H$\beta$ line
fluxes for the individual star-forming clumps falling >1-2 dex above the star
formation sequence. When combining the HST/WFC3 [OIII]$\lambda$5007/H$\beta$
emission line ratio measurements with [NII]/H$\alpha$ line ratios from Wuyts et
a. (2014), we find that the majority of the individual star-forming regions
fall along the local "normal" abundance sequence. With the first detections of
the He I $\lambda$5876 and He II $\lambda$4686 recombination lines in a distant
galaxy, we probe the massive-star content of the star-forming regions in
RCS0327. The majority of the star-forming regions have a He I $\lambda$5876 to
H$\beta$ ratio consistent with the saturated maximum value, which is only
possible if they still contain hot O-stars. Two regions have lower ratios,
implying that their last burst of new star formation ended ~5 Myr ago.
Together, the He I $\lambda$5876 and He II $\lambda$4686 to H$\beta$ line
ratios provide indirect evidence for the order in which star formation is
stopping in individual star-forming knots of this high redshift merger. We
place the spatial variations of the extinction, star formation rate and
ionization conditions in the context of the star formation history of RCS0327. | astro-ph_GA |
The Spitzer Space Telescope Survey of the Orion A and B Molecular Clouds
II: the Spatial Distribution and Demographics of Dusty Young Stellar Objects: We analyze the spatial distribution of dusty young stellar objects (YSOs)
identified in the Spitzer Survey of the Orion Molecular clouds, augmenting
these data with Chandra X-ray observations to correct for incompleteness in
dense clustered regions. We also devise a scheme to correct for spatially
varying incompleteness when X-ray data are not available. The local surface
densities of the YSOs range from 1 pc$^{-2}$ to over 10,000 pc$^{-2}$, with
protostars tending to be in higher density regions. This range of densities is
similar to other surveyed molecular clouds with clusters, but broader than
clouds without clusters. By identifying clusters and groups as continuous
regions with surface densities $\ge10$ pc$^{-2}$, we find that 59% of the YSOs
are in the largest cluster, the Orion Nebular Cluster (ONC), while 13% of the
YSOs are found in a distributed population. A lower fraction of protostars in
the distributed population is evidence that it is somewhat older than the
groups and clusters. An examination of the structural properties of the
clusters and groups show that the peak surface densities of the clusters
increase approximately linearly with the number of members. Furthermore, all
clusters with more than 70 members exhibit asymmetric and/or highly elongated
structures. The ONC becomes azimuthally symmetric in the inner 0.1 pc,
suggesting that the cluster is only $\sim 2$ Myr in age. We find the star
formation efficiency (SFE) of the Orion B cloud is unusually low, and that the
SFEs of individual groups and clusters are an order of magnitude higher than
those of the clouds. Finally, we discuss the relationship between the young low
mass stars in the Orion clouds and the Orion OB 1 association, and we determine
upper limits to the fraction of disks that may be affected by UV radiation from
OB stars or by dynamical interactions in dense, clustered regions. | astro-ph_GA |
Searching for new observational signatures of the dynamical evolution of
star clusters: We present a numerical study, based on Monte Carlo simulations, aimed at
defining new empirical parameters measurable from observations and able to
trace the different phases of star cluster dynamical evolution. As expected, a
central density cusp, deviating from the King model profile, develops during
the core collapse (CC) event. Although the slope varies during the post-CC
oscillations, the cusp remains a stable feature characterizing the central
portion of the density profile in all post-CC stages. We then investigate the
normalized cumulative radial distribution (nCRD) drawn by all the cluster stars
included within one half the tridimensional half-mass radius (R<0.5 rh),
finding that its morphology varies in time according to the cluster's dynamical
stage. To quantify these changes we defined three parameters: A5, the area
subtended by the nCRD within 5% of the half-mass radius, P5, the value of the
nCRD measured at the same distance, and S2.5, the slope of the straight line
tangent to the nCRD measured at R=2.5% rh. The three parameters evolve
similarly during the cluster's dynamical evolution: after an early phase in
which they are essentially constant, their values rapidly increase, reaching
their maximum at the CC epoch and slightly decreasing in the post-CC phase,
when their average value remains significantly larger than the initial one, in
spite of some fluctuations. The results presented in the paper suggest that
these three observable parameters are very promising empirical tools to
identify the star cluster's dynamical stage from observational data. | astro-ph_GA |
N-body simulations of the Carina dSph in MOND: The classical dwarf spheroidals (dSphs) provide a critical test for Modified
Newtonian Dynamics (MOND) because they are observable satellite galactic
systems with low internal accelerations and low, but periodically varying,
external acceleration. This varying external gravitational field is not
commonly found acting on systems with low internal acceleration. Using Jeans
modelling, Carina in particular has been demonstrated to require a V-band
mass-to-light ratio greater than 5, which is the nominal upper limit for an
ancient stellar population. We run MOND N-body simulations of a Carina-like
dSph orbiting the Milky Way to test if dSphs in MOND are stable to tidal forces
over the Hubble time and if those same tidal forces artificially inflate their
velocity dispersions and therefore their apparent mass-to-light ratio. We run
many simulations with various initial total masses for Carina, and
Galactocentric orbits (consistent with proper motions), and compare the
simulation line of sight velocity dispersions (losVDs) with the observed losVDs
of Walker et al. (2007). We find that the dSphs are stable, but that the tidal
forces are not conducive to artificially inflating the losVDs. Furthermore, the
range of mass-to-light ratios that best reproduces the observed line of sight
velocity dispersions of Carina is 5.3 to 5.7 and circular orbits are preferred
to plunging orbits. Therefore, some tension still exists between the required
mass-to-light ratio for the Carina dSph in MOND and those expected from stellar
population synthesis models. It remains to be seen whether a careful treatment
of the binary population or triaxiality might reduce this tension. | astro-ph_GA |
High-velocity stars in the cores of globular clusters: The illustrative
case of NGC 2808: We report the detection of five high-velocity stars in the core of the
globular cluster NGC 2808. The stars lie on the the red giant branch and show
total velocities between 40 and 45 km/s. For a core velocity dispersion sigma_c
= 13.4 km/s, this corresponds to up to 3.4 sigma_c. These velocities are close
to the estimated escape velocity (~ 50 km/s) and suggest an ejection from the
core. Two of these stars have been confirmed in our recent integral field
spectroscopy data and we will discuss them in more detail here. These two red
giants are located at a projected distance of ~ 0.3 pc from the center.
According to their positions on the color magnitude diagram, both stars are
cluster members. We investigate several possible origins for the high
velocities of the stars and conceivable ejection mechanisms. Since the
velocities are close to the escape velocity, it is not obvious whether the
stars are bound or unbound to the cluster. We therefore consider both cases in
our analysis. We perform numerical simulations of three-body dynamical
encounters between binaries and single stars and compare the resulting velocity
distributions of escapers with the velocities of our stars. We compare the
predictions for a single dynamical encounter with a compact object with those
of a sequence of two-body encounters due to relaxation. If the stars are
unbound, the encounter must have taken place recently, when the stars were
already in the giant phase. After including binary fractions and black-hole
retention fractions, projection effects, and detection probabilities from
Monte-Carlo simulations, we estimate the expected numbers of detections for all
the different scenarios. Based on these numbers, we conclude that the most
likely scenario is that the stars are bound and were accelerated by a single
encounter between a binary of main-sequence stars and a ~ 10 M_sun black hole. | astro-ph_GA |
Tracing the Milky Way Nuclear Wind with 21cm Atomic Hydrogen Emission: There is evidence in 21cm HI emission for voids several kpc in size centered
approximately on the Galactic centre, both above and below the Galactic plane.
These appear to map the boundaries of the Galactic nuclear wind. An analysis of
HI at the tangent points, where the distance to the gas can be estimated with
reasonable accuracy, shows a sharp transition at Galactic radii $R\lesssim 2.4$
kpc from the extended neutral gas layer characteristic of much of the Galactic
disk, to a thin Gaussian layer with FWHM $\sim 125$ pc. An anti-correlation
between HI and $\gamma$-ray emission at latitudes $10^{\circ} \leq |b| \leq
20^{\circ}$ suggests that the boundary of the extended HI layer marks the walls
of the Fermi Bubbles. With HI we are able to trace the edges of the voids from
$|z| > 2$ kpc down to $z\approx0$, where they have a radius $\sim 2$ kpc. The
extended HI layer likely results from star formation in the disk, which is
limited largely to $R \gtrsim 3$ kpc, so the wind may be expanding into an area
of relatively little HI. Because the HI kinematics can discriminate between gas
in the Galactic center and foreground material, 21cm HI emission may be the
best probe of the extent of the nuclear wind near the Galactic plane. | astro-ph_GA |
The formation of the Milky Way halo and its dwarf satellites: a NLTE-1D
abundance analysis. IV. Segue 1, Triangulum II, and Coma Berenices UFDs: We present atmospheric parameters and abundances for chemical elements from
carbon to barium in metal-poor stars in Segue 1 (seven stars), Coma Berenices
(three stars), and Triangulum II (one star) ultra-faint dwarf galaxies (UFDs).
The effective temperatures rely on new photometric observations in the visible
and infra-red bands, obtained with the 2.5 m telescope of the SAI MSU Caucasian
observatory. Abundances of up to fourteen chemical elements were derived under
the non-local thermodynamic equilibrium (NLTE) line formation, and LTE
abundances were obtained for up to five more elements. For the first time we
present abundance of oxygen in Seg 1 S1 and S4, silicon in ComaBer S2 and Tri
II S40, potassium in Seg 1 S1-S6 and ComaBer S1-S3, and barium in Seg 1 S7.
Three stars in Segue 1, two stars in Coma Berenices, and Triangulum II star
have very low [Na/Mg] of -1.08 to -1.67 dex, which is usually attributed in the
literature to an odd-even effect produced by nucleosynthesis in massive
metal-free stars. We interpret this chemical property as a footprint of first
stars, which is not blurred due to a small number of nucleosynthesis events
that contributed to chemical abundance patterns of the sample stars. Our NLTE
abundances of Sr and Ba in Coma Berenices, Segue 1, and Triangulum II report on
lower [Sr/Ba] abundance ratio in the UFDs compared to that in classical dwarf
spheroidal galaxies and the Milky Way halo. However, in UFDs, just as in
massive galaxies, [Sr/Ba] is not constant and it can be higher than the pure
r-process ratio. We suggest a hypothesis of Sr production in metal-poor
binaries at the earliest epoch of galactic evolution. | astro-ph_GA |
Radio Imaging of the NGC 2024 FIR 5/6 Region: a Hypercompact H II Region
Candidate in Orion: The NGC 2024 FIR 5/6 region was observed in the 6.9 mm continuum with an
angular resolution of about 1.5 arcsec. The 6.9 mm continuum map shows four
compact sources, FIR 5w, 5e, 6c, and 6n, as well as an extended structure of
the ionization front associated with the optical nebulosity. FIR 6c has a
source size of about 0.4 arcsec or 150 AU. The spectral energy distribution
(SED) of FIR 6c is peculiar: rising steeply around 6.9 mm and flat around 1 mm.
The possibility of a hypercompact H II region is explored. If the millimeter
flux of FIR 6c comes from hot ionized gas heated by a single object at the
center, the central object may be a B1 star of about 5800 solar luminosities
and about 13 solar masses. The 6.9 mm continuum of FIR 6n may be a mixture of
free-free emission and dust continuum emission. Archival data show that both
FIR 6n and 6c exhibit water maser activity, suggesting the existence of shocked
gas around them. The 6.9 mm continuum emission from FIR 5w has a size of about
1.8 arcsec or 760 AU. The SEDs suggest that the 6.9 mm emission of FIR 5w and
5e comes from dust, and the masses of the dense molecular gas are about 0.6 and
0.5 solar masses, respectively. | astro-ph_GA |
Properties of dense molecular gas along the major axis of M 82: Dense gas is important for galaxy evolution and star formation.
Optically-thin dense-gas tracers, such as isotopologues of HCN, HCO+, etc., are
very helpful to diagnose excitation conditions of dense molecular gas. However,
previous studies of optically-thin dense-gas tracers were mostly focusing on
average properties of galaxies as a whole, due to limited sensitivity and
angular resolution. M82, a nearby prototype starburst galaxy, offers a unique
case for spatially-resolved studies with single-dish telescopes. With the IRAM
30-m telescope, we observed the J = 1 - 0 transition of H13CN, HC15N, H13CO+,
HN13C, H15NC, and SiO J = 2 - 1, HC3N J= 10 - 9, H2CO J = 2 - 1 toward five
positions along the major axis of M82. The intensity ratios of I(HCN)/I(H13CN)
and I(HCO+)/I(H13CO+) show a significant spatial variation along the major
axis, with lower values in the central region than those on the disk,
indicating higher optical depths in the central region. The optical depths of
HCO+ lines are found to be systematically higher than those of HCN lines at all
positions. Futhermore, we find that the 14N/15N ratios have an increasing
gradient from the center to the outer disk. | astro-ph_GA |
VINTERGATAN III: how to reset the metallicity of the Milky Way: Using the cosmological zoom simulation VINTERGATAN, we present a new scenario
for the onset of star formation at the metal-poor end of the low-[$\alpha$/Fe]
sequence in a Milky Way-like galaxy. In this scenario, the galaxy is fueled by
two distinct gas flows. One is enriched by outflows from massive galaxies, but
not the other. While the former feeds the inner galactic region, the latter
fuels an outer gas disk, inclined with respect to the main galactic plane, and
with a significantly poorer chemical content. The first passage of the last
major merger galaxy triggers tidal compression in the outer disk, which
increases the gas density and eventually leads to star formation, at a
metallicity 0.75 dex lower than the inner galaxy. This forms the first stars of
the low-[$\alpha$/Fe] sequence. These in situ stars have halo-like kinematics,
similarly to what is observed in the Milky Way, due to the inclination of the
outer disk which eventually aligns with the inner one via gravitational
torques. We show that this tilting disk scenario is likely to be common in
Milky-Way like galaxies. This process implies that the low-[$\alpha$/Fe]
sequence is populated in situ, simultaneously from two formation channels, in
the inner and the outer galaxy, with distinct metallicities. This contrasts
with purely sequential scenarios for the assembly of the Milky Way disk and
could be tested observationally. | astro-ph_GA |
Effects of radial flows on the chemical evolution of the Milky Way disk: The majority of chemical evolution models assume that the Galactic disk forms
by means of infall of gas and divide the disk into several independent rings
without exchange of matter between them. However, if gas infall is important,
radial gas flows should be taken into account as a dynamical consequence of
infall. The aim of this paper is to test the effect of radial gas flows on
detailed chemical evolution models (one-infall and two-infall) for the Milky
Way disk with different prescriptions for the infall law and star formation
rate. We found, that with a gas radial inflow of constant speed the metallicity
gradient tends to steepen. Taking into account a constant time scale for the
infall rate along the Galaxy disk and radial flows with a constant speed, we
obtained a too flat gradient, at variance with data, implying that an
inside-out formation and/or a variable gas flow speed are required. To
reproduce the observed gradients the gas flow should increase in modulus with
the galactocentric distance, both in the one-infall and two-infall models.
However, the inside-out disk formation coupled with a threshold in the gas
density (only in the two-infall model) for star formation and/or a variable
efficiency of star formation with galactocentric distance can also reproduce
the observed gradients without radial flows. We showed that the radial flows
can be the most important process in reproducing abundance gradients but only
with a variable gas speed. Finally, one should consider that uncertainties in
the data concerning gradients prevent us to draw firm conclusions. Future more
detailed data will help to ascertain whether the radial flows are a necessary
ingredient in the formation and evolution of the Galactic disk and disks in
general. | astro-ph_GA |
Galactic disk winds driven by cosmic ray pressure: Cosmic ray pressure gradients transfer energy and momentum to extraplanar gas
in disk galaxies, potentially driving significant mass loss as galactic winds.
This may be particularly important for launching high-velocity outflows of
"cool" (T < 10^4 K) gas. We study cosmic-ray driven disk winds using a
simplified semi-analytic model assuming streamlines follow the large-scale
gravitational potential gradient. We consider scaled Milky Way-like potentials
including a disk, bulge, and halo with a range of halo velocities V_H = 50-300
km/s, and streamline footpoints with radii in the disk R_0=1-16 kpc at height 1
kpc. Our solutions cover a wide range of footpoint gas velocity u_0,
magnetic-to-cosmic-ray pressure ratio, gas-to-cosmic-ray pressure ratio, and
angular momentum. Cosmic ray streaming at the Alfv\'en speed enables the
effective sound speed C_eff to increase from the footpoint to a critical point
where C_eff,c = u_c ~ V_H; this differs from thermal winds in which C_eff
decreases outward. The critical point is typically at a height of 1-6 kpc from
the disk, increasing with V_H, and the asymptotic wind velocity exceeds the
escape speed of the halo. Mass loss rates are insensitive to the footpoint
values of the magnetic field and angular momentum. In addition to numerical
parameter space exploration, we develop and compare to analytic scaling
relations. We show that winds have mass loss rates per unit area up to ~ Pi_0
V_H^-5/3 u_0^2/3 where Pi_0 is the footpoint cosmic ray pressure and u_0 is set
by the upwelling of galactic fountains. The predicted wind mass-loss rate
exceeds the star formation rate for V_H < 200 km/s and u_0 = 50 km/s, a typical
fountain velocity. | astro-ph_GA |
VERA astrometry toward the Perseus arm gap: The Perseus arm has a gap in Galactic longitudes (l) between 50 and 80 deg
(hereafter the Perseus arm gap) where the arm has little star formation
activity. To better understand the gap, we conducted astrometric observations
with VERA and analyzed archival H I data. We report on parallax and proper
motion results from four star-forming regions, of which G050.28-00.39 and
G070.33+01.59 are likely associated with the gap. The measured parallaxes are
0.140+/-0.018 (mas), 0.726+/-0.038 (mas), 0.074+/-0.037 (mas), and
0.118+/-0.035 (mas) for G050.28-00.39, G053.14+00.07, G070.33+01.59, and
G079.08+01.33, respectively. Since the fractional parallax error of
G070.33+01.59 is large (0.5), we estimated a 3D kinematic distance of the
source to be 7.7+/-1.0 kpc using both the LSR velocity (VLSR) and the measured
proper motion. Perseus-arm sources G049.41+00.32 and G050.28-00.39 lag relative
to a Galactic rotation by 77+/-17 km/s and 31+/-10 km/s, respectively. The
noncircular motion of G049.41+00.32 cannot be explained by the gravitational
potential of the Perseus arm. We discovered rectangular holes with integrated
brightness temperatures of < 30 K arcdeg in l vs. VLSR of the H I data. One of
the holes is centered near (l, VLSR) = (47 deg, -15 km/s), and G049.41+00.32 is
associated with the rim of the hole. However, G050.28-00.39 is not associated
with the hole. We found extended H I emission on one side of the Galactic plane
when integrating the H I data over the velocity range covering the hole (i.e.,
VLSR = [-25, -5] km/s). G049.41+00.32 and G050.28-00.39 are moving toward the
emission. The Galactic H I disk at the same velocity range showed an arc
structure, indicating that the disk was pushed from the lower side of the disk.
All the observational results might be explained by a cloud collision with the
Galactic disk. | astro-ph_GA |
Universes without the Weak Force: Astrophysical Processes with Stable
Neutrons: We investigate a class of universes in which the weak interaction is not in
operation. We consider how astrophysical processes are altered in the absence
of weak forces, including Big Bang Nucleosynthesis (BBN), galaxy formation,
molecular cloud assembly, star formation, and stellar evolution. Without weak
interactions, neutrons no longer decay, and the universe emerges from its early
epochs with a mixture of protons, neutrons, deuterium, and helium. The
baryon-to-photon ratio must be smaller than the canonical value in our universe
to allow free nucleons to survive the BBN epoch without being incorporated into
heavier nuclei. At later times, the free neutrons readily combine with protons
to make deuterium in sufficiently dense parts of the interstellar medium, and
provide a power source before they are incorporated into stars. Almost all of
the neutrons are incorporated into deuterium nuclei before stars are formed. As
a result, stellar evolution proceeds primarily through strong interactions,
with deuterium first burning into helium, and then helium fusing into carbon.
Low-mass deuterium-burning stars can be long-lived, and higher mass stars can
synthesize the heavier elements necessary for life. Although somewhat different
from our own, such universes remain potentially habitable. | astro-ph_GA |
Star Formation in CALIFA survey perturbed galaxies. I. Effects of Tidal
Interactions: We explore the effects of tidal interactions on star formation (SF) by
analysing a sample of CALIFA survey galaxies. The sample consists of tidally
and non-tidally perturbed galaxies, paired at the closest stellar mass
densities for the same galaxy type between subsamples. They are then compared,
both on the resolved Star Formation Main Sequence (SFMS) plane and in annular
property profiles. Star-forming regions in tidally perturbed galaxies exhibit
flatter SFMS slopes compared to star-forming regions in non-tidally perturbed
galaxies. Despite that the annular profiles show star-forming regions in
tidally perturbed galaxies as being mostly older, their SF properties are never
reduced against those ones proper of non-tidally perturbed galaxies.
Star-forming regions in non-tidally perturbed galaxies are better candidates
for SF suppression (quenching). The lowered SF with increasing stellar mass
density in tidally perturbed galaxies may suggest a lower dependence of SF on
stellar mass. Though the SFMS slopes, either flatter or steeper, are found
independent of stellar mass density, the effect of global stellar mass can not
be ignored when distinguishing among galaxy types. Since a phenomenon or
property other than local/global stellar mass may be taking part in the
modulation of SF, the integrated SF properties are related to the tidal
perturbation parameter. We find weak, but detectable, positive correlations for
perturbed galaxies suggesting that tidal perturbations induced by close
companions increase the gas accretion rates of these objects. | astro-ph_GA |
Tracing the general structure of Galactic molecular clouds using Planck
data: I. The Perseus region as a test case: We present an analysis of probability distribution functions (pdfs) of column
density in different zones of the star-forming region Perseus and its diffuse
environment based on the map of dust opacity at 353 GHz available from the
Planck archive. The pdf shape can be fitted by a combination of a lognormal
function and an extended power-law tail at high densities, in zones centred at
the molecular cloud Perseus. A linear combination of several lognormals fits
very well the pdf in rings surrounding the cloud or in zones of its diffuse
neighbourhood. The slope of the mean density scaling law $\langle\rho\rangle_L
\propto L^\alpha$ is steep ($\alpha=-1.93$) in the former case and rather
shallow ($\alpha=-0.77\pm0.11$) in the rings delineated around the cloud. We
interpret these findings as signatures of two distinct physical regimes: i) a
gravoturbulent one which is characterized by nearly linear scaling of mass and
practical lack of velocity scaling; and ii) a predominantly turbulent one which
is best described by steep velocity scaling and by invariant for compressible
turbulence $\langle\rho\rangle_L u_L^3/L$, describing a scale-independent flux
of the kinetic energy per unit volume through turbulent cascade. The
gravoturbulent spatial domain can be identified with the molecular cloud
Perseus while a relatively sharp transition to predominantly turbulent regime
occurs in its vicinity. | astro-ph_GA |
Galactic planetary nebulae in the AKARI far-infrared surveyor bright
source catalog: We present the results of our preliminary study of all known Galactic PNe
(included in the Kerber 2003 catalog) which are detected by the AKARI/FIS
All-Sky Survey as identified in the AKARI/FIS Bright Source Catalog (BSC)
Version Beta-1. | astro-ph_GA |
Predicting the hypervelocity star population in Gaia: Hypervelocity stars (HVSs) are amongst the fastest objects in our Milky Way.
These stars are predicted to come from the Galactic center (GC) and travel
along unbound orbits across the Galaxy. In the coming years, the ESA satellite
Gaia will provide the most complete and accurate catalogue of the Milky Way,
with full astrometric parameters for more than $1$ billion stars. In this
paper, we present the expected sample size and properties (mass, magnitude,
spatial, velocity distributions) of HVSs in the Gaia stellar catalogue. We
build three Gaia mock catalogues of HVSs anchored to current observations,
exploring different ejection mechanisms and GC stellar population properties.
In all cases, we predict hundreds to thousands of HVSs with precise proper
motion measurements within a few tens of kpc from us. For stars with a relative
error in total proper motion below $10 \%$, the mass range extends to ~$10
M_{\odot}$ but peaks at ~$1$ $M_\odot$. The majority of Gaia HVSs will
therefore probe a different mass and distance range compared to the current
non-Gaia sample. In addition, a subset of a few hundreds to a few thousands of
HVSs with $M$ ~ $3$ $M_\odot$ will be bright enough to have a precise
measurement of the three-dimensional velocity from Gaia alone. Finally, we show
that Gaia will provide more precise proper motion measurements for the current
sample of HVS candidates. This will help identifying their birthplace narrowing
down their ejection location, and confirming or rejecting their nature as HVSs.
Overall, our forecasts are extremely encouraging in terms of quantity and
quality of HVS data that can be exploited to constrain both the Milky Way
potential and the GC properties. | astro-ph_GA |
The Faint Satellite System of NGC 253: Insights into Low-Density
Environments and No Satellite Plane: We have conducted a systematic search around the Milky Way (MW) analog NGC
253 (D=3.5 Mpc), as a part of the Panoramic Imaging Survey of Centaurus and
Sculptor (PISCeS) - a Magellan+Megacam survey to identify dwarfs and other
substructures in resolved stellar light around MW-mass galaxies outside of the
Local Group. In total, NGC 253 has five satellites identified by PISCeS within
100 kpc with an absolute V-band magnitude $M_V<-7$. We have additionally
obtained deep Hubble Space Telescope imaging of four reported candidates beyond
the survey footprint: Do III, Do IV, and dw0036m2828 are confirmed to be
satellites of NGC 253, while SculptorSR is found to be a background galaxy. We
find no convincing evidence for the presence of a plane of satellites
surrounding NGC 253. We construct its satellite luminosity function, which is
complete down to $M_V$$\lesssim$$-8$ out to 100 kpc and $M_V$$\lesssim$$-9$ out
to 300 kpc, and compare it to those calculated for other Local Volume galaxies.
Exploring trends in satellite counts and star-forming fractions among satellite
systems, we find relationships with host stellar mass, environment, and
morphology, pointing to a complex picture of satellite formation, and a
successful model has to reproduce all of these trends. | astro-ph_GA |
Optical Properties of High-Frequency Radio Sources from the Australia
Telescope 20 GHz (AT20G) Survey: Our current understanding of radio-loud AGN comes predominantly from studies
at frequencies of 5 GHz and below. With the recent completion of the Australia
Telescope 20 GHz (AT20G) survey, we can now gain insight into the
high-frequency radio properties of AGN. This paper presents supplementary
information on the AT20G sources in the form of optical counterparts and
redshifts. Optical counterparts were identified using the SuperCOSMOS database
and redshifts were found from either the 6dF Galaxy survey or the literature.
We also report 144 new redshifts. For AT20G sources outside the Galactic plane,
78.5% have optical identifications and 30.9% have redshift information. The
optical identification rate also increases with increasing flux density.
Targets which had optical spectra available were examined to obtain a spectral
classification.
There appear to be two distinct AT20G populations; the high luminosity
quasars that are generally associated with point-source optical counterparts
and exhibit strong emission lines in the optical spectrum, and the lower
luminosity radio galaxies that are generally associated with passive galaxies
in both the optical images and spectroscopic properties. It is suggested that
these different populations can be associated with different accretion modes
(cold-mode or hot-mode). We find that the cold-mode sources have a steeper
spectral index and produce more luminous radio lobes, but generally reside in
smaller host galaxies than their hot-mode counterparts. This can be attributed
to the fact that they are accreting material more efficiently. Lastly, we
compare the AT20G survey with the S-cubed semi-empirical (S3-SEX) models and
conclude that the S3-SEX models need refining to correctly model the compact
cores of AGN. The AT20G survey provides the ideal sample to do this. | astro-ph_GA |
Aligned Grains and Scattered Light Found in Gaps of Planet-Forming Disk: Polarized (sub)millimeter emission from dust grains in circumstellar disks
was initially thought to be due to grains aligned with the magnetic field.
However, higher resolution multi-wavelength observations along with improved
models found that this polarization is dominated by self-scattering at shorter
wavelengths (e.g., 870 $\mu$m) and by grains aligned with something other than
magnetic fields at longer wavelengths (e.g., 3 mm). Nevertheless, the
polarization signal is expected to depend on the underlying substructure, and
observations hitherto have been unable to resolve polarization in multiple
rings and gaps. HL Tau, a protoplanetary disk located 147.3 $\pm$ 0.5 pc away,
is the brightest Class I or Class II disk at millimeter/submillimeter
wavelengths. Here we show deep, high-resolution 870 $\mu$m polarization
observations of HL Tau, resolving polarization in both the rings and gaps. We
find that the gaps have polarization angles with a significant azimuthal
component and a higher polarization fraction than the rings. Our models show
that the disk polarization is due to both scattering and emission from aligned
effectively prolate grains. The intrinsic polarization of aligned dust grains
is likely over 10%, which is much higher than what was expected in low
resolution observations (~1%). Asymmetries and dust features are revealed in
the polarization observations that are not seen in non-polarimetric
observations. | astro-ph_GA |
The destruction and survival of dust in the shell around SN 2008S: SN 2008S erupted in early 2008 in the grand design spiral galaxy NGC 6946.
The progenitor was detected by Prieto et al. in Spitzer Space Telescope images
taken over the four years prior to the explosion, but was not detected in deep
optical images, from which they inferred a self-obscured object with a mass of
about 10 Msun. We obtained Spitzer observations of SN 2008S five days after its
discovery, as well as coordinated Gemini and Spitzer optical and infrared
observations six months after its outburst.
We have constructed radiative transfer dust models for the object before and
after the outburst, using the same r^-2 density distribution of pre-existing
amorphous carbon grains for all epochs and taking light-travel time effects
into account for the early post-outburst epoch. We rule out silicate grains as
a significant component of the dust around SN 2008S. The inner radius of the
dust shell moved outwards from its pre-outburst value of 85 AU to a
post-outburst value of 1250 AU, attributable to grain vaporisation by the light
flash from SN 2008S. Although this caused the circumstellar extinction to
decrease from Av = 15 before the outburst to 0.8 after the outburst, we
estimate that less than 2% of the overall circumstellar dust mass was
destroyed.
The total mass-loss rate from the progenitor star is estimated to have been
(0.5-1.0)x10^-4 Msun yr^-1. The derived dust mass-loss rate of 5x10^-7 Msun
yr^-1 implies a total dust injection into the ISM of up to 0.01 Msun over the
suggested duration of the self-obscured phase. We consider the potential
contribution of objects like SN 2008S to the dust enrichment of galaxies. | astro-ph_GA |
Constraints on the assembly history of the Milky Way's smooth, diffuse
stellar halo from the metallicity-dependent, radially-dominated velocity
anisotropy profiles probed with K giants and BHB stars using LAMOST,
SDSS/SEGUE, and Gaia: We analyze the anisotropy profile of the Milky Way's smooth, diffuse stellar
halo using SDSS/SEGUE blue horizontal branch stars and SDSS/SEGUE and LAMOST K
giants. These intrinsically luminous stars allow us to probe the halo to
approximately 100 kpc from the Galactic center. Line-of-sight velocities,
distances, metallicities, and proper motions are available for all stars via
SDSS/SEGUE, LAMOST, and Gaia, and we use these data to construct a full 7D set
consisting of positions, space motions, and metallicity. We remove substructure
from our samples using integrals of motion based on the method of Xue et al. We
find radially dominated kinematic profiles with nearly constant anisotropy
within 20 kpc, beyond which the anisotropy profile gently declines although
remains radially dominated to the furthest extents of our sample. Independent
of star type or substructure removal, the anisotropy depends on metallicity,
such that the orbits of the stars become less radial with decreasing
metallicity. For $-1.7<$ [Fe/H] $<-1$, the smooth, diffuse halo anisotropy
profile begins to decline at Galactocentric distances $\sim20$ kpc, from
$\beta\sim0.9$ to 0.7 for K giants and from $\beta\sim0.8$ to 0.1 for blue
horizontal branch stars. For [Fe/H] $<-1.7$, the smooth, diffuse halo
anisotropy remains constant along all distances with $0.2<\beta<0.7$ depending
on the metallicity range probed, although independent on star type. These
samples are ideal for estimating the total Galactic mass as they represent the
virialized stellar halo system. | astro-ph_GA |
Kiloparsec-scale jets in three radio-loud narrow-line Seyfert 1 galaxies: We have discovered kiloparsec-scale extended radio emission in three
narrow-line Seyfert 1 galaxies (NLS1s) in sub-arcsecond resolution 9 GHz images
from the Karl G. Jansky Very Large Array (VLA). We find all sources show
two-sided, mildly core-dominated jet structures with diffuse lobes dominated by
termination hotspots. These span 20-70 kpc with morphologies reminiscent of FR
II radio galaxies, while the extended radio luminosities are intermediate
between FR I and FR II sources. In two cases the structure is linear, while a
$45^{\circ}$ bend is apparent in the third. Very Long Baseline Array images at
7.6 GHz reveal parsec-scale jet structures, in two cases with extended
structure aligned with the inner regions of the kiloparsec-scale jets. Based on
this alignment, the ratio of the radio core luminosity to the optical
luminosity, the jet/counter-jet intensity and extension length ratios, and
moderate core brightness temperatures ($\lesssim10^{10}$ K), we conclude these
jets are mildly relativistic ($\beta\lesssim0.3$, $\delta\sim1$-$1.5$) and
aligned at moderately small angles to the line of sight (10-15$^{\circ}$). The
derived kinematic ages of $\sim10^6$-$10^7$ y are much younger than radio
galaxies but comparable to other NLS1s. Our results increase the number of
radio-loud NLS1s with known kiloparsec-scale extensions from seven to ten and
suggest that such extended emission may be common, at least among the brightest
of these sources. | astro-ph_GA |
AlFoCS + Fornax3D: resolved star formation in the Fornax cluster with
ALMA and MUSE: We combine data from ALMA and MUSE to study the resolved (~300 pc scale) star
formation relation (star formation rate vs. molecular gas surface density) in
cluster galaxies. Our sample consists of 9 Fornax cluster galaxies, including
spirals, ellipticals, and dwarfs, covering a stellar mass range of ~10^8.8 -
10^11 M_Sun. CO(1-0) and extinction corrected Halpha were used as tracers for
the molecular gas mass and star formation rate, respectively. We compare our
results with Kennicutt (1998) and Bigiel et al. (2008). Furthermore, we create
depletion time maps to reveal small-scale variations in individual galaxies. We
explore these further in FCC290, using the 'uncertainty principle for star
formation' (Kruijssen & Longmore, 2014a) to estimate molecular cloud lifetimes,
which we find to be short (<10 Myr) in this galaxy. Galaxy-averaged depletion
times are compared with other parameters such as stellar mass and
cluster-centric distance. We find that the star formation relation in the
Fornax cluster is close to those from Kennicutt (1998) and Bigiel et al.
(2008}), but overlaps mostly with the shortest depletion times predicted by
Bigiel et al. (2008). This slight decrease in depletion time is mostly driven
by dwarf galaxies with disturbed molecular gas reservoirs close to the virial
radius. In FCC90, a dwarf galaxy with a molecular gas tail, we find that
depletion times are a factor >~10 higher in its tail than in its stellar body. | astro-ph_GA |
Molecular outflows in local ULIRGs: energetics from multi-transition OH
analysis: We report on the energetics of molecular outflows in 14 local Ultraluminous
Infrared Galaxies (ULIRGs) that show unambiguous outflow signatures (P-Cygni
profiles or high-velocity absorption wings) in the far-infrared lines of OH
measured with the Herschel/PACS spectrometer. Detection of both ground-state
(at 119 and 79 um) and one or more radiatively-excited (at 65 and 84 um) lines
allows us to model the nuclear gas (<~300 pc) as well as the more extended
components using spherically symmetric radiative transfer models. The highest
molecular outflow velocities are found in buried sources, in which slower but
massive expansion of the nuclear gas is also observed. With the exception of a
few outliers, the outflows have momentum fluxes of (2-5)xL_IR/c and mechanical
luminosities of (0.1-0.3)% of L_IR. The moderate momentum boosts in these
sources (<~3) suggest that the outflows are mostly momentum-driven by the
combined effects of AGN and nuclear starbursts, as a result of radiation
pressure, winds, and supernovae remnants. In some sources (~20%), however,
powerful (10^{10.5-11} Lsun) AGN feedback and (partially) energy-conserving
phases are required, with momentum boosts in the range 3-20. These outflows
appear to be stochastic strong-AGN feedback events that occur throughout the
merging process. In a few sources, the outflow activity in the innermost
regions has subsided in the last ~1 Myr. While OH traces the molecular outflows
at sub-kpc scales, comparison of the masses traced by OH with those previously
inferred from tracers of more extended outflowing gas suggests that most mass
is loaded (with loading factors of Mdot/SFR=1-10) from the central galactic
cores (a few x 100 pc). Outflow depletion timescales are <10^8 yr, shorter than
the gas consumption timescales by factors of 1.1-15, and are anti-correlated
with the AGN luminosity. | astro-ph_GA |
The anti-correlation between the hard X-ray photon index and the
Eddington ratio in LLAGNs: We find a significant anti-correlation between the hard X-ray photon index
and the Eddington ratio L_Bol/L_Edd for a sample of Low-Ionization Nuclear
Emission-line Regions (LINERs) and local Seyfert galaxies, compiled from
literatures with Chandra or XMM-Newton observations. This result is in contrast
with the positive correlation found in luminous active galactic nuclei (AGNs),
while it is similar to that of X-ray binaries (XRBs) in low/hard state. Our
result is qualitatively consistent with the spectra produced from advection
dominated accretion flows (ADAFs). It implies that the X-ray emission of
low-luminosity active galactic nuclei (LLAGNs) may originate from the
Comptonization process in ADAF, and the accretion process in LLAGNs may be
similar to that of XRBs in the low/hard state, which is different from that in
luminous AGNs. | astro-ph_GA |
Missing dark matter in dwarf galaxies?: We use cosmological hydrodynamical simulations of the APOSTLE project along
with high-quality rotation curve observations to examine the fraction of
baryons in {\Lambda}CDM haloes that collect into galaxies. This 'galaxy
formation efficiency' correlates strongly and with little scatter with halo
mass, dropping steadily towards dwarf galaxies. The baryonic mass of a galaxy
may thus be used to place a lower limit on total halo mass and, consequently,
on its asymptotic maximum circular velocity. A number of observed dwarfs seem
to violate this constraint, having baryonic masses up to ten times higher than
expected from their rotation speeds, or, alternatively, rotating at only half
the speed expected for their mass. Taking the data at face value, either these
systems have formed galaxies with extraordinary efficiency - highly unlikely
given their shallow potential wells - or their dark matter content is much
lower than expected from {\Lambda}CDM haloes. This 'missing dark matter' is
reminiscent of the inner mass deficit of galaxies with slowly-rising rotation
curves, but cannot be explained away by star formation-induced 'cores' in the
dark mass profile, since the anomalous deficit applies to regions larger than
the luminous galaxies themselves. We argue that explaining the structure of
these galaxies would require either substantial modification of the standard
Lambda cold dark matter paradigm or else significant revision to the
uncertainties in their inferred mass profiles, which should be much larger than
reported. Systematic errors in inclination may provide a simple resolution to
what would otherwise be a rather intractable problem for the current paradigm. | astro-ph_GA |
GASP XXXIII. The ability of spatially resolved data to distinguish among
the different physical mechanisms affecting galaxies in low-density
environments: Galaxies inhabit a wide range of environments and therefore are affected by
different physical mechanisms. Spatially resolved maps combined with the
knowledge of the hosting environment are very powerful to classify galaxies by
physical process. In the context of the GAs Stripping Phenomena in galaxies
(GASP), we present a study of 27 non-cluster galaxies: 24 of them were selected
for showing asymmetries and disturbances in the optical morphology, suggestive
of gas stripping, three of them are passive galaxies and were included to
characterize the final stages of galaxy evolution. We therefore provide a
panorama of the different processes taking place in low-density environments.
The analysis of VLT/MUSE data allows us to separate galaxies into the following
categories: Galaxy-galaxy interactions (2 galaxies), mergers (6), ram pressure
stripping (4), cosmic web stripping (2), cosmic web enhancement (5), gas
accretion (3), starvation (3). In one galaxy we identify the combination of
merger and ram pressure stripping. Only 6/27 of these galaxies have just a
tentative classification. We then investigate where these galaxies are located
on scaling relations determined for a sample of undisturbed galaxies. Our
analysis shows the successes and limitations of a visual optical selection in
identifying the processes that deplete galaxies of their gas content and probes
the power of IFU data in pinning down the acting mechanism. | astro-ph_GA |
Mass and Magnetic distributions in Self Gravitating Super Alfvenic
Turbulence with AMR: In this work, we present the mass and magnetic distributions found in a
recent Adaptive Mesh Refinement (AMR) MHD simulation of supersonic, \sa, self
gravitating turbulence. Powerlaw tails are found in both volume density and
magnetic field probability density functions, with $P(\rho) \propto
\rho^{-1.67}$ and $P(B)\propto B^{-2.74}$. A power law is also found between
magnetic field strength and density, with $B\propto \rho^{0.48}$, throughout
the collapsing gas. The mass distribution of gravitationally bound cores is
shown to be in excellent agreement with recent observation of prestellar cores.
The mass to flux distribution of cores is also found to be in excellent
agreement with recent Zeeman splitting measurements. | astro-ph_GA |
Constraining the Galactic millisecond pulsar population using Fermi
Large Area Telescope: The Fermi Large Area Telescope (Fermi-LAT) has recently revealed a large
population of gamma-ray emitting millisecond pulsars (MSPs) in our Galaxy. We
aim to infer the properties of the Galactic population of gamma-ray emitting
MSPs from the samples detected by the Fermi-LAT. We developed a Monte Carlo
model to predict the spatial and gamma-ray luminosity distribution of the
Galactic MSP population. Based on the estimated detection sensitivity of
Fermi-LAT, we split the model population into detectable and undetectable
samples of MSPs. Using a maximum likelihood method, we compared the detectable
sample to a set of 36 MSPs detected by Fermi-LAT, and we derived the parameters
of the spatial distribution and the total number of gamma-ray emitting MSPs in
the Galaxy. The corresponding undetectable sample provided us with an estimate
for the expected diffuse emission from unresolved MSPs in the Milky Way. We
also applied our method to an extended sample of 66 MSPs that combines firmly
detected MSPs and gamma-ray sources that show characteristics reminiscent of
MSPs. For the first time our analysis provides gamma-ray based constraints on
the Galactic population of MSPs. The radial scale length and vertical scale
height of the population is consistent with estimates based on radio data. Our
analysis suggests that MSPs do not provide any significant contribution to the
isotropic diffuse gamma-ray background emission. | astro-ph_GA |
Hierarchical structures in the Large and Small Magellanic Clouds: We investigate the degree of spatial correlation among extended structures in
the LMC and SMC. To this purpose we work with sub-samples characterised by
different properties such as age and size, taken from the updated catalogue of
Bica et al. or gathered in the present work. The structures are classified as
star clusters or non-clusters (basically, nebular complexes and their stellar
associations). The radius distribution functions follow power-laws
($dN/dR\propto R^{-\alpha}$) with slopes and maximum radius ($R_{max}$) that
depend on object class (and age). Non-clusters are characterised by
$\alpha\approx1.9$ and $R_{max}\la472$ pc, while young clusters (age $\la10$
Myr) have $\alpha\approx3.6$ and $R_{max}\la15$ pc, and old ones (age $\ga600$
Myr) have $\alpha\approx2.5$ and $R_{max}\la40$ pc. Young clusters present a
high degree of spatial self-correlation and, especially, correlate with
star-forming structures, which does not occur with the old ones. This is
consistent with the old clusters having been heavily mixed up, since their ages
correspond to several LMC and SMC crossing times. On the other hand, with ages
corresponding to fractions of the respective crossing times, the young clusters
still trace most of their birthplace structural pattern. Also, small clusters
($R<10$ pc), as well as small non-clusters ($R<100$ pc), are spatially
self-correlated, while their large counterparts of both classes are not. The
above results are consistent with a hierarchical star-formation scenario for
the LMC and SMC. | astro-ph_GA |
New insights from deep VLA data on the potentially recoiling black hole
CID-42 in the COSMOS field: We present deep 3 GHz VLA observations of the potentially recoiling black
hole CID-42 in the COSMOS field. This galaxy shows two optical nuclei in the
HST/ACS image and a large velocity offset of ~ 1300 km/s between the broad and
narrow H beta emission line although the spectrum is not spacially resolved
(Civano et al. 2010). The new 3 GHz VLA data has a bandwidth of 2 GHz and to
correctly interpret the flux densities imaging was done with two different
methods: multi-scale multi-frequency synthesis and spectral windows stacking.
The final resolutions and sensitivities of these maps are 0.7" with rms = 4.6
muJy/beam and 0.9" with rms = 4.8 muJy/beam respectively. With a 7 sigma
detection we find that the entire observed 3 GHz radio emission can be
associated with the South-Eastern component of CID-42, coincident with the
detected X-ray emission. We use our 3 GHz data combined with other radio data
from the literature ranging from 320 MHz to 9 GHz, which include the VLA, VLBA
and GMRT data, to construct a radio synchrotron spectrum of CID-42. The radio
spectrum suggests a type I unobscured radio-quiet flat-spectrum AGN in the
South-Eastern component which may be surrounded by a more extended region of
old synchrotron electron population or shocks generated by the outflow from the
supermassive black hole. Our data are consistent with the recoiling black hole
picture but cannot rule out the presence of an obscured and radio-quiet SMBH in
the North-Western component. | astro-ph_GA |
The Catalogue for Astrophysical Turbulence Simulations (CATS): Turbulence is a key process in many fields of astrophysics. Advances in
numerical simulations of fluids over the last several decades have
revolutionized our understanding of turbulence and related processes such as
star formation and cosmic ray propagation. However, data from numerical
simulations of astrophysical turbulence are often not made public. We introduce
a new simulation-oriented database for the astronomical community: The
Catalogue for Astrophysical Turbulence Simulations (CATS), located at
www.mhdturbulence.com. CATS includes magnetohydrodynamic (MHD) turbulent box
simulation data products generated by the public codes athena++, arepo, enzo,
and flash. CATS also includes several synthetic observational data sets, such
as turbulent HI data cubes. We also include measured power spectra and 3-point
correlation functions from some of these data. We discuss the importance of
open source statistical and visualization tools for the analysis of turbulence
simulations such as those found in CATS. | astro-ph_GA |
Exploring the Galactic Anticenter substructure with LAMOST & Gaia DR2: We characterize the kinematic and chemical properties of 589 Galactic
Anticenter Substructure Stars (GASS) with K-/M- giants in Integrals-of-Motion
space. These stars likely include members of previously identified
substructures such as Monoceros, A13, and the Triangulum-Andromeda cloud
(TriAnd). We show that these stars are on nearly circular orbits on both sides
of the Galactic plane. We can see velocity($V_{Z}$) gradient along Y-axis
especially for the south GASS members. Our GASS members have similar energy and
angular momentum distributions to thin disk stars. Their location in
[$\alpha$/M] vs. [M/H] space is more metal poor than typical thin disk stars,
with [$\alpha$/M] \textbf{lower} than the thick disk. We infer that our GASS
members are part of the outer metal-poor disk stars, and the outer-disk extends
to 30 kpc. Considering the distance range and $\alpha$-abundance features, GASS
could be formed after the thick disk was formed due to the molecular cloud
density decreased in the outer disk where the SFR might be less efficient than
the inner disk. | astro-ph_GA |
The GLASS James Webb Space Telescope Early Release Science Program. I.
Survey Design and Release Plans: The GLASS James Webb Space Telescope Early Release Science (hereafter
GLASS-JWST-ERS) Program will obtain and make publicly available the deepest
extragalactic data of the ERS campaign. It is primarily designed to address two
key science questions, namely, "what sources ionized the universe and when?"
and "how do baryons cycle through galaxies?", while also enabling a broad
variety of first look scientific investigations. In primary mode, it will
obtain NIRISS and NIRSpec spectroscopy of galaxies lensed by the foreground
Hubble Frontier Field cluster, Abell 2744. In parallel, it will use NIRCam to
observe two fields that are offset from the cluster center, where lensing
magnification is negligible, and which can thus be effectively considered blank
fields. In order to prepare the community for access to this unprecedented
data, we describe the scientific rationale, the survey design (including target
selection and observational setups), and present pre-commissioning estimates of
the expected sensitivity. In addition, we describe the planned public releases
of high-level data products, for use by the wider astronomical community. | astro-ph_GA |
The effect of primordial mass segregation on the size scale of globular
clusters: We use direct $N$-body calculations to investigate the impact of primordial
mass segregation on the size scale and mass-loss rate of star clusters in a
galactic tidal field. We run a set of simulations of clusters with varying
degrees of primordial mass segregation at various galactocentric radii and show
that, in primordially segregated clusters, the early, impulsive mass-loss from
stellar evolution of the most massive stars in the innermost regions of the
cluster leads to a stronger expansion than for initially non-segregated
clusters. Therefore, models in stronger tidal fields dissolve faster due to an
enhanced flux of stars over the tidal boundary. Throughout their lifetimes, the
segregated clusters are more extended by a factor of about 2, suggesting that
(at least) some of the very extended globular clusters in the outer halo of the
Milky Way may have been born with primordial mass segregation. We finally
derive a relation between star-cluster dissolution time, $T_{diss}$, and
galactocentric radius, $R_G$, and show how it depends on the degree of
primordial mass segregation. | astro-ph_GA |
Gas expulsion in massive star clusters? Constraints from observations of
young and gas-free objects: Gas expulsion is a central concept in some of the models for multiple
populations and the light-element anticorrelations in globular clusters. If the
star formation efficiency was around 30 per cent and the gas expulsion happened
on the crossing timescale, this process could expel preferentially stars born
with the chemical composition of the proto-cluster gas, while stars with
special composition born in the centre would remain bound. Recently, a sample
of extragalactic, gas-free, young massive clusters has been identified that has
the potential to test the conditions for gas expulsion. We compute a large
number of thin shell models, and calculate if the Rayleigh-Taylor instability
is able to disrupt the shell before it reaches the escape speed. We show that
the success of gas expulsion depends on the compactness index of a star cluster
C5, proportionate to stellar mass over half-mass radius. For given C5, a
certain critical, local star formation efficiency is required to remove the
rest of the gas. Common stellar feedback processes may not lead to gas
expulsion with significant loss of stars above C5 = 1. Considering pulsar winds
and hypernovae, the limit increases to C5 = 30. If successful, gas expulsion
generally takes place on the crossing timescale. Some observed young massive
clusters have 1 < C5 < 10 and are gas-free at 10 Myr. This suggests that gas
expulsion does not affect their stellar mass significantly, unless powerful
pulsar winds and hypernovae are common in such objects. By comparison to
observations, we show that C5 is a better predictor for the expression of
multiple populations than stellar mass. The best separation between star
clusters with and without multiple populations is achieved by a stellar
winds-based gas expulsion model, where gas expulsion would occur exclusively in
star clusters without multiple populations. | astro-ph_GA |
Polarization, Polarizing Efficiency, and Grain alignment towards the
direction of the cluster NGC 2345: We have investigated the grain alignment and dust properties towards the
direction of the cluster NGC 2345 using the multi-band optical polarimetric
observations. For the majority of the stars, the observed polarization is found
to be due to the interstellar medium with average values of maximum
polarization and wavelength corresponding to it as 1.55% and 0.58 $\mu m$,
respectively. This reveals a similar size distribution of dust grains to that
in the general interstellar medium in the direction of NGC 2345. Alteration of
dust properties near the distance of 1.2 kpc towards the direction of NGC 2345
has been noticed. The dust grains located beyond this distance are found to be
aligned with the Galactic magnetic field, whereas a dispersion in orientation
of the dust grains lying in the foreground of this distance is found.
Polarizing efficiency of grains in this direction is found to be close to the
average efficiency for our Galaxy. The decreased grain size along with the
increased polarizing efficiency towards the core region of the cluster
indicates the local radiation field is higher within the cluster which is
responsible for the increased alignment efficiency of small grains. The
wavelength of maximum polarization (associated with the average size of aligned
grains) is also found to increase with extinction and reduces with the increase
in polarizing efficiency, which can be explained by the radiative torque
alignment mechanism. | astro-ph_GA |
The rapid transition from star-formation to AGN dominated rest-frame UV
light at z ~ 4: With the advent of deep optical-to-near-infrared extragalactic imaging on the
degree scale, samples of high-redshift sources are being selected that contain
both bright star-forming (SF) galaxies and faint active galactic nuclei (AGN).
In this study we investigate the transition between SF and AGN-dominated
systems at $z \simeq 4$ in the rest-frame UV. We find a rapid transition to
AGN-dominated sources bright-ward of $M_{\rm UV} \simeq -23.2$. The effect is
observed in the rest-frame UV morphology and size-luminosity relation, where
extended clumpy systems become point-source dominated, and also in the
available spectra for the sample. These results allow us to derive the
rest-frame UV luminosity function for the SF and AGN-dominated sub-samples. We
find the SF-dominated LF is best fit with a double-power law, with a lensed
Schechter function being unable to explain the existence of extremely luminous
SF galaxies at $M_{\rm UV} \simeq -23.5$. If we identify AGN-dominated sources
according to a point-source morphology criterion we recover the relatively flat
faint-end slope of the AGN LF determined in previous studies. If we instead
separate the LF according to the current spectroscopic AGN fraction, we find a
steeper faint-end slope of $\alpha = -1.83 \pm 0.11$. Using a simple model to
predict the rest-frame AGN LF from the $z = 4 $ galaxy LF we find that the
increasing impact of host galaxy light on the measured morphology of faint AGN
can explain our observations. | astro-ph_GA |
Evidence for photoionization-driven variability in narrow absorption
lines: In this Letter, we report the discovery of a strong correlation between the
variability of narrow absorption lines (NALs) and the ionizing continuum from a
two-epoch spectra sample of 40 quasars containing 52 variable C iv
{\lambda}{\lambda}1548; 1551 absorption doublets. According to the concordance
index, this sample is classified into two subsamples. Subsample I shows an
anti-correlation between the variations of absorption lines and the continuum,
while Subsample II exhibits a positive correlation. These results imply that
these variable C iv {\lambda}{\lambda}1548; 1551 absorption doublets are
intrinsic to the corresponding quasars and that their variations are caused
primarily by the fluctuations of the ionizing continuum. Based on our analysis,
we propose that there might be two kinds of absorption gas: one that is very
sensitive to the continuum variations, the another that is not. In addition, we
suggest that in many cases the emergence or disappearance of NALs is caused by
fluctuations of the ionizing continuum. | astro-ph_GA |
Local Stability of Galactic Discs in Modified Dynamics: The local stability of stellar and fluid discs, under a new modified
dynamical model, is surveyed by using WKB approximation. The exact form of the
modified Toomre criterion is derived for both types of systems and it is shown
that the new model is, in all situations, more locally stable than Newtonian
model. In addition, it has been proved that the central surface density of the
galaxies plays an important role in the local stability in the sense that LSB
galaxies are more stable than HSBs. Furthermore, the growth rate in the new
model is found to be lower than the Newtonian one. We found that, according to
this model, the local instability is related to the ratio of surface density of
the disc to a critical surface density $\Sigma^{crit}$. We provide
observational evidence to support this result based on star formation rate in
HSBs and LSBs. | astro-ph_GA |
Multivariate Approaches to Classification in Extragalactic Astronomy: Clustering objects into synthetic groups is a natural activity of any
science. Astrophysics is not an exception and is now facing a deluge of data.
For galaxies, the one-century old Hubble classification and the Hubble tuning
fork are still largely in use, together with numerous mono-or bivariate
classifications most often made by eye. However, a classification must be
driven by the data, and sophisticated multivariate statistical tools are used
more and more often. In this paper we review these different approaches in
order to situate them in the general context of unsupervised and supervised
learning. We insist on the astrophysical outcomes of these studies to show that
multivariate analyses provide an obvious path toward a renewal of our
classification of galaxies and are invaluable tools to investigate the physics
and evolution of galaxies. | astro-ph_GA |
Systematic variations of interstellar linear polarization and growth of
dust grains: A quantitative interpretation of the observed relation between the
interstellar linear polarization curve parameters $K$ and $\lambda_{\max}$
characterizing the width and the wavelength of a polarization maximum,
respectively, is given. The observational data available for 57 stars located
in the dark clouds in Taurus, Chamaeleon, around the stars $\rho$ Oph and R CrA
are considered. The spheroidal particle model of interstellar dust grains
earlier applied to simultaneously interpret the interstellar extinction and
polarization curves in a wide spectral range is utilized. The observed trend $K
\approx 1.7 \lambda_{\max}$ is shown to be most likely related to a growth of
dust grains due to coagulation rather than mantle accretion. The relation of
the parameters $K$ and $\lambda_{\max}$ with an average size of silicate dust
grains is discussed. | astro-ph_GA |
Detecting the effect of non-thermal sources on the warm-hot Galactic
halo: We report the first detection of non-thermal broadening of OVII lines in the
warm-hot $\approx 10^6$ K circumgalactic medium (CGM) of the Milky Way. We use
$z$=0 absorption of OVII K$\alpha$, OVII K$\beta$, and OVIII K$\alpha$ lines in
archival grating data of $b>$15$^\circ$ quasar sightlines from $Chandra$ and
$XMM$-$Newton$. Non-thermal line broadening is evident in two-third of the
sightlines considered, and on average is constrained at 4.6$\sigma$
significance. Non-thermal line broadening dominates over thermal broadening. We
extensively test whether the appearance of non-thermal line broadening could
instead be because of multiple thermally broadened velocity components and
robustly rule it out. Non-thermal line broadening is more evident toward
sightlines at lower galactic latitude indicating the Galactic disk origin of
the nonthermal sources. There is weak/no correlation between non-thermal line
broadening and the angular separation of sightlines from the Galactic center,
indicating that the nuclear region might not be a major source of non-thermal
factors. | astro-ph_GA |
Dust-depletion sequences in damped Ly-α absorbers II. The
composition of cosmic dust, from low-metallicity systems to the Galaxy: We aim at assessing what are the most dominant dust species or types,
including silicate and iron oxide grains present in the ISM, by using recent
observations of dust depletion of galaxies at various evolutionary stages. We
use the observed elemental abundances in dust of several metals (O, S, Si, Mg,
and Fe) in different environments, considering systems with different
metallicities and dust content, namely damped Lyman-{\alpha} absorbers (DLAs)
towards quasars and the Galaxy. We derive a possible dust composition by
computationally finding the statistically expected elemental abundances in dust
assuming a set of key dust species with the iron content as a free parameter.
Carbonaceous dust is not considered in the present study. Metallic iron (likely
in the form of inclusions in silicate grains) and iron oxides is an important
component of the mass composition of carbon-free dust. Iron oxides make up a
significant mass fraction (~1/4 in some cases) of the oxygen-bearing dust and
there are good reasons to believe that metallic iron constitutes a similar mass
fraction of dust. W\"ustite (FeO) could be a simple explanation for the
depletion of iron and oxygen because it is easily formed. There appears to be
no silicate species clearly dominating the silicate mass, but rather a mix of
iron-poor as well as iron-rich olivine and pyroxene. To what extent sulphur
depletion is due to sulfides remains unclear. In general, there seems to be
little evolution of the dust composition (not considering carbonaceous dust)
from low-metallicity systems to the Galaxy. | astro-ph_GA |
Galaxy Zoo: constraining the origin of spiral arms: Since the discovery that the majority of low-redshift galaxies exhibit some
level of spiral structure, a number of theories have been proposed as to why
these patterns exist. A popular explanation is a process known as swing
amplification, yet there is no observational evidence to prove that such a
mechanism is at play. By using a number of measured properties of galaxies, and
scaling relations where there are no direct measurements, we model samples of
SDSS and S$^4$G spiral galaxies in terms of their relative halo, bulge and disc
mass and size. Using these models, we test predictions of swing amplification
theory with respect to directly measured spiral arm numbers from Galaxy Zoo 2.
We find that neither a universal cored or cuspy inner dark matter profile can
correctly predict observed numbers of arms in galaxies. However, by invoking a
halo contraction/expansion model, a clear bimodality in the spiral galaxy
population emerges. Approximately 40 per cent of unbarred spiral galaxies at $z
\lesssim 0.1$ and $\mathrm{M_*} \gtrsim 10^{10} \mathrm{M_\odot}$ have spiral
arms that can be modelled by swing amplification. This population display a
significant correlation between predicted and observed spiral arm numbers,
evidence that they are swing amplified modes. The remainder are dominated by
two-arm systems for which the model predicts significantly higher arm numbers.
These are likely driven by tidal interactions or other mechanisms. | astro-ph_GA |
Gas Dynamics in the Galaxy: Total Mass Distribution and the Bar Pattern
Speed: Gas morphology and kinematics in the Milky Way contain key information for
understanding the formation and evolution of our Galaxy. We present a high
resolution hydrodynamical simulation based on a realistic barred Milky Way
potential constrained by recent observations. Our model can reproduce most
features in the observed longitude-velocity diagram, including the Central
Molecular Zone, the Near and Far 3-kpc arms, the Molecular Ring, and the spiral
arm tangents. It can also explain the non-circular motions of masers obtained
by the recent BeSSeL2 survey. The central gas kinematics are consistent with a
mass of $6.9\times10^8\; {\rm M}_{\odot}$ in the Nuclear Stellar Disk. Our
model predicts the formation of an elliptical gaseous ring surrounding the bar,
which is composed of the 3-kpc arms, Norma arm, and the bar-spiral interfaces.
This ring is similar to those "inner" rings in some Milky Way analogs with a
boxy/peanut-shaped bulge. The kinematics of gas near the solar neighbourhood
are governed by the Local arm, which is induced by the four major stellar
spiral arms. The bar pattern speed constrained by our gas model is $37.5-40\;
{\rm km}\;{\rm s}^{-1}\;{\rm kpc}^{-1}$, corresponding to a corotation radius
of $R_{\rm CR}=6.0-6.4\;{\rm kpc}$. The rotation curve of our model rises
gently within the central $\sim5\;{\rm kpc}$, which is significantly less steep
than those predicted by modern zoom-in cosmological simulations such as Auriga. | astro-ph_GA |
Rapid Black Hole Growth under Anisotropic Radiation Feedback: Discovery of high-redshift (z > 6) supermassive black holes (BHs) may
indicate that the rapid (or super-Eddington) gas accretion has aided their
quick growth. Here, we study such rapid accretion of the primordial gas on to
intermediate-mass (10^2 - 10^5 M_sun) BHs under anisotropic radiation feedback.
We perform two-dimensional radiation hydrodynamics simulations that solve the
flow structure across the Bondi radius, from far outside of the Bondi radius
down to a central part which is larger than a circum-BH accretion disc. The
radiation from the unresolved circum-BH disc is analytically modeled
considering self-shadowing effect. We show that the flow settles into a steady
state, where the flow structure consists of two distinct parts: (1) bipolar
ionized outflowing regions, where the gas is pushed outward by thermal gas
pressure and super-Eddington radiation pressure, and (2) an equatorial neutral
inflowing region, where the gas falls toward the central BH without affected by
radiation feedback. The resulting accretion rate is much higher than that in
the case of isotropic radiation, far exceeding the Eddington-limited rate to
reach a value slightly lower than the Bondi one. The opening angle of the
equatorial inflowing region is determined by the luminosity and directional
dependence of the central radiation. We find that photoevaporation from its
surfaces set the critical opening angle of about ten degrees below which the
accretion to the BH is quenched. We suggest that the shadowing effect allows
even stellar-remnant BHs to grow rapidly enough to become high-redshift
supermassive BHs. | astro-ph_GA |
Spectroscopic confirmation of the dwarf spheroidal galaxy d0944+71 as a
member of the M81 group of galaxies: We use Keck/DEIMOS spectroscopy to measure the first velocity and metallicity
of a dwarf spheroidal (dSph) galaxy beyond the Local Group using resolved
stars. Our target, d0944+71, is a faint dSph found in the halo of the massive
spiral galaxy M81 by Chiboucas et al. We coadd the spectra of 27 individual
stars and measure a heliocentric radial velocity of $-38\pm10$~km/s. This
velocity is consistent with d0944+71 being gravitationally bound to M81. We
coadd the spectra of the 23 stars that are consistent with being red giant
branch stars and measure an overall metallicity of ${\rm [Fe/H]}=-1.3 \pm 0.3$
based on the calcium triplet lines. This metallicity is consistent with
d0944+71 following the metallicity$-$luminosity relation for Local Group dSphs.
We investigate several potential sources of observational bias but find that
our sample of targeted stars is representative of the metallicity distribution
function of d0944+71 and any stellar contamination due to seeing effects is
negligible. The low ellipticity of the galaxy and its position in the
metallicity$-$luminosity relation suggest that d0944+71 has not been affected
by strong tidal stripping. | astro-ph_GA |
The Relation Between [OIII]/H$β$ and Specific Star Formation Rate in
Galaxies at $z \sim 2$: Recent surveys have identified a seemingly ubiquitous population of galaxies
with elevated [OIII]/H$\beta$ emission line ratios at $z > 1$, though the
nature of this phenomenon continues to be debated. The [OIII]/H$\beta$ line
ratio is of interest because it is a main component of the standard diagnostic
tools used to differentiate between active galactic nuclei (AGN) and
star-forming galaxies, as well as the gas-phase metallicity indicators $O_{23}$
and $R_{23}$. Here, we investigate the primary driver of increased
[OIII]/H$\beta$ ratios by median-stacking rest-frame optical spectra for a
sample of star-forming galaxies in the 3D-HST survey in the redshift range
$z\sim1.4-2.2$. Using $N = 4220$ star-forming galaxies, we stack the data in
bins of mass and specific star formation rates (sSFR) respectively. After
accounting for stellar Balmer absorption, we measure
[OIII]$\lambda5007$\AA/H$\beta$ down to $\mathrm{M} \sim 10^{9.2} \
\mathrm{M_\odot}$ and sSFR $\sim 10^{-9.6} \ \mathrm{yr}^{-1}$, more than an
order of magnitude lower than previous work at similar redshifts. We find an
offset of $0.59\pm0.05$ dex between the median ratios at $z\sim2$ and $z\sim0$
at fixed stellar mass, in agreement with existing studies. However, with
respect to sSFR, the $z \sim 2$ stacks all lie within 1$\sigma$ of the median
SDSS ratios, with an average offset of only $-0.06\pm 0.05$. We find that the
excitation properties of galaxies are tightly correlated with their sSFR at
both $z\sim2$ and $z\sim0$, with a relation that appears to be roughly constant
over the last 10 Gyr of cosmic time. | astro-ph_GA |
JWST Reveals Widespread AGN-Driven Neutral Gas Outflows in Massive z ~ 2
Galaxies: We use deep JWST/NIRSpec R~1000 slit spectra of 113 galaxies at 1.7 < z <
3.5, selected from the mass-complete Blue Jay survey, to investigate the
prevalence and typical properties of neutral gas outflows at cosmic noon. We
detect excess Na I D absorption (beyond the stellar contribution) in 46% of
massive galaxies ($\log$ M$_*$/M$_\odot >$ 10), with similar incidence rates in
star-forming and quenching systems. Half of the absorption profiles are
blueshifted by at least 100 km/s, providing unambiguous evidence for neutral
gas outflows. Galaxies with strong Na I D absorption are distinguished by
enhanced emission line ratios consistent with AGN ionization. We conservatively
measure mass outflow rates of 3 - 100 $M_\odot$ yr$^{-1}$; comparable to or
exceeding ionized gas outflow rates measured for galaxies at similar stellar
mass and redshift. The outflows from the quenching systems
(log(sSFR)[yr$^{-1}$] $\lesssim$ -10) have mass loading factors of 4 - 360, and
the energy and momentum outflow rates exceed the expected injection rates from
supernova explosions, suggesting that these galaxies could possibly be caught
in a rapid blowout phase powered by the AGN. Our findings suggest that
AGN-driven ejection of cold gas may be a dominant mechanism for fast quenching
of star formation at z~2. | astro-ph_GA |
SDSS-IV MaNGA: the indispensable role of bars in enhancing the central
star formation of low-$z$ galaxies: We analyse two-dimensional maps and radial profiles of EW(H$\alpha$),
EW(H$\delta_A$), and D$_n$(4000) of low-redshift galaxies using integral field
spectroscopy from the MaNGA survey. Out of $\approx1400$ nearly face-on
late-type galaxies with a redshift $z<0.05$, we identify 121 "turnover"
galaxies that each have a central upturn in EW(H$\alpha$), EW(H$\delta_A$)
and/or a central drop in D$_n$(4000), indicative of ongoing/recent star
formation. The turnover features are found mostly in galaxies with a stellar
mass above $\sim$10$^{10}$ M$_{\odot}$ and NUV-$r$ colour less than $\approx5$.
The majority of the turnover galaxies are barred, with a bar fraction of
89$\pm$3\%. Furthermore, for barred galaxies the radius of the central turnover
region is found to tightly correlate with one third of the bar length.
Comparing the observed and the inward extrapolated star formation rate surface
density, we estimate that the central SFR have been enhanced by an order of
magnitude. Conversely, only half of the barred galaxies in our sample have a
central turnover feature, implying that the presence of a bar is not sufficient
to lead to a central SF enhancement. We further examined the SF enhancement in
paired galaxies, as well as the local environment, finding no relation. This
implies that environment is not a driving factor for central SF enhancement in
our sample. Our results reinforce both previous findings and theoretical
expectation that galactic bars play a crucial role in the secular evolution of
galaxies by driving gas inflow and enhancing the star formation and bulge
growth in the center. | astro-ph_GA |
Constraints on interstellar dust models from extinction and
spectro-polarimetry: We present polarisation spectra of seven stars in the lines-of-sight towards
the Sco OB1 association. Our spectra were obtained within the framework of the
Large Interstellar Polarization Survey carried out with the FORS instrument of
the ESO VLT. We have modelled the wavelength-dependence of extinction and
linear polarisation with a dust model for the diffuse interstellar medium which
consists of a mixture of particles with size ranging from the molecular domain
of 0.5 nm up to 350 nm. We have included stochastically heated small dust
grains with radii between 0.5 and 6 nm made of graphite and silicate, as well
as polycyclic aromatic hydrocarbon molecules (PAHs), and we have assumed that
larger particles are prolate spheroids made of amorphous carbon and silicate.
Overall, a dust model with eight free parameters best reproduces the
observations. Reducing the number of free parameters leads to results that are
inconsistent with cosmic abundance constraints. We found that aligned silicates
are the dominant contributor to the observed polarisation, and that the
polarisation spectra are best-fit by a lower limit of the equivolume sphere
radius of aligned grains of 70 - 200nm. | astro-ph_GA |
Sub-arcsecond imaging of the water emission in Arp 220: Extragalactic observations of water emission can provide valuable insights
into the excitation of the interstellar medium. In addition, extragalactic
megamasers are powerful probes of kinematics close to active nuclei. Therefore,
it is paramount to determine the true origin of the water emission, whether it
is excited by processes close to an AGN or in star-forming regions. We use ALMA
Band 5 science verification observations to analyse the emission of the 183 GHz
water line in Arp 220 on sub-arcsecond scales, in conjunction with new ALMA
Band 7 data at 325 GHz. Specifically, the nature of the process leading to the
excitation of emission at these water lines is studied in this context.
Supplementary 22 GHz VLA observations are used to better constrain the
parameter space in the excitation modelling of the water lines. We detect 183
GHz H2O and 325 GHz water emission towards the two compact nuclei at the center
of Arp 220, being brighter in Arp 220 West. The emission at these two
frequencies is compared to previous single-dish data and does not show evidence
of variability. The 183 and 325 GHz lines show similar spectra and kinematics,
but the 22 GHz profile is significantly different in both nuclei due to a blend
with an NH3 absorption line. Our findings suggest that the most likely scenario
to cause the observed water emission in Arp 220 is a large number of
independent masers originating from numerous star-forming regions. | astro-ph_GA |
KMOS LENsing Survey (KLENS) : morpho-kinematic analysis of star-forming
galaxies at $z \sim 2$: We present results from the KMOS lensing survey-KLENS which is exploiting
gravitational lensing to study the kinematics of 24 star forming galaxies at
$1.4<z<3.5$ with a median mass of $\rm log(M_\star/M_\odot)=9.6$ and median
star formation rate (SFR) of $\rm 7.5\,M_\odot\,yr^{-1}$. We find that 25% of
these low-mass/low-SFR galaxies are rotation dominated, while the majority of
our sample shows no velocity gradient. When combining our data with other
surveys, we find that the fraction of rotation dominated galaxies increases
with the stellar mass, and decreases for galaxies with a positive offset from
the main sequence. We also investigate the evolution of the intrinsic velocity
dispersion, $\sigma_0$, as a function of the redshift, $z$, and stellar mass,
$\rm M_\star$, assuming galaxies in quasi-equilibrium (Toomre Q parameter equal
to 1). From the $z-\sigma_0$ relation, we find that the redshift evolution of
the velocity dispersion is mostly expected for massive galaxies ($\rm
log(M_\star/M_\odot)>10$). We derive a $\rm M_\star-\sigma_0$ relation, using
the Tully-Fisher relation, which highlights that a different evolution of the
velocity dispersion is expected depending on the stellar mass, with lower
velocity dispersions for lower masses, and an increase for higher masses,
stronger at higher redshift. The observed velocity dispersions from this work
and from comparison samples spanning $0<z<3.5$ appear to follow this relation,
except at higher redshift ($z>2$), where we observe higher velocity dispersions
for low masses ($\rm log(M_\star/M_\odot)\sim 9.6$) and lower velocity
dispersions for high masses ($\rm log(M_\star/M_\odot)\sim 10.9$) than
expected. This discrepancy could, for instance, suggest that galaxies at
high-$z$ do not satisfy the stability criterion, or that the adopted
parametrisation of the specific star formation rate and molecular properties
fail at high redshift. | astro-ph_GA |
Radio Loud and Radio Quiet Quasars: We discuss 6 GHz JVLA observations covering a volume-limited sample of 178
low redshift ($0.2 < z < 0.3$) optically selected QSOs. Our 176 radio
detections fall into two clear categories: (1) About $20$\% are radio-loud QSOs
(RLQs) having spectral luminosities $L_6 \gtrsim 10^{\,23.2}
\mathrm{~W~Hz}^{-1}$ primarily generated in the active galactic nucleus (AGN)
responsible for the excess optical luminosity that defines a \emph{bona fide}
QSO. (2) The radio-quiet QSOs (RQQs) have $10^{\,21} \lesssim L_6 \lesssim
10^{\,23.2} \mathrm{~W~Hz}^{-1}$ and radio sizes $\lesssim 10 \mathrm{~kpc}$,
and we suggest that the bulk of their radio emission is powered by star
formation in their host galaxies. "Radio silent" QSOs ($L_6 \lesssim 10^{\,21}
\mathrm{~W~Hz}^{-1}$) are rare, so most RQQ host galaxies form stars faster
than the Milky Way; they are not "red and dead" ellipticals. Earlier radio
observations did not have the luminosity sensitivity $L_6 \lesssim 10^{\,21}
\mathrm{~W~Hz}^{-1}$ needed to distinguish between such RLQs and RQQs. Strong,
generally double-sided, radio emission spanning $\gg 10 \mathrm{~kpc}$ was
found associated with 13 of the 18 RLQ cores having peak flux densities
$S_\mathrm{p} > 5 \mathrm{~mJy~beam}^{-1}$ ($log(L) \gtrsim 24$). The radio
luminosity function of optically selected QSOs and the extended radio emission
associated with RLQs are both inconsistent with simple "unified" models that
invoke relativistic beaming from randomly oriented QSOs to explain the
difference between RLQs and RQQs. Some intrinsic property of the AGNs or their
host galaxies must also determine whether or not a QSO appears radio loud. | astro-ph_GA |
Molecular gas and a new young stellar cluster in the far outer Galaxy: We investigate the star-formation ocurring in the region towards
IRAS07527-3446 in the molecular cloud [MAB97]250.63-3.63, in the far outer
Galaxy. We report the discovery of a new young stellar cluster, and describe
its properties and those of its parent molecular cloud. Near-infrared JHKS
images were obtained with VLT/ISAAC, and millimetre line CO spectra were
obtained with the SEST telescope. VLA archive date were also used. The cloud
and cluster are located at a distance of 10.3 kpc and a Galactocentric distance
of 15.4 kpc, in the far outer Galaxy. Morphologically, IRAS 07527-3446 appears
as a young embedded cluster of a few hundred stars seen towards the position of
the IRAS source, extending for about 2-4 pc and exhibiting sub-clustering. The
cluster contains low and intermediate-mass young reddened stars, a large
fraction having cleared the inner regions of their circumstellar discs
responsible for (H-Ks) colour excess. The observations are compatible with a <
5 Myr cluster with variable spatial extinction of between Av = 5 and Av = 11.
Decomposition of CO emission in clumps, reveals a clump clearly associated with
the cluster position, of mass 3.3 x 10^3 M(solar). Estimates of the slopes of
the Ks-band luminosity function and of the star-formation efficiency yield
values similar to those seen in nearby star-formation sites. These findings
reinforce previous results that the distant outer Galaxy continues to be active
in the production of new and rich stellar clusters, with the physical
conditions required for the formation of rich clusters continuing to be met in
the very distant environment of the outer Galactic disc. | astro-ph_GA |
The impact of baryonic physics on the subhalo mass function and
implications for gravitational lensing: We investigate the impact of baryonic physics on the subhalo population by
analyzing the results of two recent hydrodynamical simulations (EAGLE and
Illustris), which have very similar configuration, but a different model of
baryonic physics. We concentrate on haloes with a mass between $10^{12.5}$ and
$10^{14}M_{\odot}h^{-1}$ and redshift between 0.2 and 0.5, comparing with
observational results and subhalo detections in early-type galaxy lenses. We
compare the number and the spatial distribution of subhaloes in the fully hydro
runs and in their dark matter only counterparts, focusing on the differences
between the two simulations. We find that the presence of baryons reduces the
number of subhaloes, especially at the low mass end ($\leq
10^{10}M_{\odot}h^{-1}$), by different amounts depending on the model. The
variations in the subhalo mass function are strongly dependent on those in the
halo mass function, which is shifted by the effect of stellar and AGN feedback.
Finally, we search for analogues of the observed lenses (SLACS) in the
simulations, selecting them in velocity dispersion and dynamical properties. We
use the selected galaxies to quantify detection expectations based on the
subhalo populations in the different simulations, calculating the detection
probability and the predicted values for the projected dark matter fraction in
subhaloes $f_{DM}$ and the slope of the mass function $\alpha$. We compare
these values with those derived from subhalo detections in observations and
conclude that the dark-matter-only and hydro EAGLE runs are both compatible
with observational results, while results from the hydro Illustris run do not
lie within the errors. | astro-ph_GA |
Unlocking the Full Potential of Extragalactic Ly$α$ through Its
Polarization Properties: Lyman-$\alpha$ (Ly$\alpha$) is a powerful astrophysical probe. Not only is it
ubiquitous at high redshifts, it is also a resonant line, making Ly$\alpha$
photons scatter. This scattering process depends on the physical conditions of
the gas through which Ly$\alpha$ propagates, and these conditions are imprinted
on observables such as the Ly$\alpha$ spectrum and its surface brightness
profile. In this work, we focus on a less-used observable capable of probing
any scattering process: polarization. We implement the density matrix formalism
of polarization into the Monte Carlo radiative transfer code tlac. This allows
us to treat it as a quantum mechanical process where single photons develop and
lose polarization from scatterings in arbitrary gas geometries. We explore
static and expanding ellipsoids, biconical outflows, and clumpy multiphase
media. We find that photons become increasingly polarized as they scatter and
diffuse into the wings of the line profiles, making scattered Ly$\alpha$
polarized in general. The degree and orientation of Ly$\alpha$ polarization
depends on the kinematics and distribution of the scattering HI gas. We find
that it generally probes spatial or velocity space asymmetries and aligns
itself tangentially to the emission source. We show that the mentioned
observables, when studied separately, can leave similar signatures for
different source models. We conclude by revealing how a joint analysis of the
Ly$\alpha$ spectra, surface brightness profiles, and polarization can break
these degeneracies and help us extract unique physical information on galaxies
and their environments from their strongest, most prominent emission line. | astro-ph_GA |
The Evolution of Environmental Quenching Timescales to $z\sim1.6$: Using a sample of 4 galaxy clusters at $1.35 < z < 1.65$ and 10 galaxy
clusters at $0.85 < z < 1.35$, we measure the environmental quenching
timescale, $t_Q$, corresponding to the time required after a galaxy is accreted
by a cluster for it to fully cease star formation. Cluster members are selected
by a photometric-redshift criterion, and categorized as star-forming,
quiescent, or intermediate according to their dust-corrected rest-frame colors
and magnitudes. We employ a "delayed-then-rapid" quenching model that relates a
simulated cluster mass accretion rate to the observed numbers of each type of
galaxy in the cluster to constrain $t_Q$. For galaxies of mass $M_* \gtrsim
10^{10.5}~ \mathrm{M}_\odot$, we find a quenching timescale of $t_Q=$ 1.24 Gyr
in the $z\sim1.5$ cluster sample, and $t_Q=$ 1.50 Gyr at $z\sim1$. Using values
drawn from the literature, we compare the redshift evolution of $t_Q$ to
timescales predicted for different physical quenching mechanisms. We find $t_Q$
to depend on host halo mass such that quenching occurs over faster timescales
in clusters relative to groups, suggesting that properties of the host halo are
responsible for quenching high-mass galaxies. Between $z=0$ and $z=1.5$, we
find that $t_Q$ evolves faster than the molecular gas depletion timescale and
slower than an SFR-outflow timescale, but is consistent with the evolution of
the dynamical time. This suggests that environmental quenching in these
galaxies is driven by the motion of satellites relative to the cluster
environment, although due to uncertainties in the atomic gas budget at high
redshift, we cannot rule out quenching due to simple gas depletion. | astro-ph_GA |
Science with an ngVLA: Understanding Massive Star Formation through
Maser Imaging: Imaging the bright maser emission produced by several molecular species at
centimeter wavelengths is an essential tool for understanding the process of
massive star formation because it provides a way to probe the kinematics of
dense molecular gas at high angular resolution. Unimpeded by the high dust
optical depths that affect shorter wavelength observations, the high brightness
temperature of these emission lines offers a way to resolve accretion and
outflow motions down to scales as fine as $\sim$1-10 au in deeply embedded
Galactic star-forming regions, and at sub-pc scales in nearby galaxies. The
Next Generation Very Large Array will provide the capabilities needed to fully
exploit these powerful tracers. | astro-ph_GA |
Spatial variations of magnetic field along active galactic nuclei jets
on sub-pc to Mpc scales: We report the systematic analysis of knots, hotspots, and lobes in 57 active
galactic nuclei (AGNs) to investigate the variation of the magnetic field along
with the jet from the sub-pc base to the terminus in kpc-to-Mpc scales.
Expanding the number of radio/X-ray samples in Kataoka & Stawarz (2005), we
analyzed the data in 12 FR I and 30 FR II radio galaxies, 12 quasars, and 3 BL
Lacs that contained 76 knots, 42 hotspots, and 29 radio lobes. We first derived
the equipartition magnetic fields in the cores and then estimated those in
various jet components by assuming $B_{\rm est}$ $\propto$ $d^{-1}$, where $d$
is the distance from the jet base. On the other hand, the magnetic field in
large-scale jets (knots, hotspots, and lobes), $B_{\rm eq}$, can be estimated
from the observed flux and spatial extent under the equipartition hypothesis.
We show that the magnetic field decreases as the distance along the jet
increases, but generally gentler than $\propto d^{-1}$. The increase in $B_{\rm
eq}/B_{\rm est}$ at a larger $d$ may suggest the deceleration of the jet around
the downstream, but there is no difference between FR I and FR II jets.
Moreover, the magnetic fields in the hotspots are systematically larger than
those of knots and lobes. Finally, we applied the same analysis to knots and
lobes in Centaurus A to check whether the above discussion will hold even in a
single jet source. | astro-ph_GA |
Disruption of the Orion Molecular Core 1 by the stellar wind of the
massive star $θ^1$ Ori C: Massive stars inject mechanical and radiative energy into the surrounding
environment, which stirs it up, heats the gas, produces cloud and intercloud
phases in the interstellar medium, and disrupts molecular clouds (the birth
sites of new stars). Stellar winds, supernova explosions and ionization by
ultraviolet photons control the lifetimes of molecular clouds. Theoretical
studies predict that momentum injection by radiation should dominate that by
stellar winds, but this has been difficult to assess observationally.
Velocity-resolved large-scale images in the fine-structure line of ionized
carbon ([C II]) provide an observational diagnostic for the radiative energy
input and the dynamics of the interstellar medium around massive stars. Here we
report observations of a one-square-degree region (about 7 parsecs in diameter)
of Orion molecular core -- the region nearest to Earth that exhibits
massive-star formation -- at a resolution of 16 arcseconds (0.03 parsecs) in
the [C II] line at 1.9 terahertz (158 micrometres). The results reveal that the
stellar wind originating from the massive star ${\theta}^{1}$ Orionis C has
swept up the surrounding material to create a bubble roughly four parsecs in
diameter with a 2,600-solar-mass shell, which is expanding at 13 kilometres per
second. This finding demonstrates that the mechanical energy from the stellar
wind is converted very efficiently into kinetic energy of the shell and causes
more disruption of the Orion molecular core 1 than do photo-ionization and
evaporation or future supernova explosions. | astro-ph_GA |
Probabilities for Solar Siblings: We have shown previously (Bobylev et al 2011) that some of the stars in the
Solar neighborhood today may have originated in the same star cluster as the
Sun, and could thus be called Solar Siblings. In this work we investigate the
sensitivity of this result to Galactic models and to parameters of these
models, and also extend the sample of orbits. There are a number of good
candidates for the Sibling category, but due to the long period of orbit
evolution since the break-up of the birth cluster of the Sun, one can only
attach probabilities of membership. We find that up to 10% (but more likely
around 1 %) of the members of the Sun's birth cluster could be still found
within 100 pc from the Sun today. | astro-ph_GA |
The Optically Unbiased Gamma-Ray Burst Host (TOUGH) Survey. VII. The
Host Galaxy Luminosity Function: Probing the Relationship Between GRBs and
Star Formation to Redshift $\sim6$: Gamma-ray bursts (GRBs) offer a route to characterizing star-forming galaxies
and quantifying high-$z$ star formation that is distinct from the approach of
traditional galaxy surveys: GRB selection is independent of dust and probes
even the faintest galaxies that can evade detection in flux-limited surveys.
However, the exact relation between the GRB rate and the star formation rate
(SFR) throughout all redshifts is controversial. The Optically Unbiased GRB
Host (TOUGH) survey includes observations of all GRB hosts (69) in an optically
unbiased sample of Swift GRBs and we utilize these to constrain the evolution
of the UV GRB-host-galaxy luminosity function (LF) between $z=0$ and $z=4.5$,
and compare this with LFs derived from both Lyman-break galaxy (LBG) surveys
and simulation modeling. At all redshifts we find the GRB hosts to be most
consistent with a luminosity function derived from SFR weighted models
incorporating GRB production via both metallicity-dependent and independent
channels with a relatively high level of bias toward low metallicity hosts. In
the range $1<z<3$ an SFR weighted LBG derived (i.e., non-metallicity biased) LF
is also a reasonable fit to the data. Between $z\sim3$ and $z\sim6$, we observe
an apparent lack of UV bright hosts in comparison with LBGs, though the
significance of this shortfall is limited by nine hosts of unknown redshift. | astro-ph_GA |
Optical Discovery of an Apparent Galactic Supernova Remnant G159.6+7.3: Deep Halpha images of portions of a faint 3 x 4 degree Halpha shell centered
at l = 159.6 deg, b = 7.3 deg seen on the Virginia Tech Spectral Line Survey
images revealed the presence of several thin emission filaments along its
eastern limb. Low-dispersion optical spectra of two of these filaments covering
the wavelength range of 4500 - 7500 Angstroms show narrow Halpha line emissions
with velocities around -170 +/- 30 km/s. Both the morphology and spectra of
these filaments are consistent with a Balmer dominated shock interpretation and
we propose these optical filaments indicate that the large Halpha emission
shell is a previously unrecognized supernova remnant. ROSAT All Sky Survey
images indicate the possible presence of extremely faint, diffuse emission from
the shell's central region. The shell's location more than seven degrees off
the Galactic plane in a region of relatively low interstellar density may
account for the lack of any reported associated nonthermal radio emissions. The
rare discovery of a Galactic SNR at optical wavelengths suggests that
additional high latitude SNRs may have escaped radio and X-ray detection. | astro-ph_GA |
Morpho-kinematic properties of field S0 bulges in the CALIFA survey: We study a sample of 28 S0 galaxies extracted from the integral-field
spectroscopic (IFS) survey CALIFA. We combine an accurate two-dimensional (2D)
multi-component photometric decomposition with the IFS kinematic properties of
their bulges to understand their formation scenario. Our final sample is
representative of S0s with high stellar masses ($M_{star}/M_{sun} > 10^{10}$).
They lay mainly on the red sequence and live in relatively isolated
environments similar to that of the field and loose groups. We use our 2D
photometric decomposition to define the size and photometric properties of the
bulges, as well as their location within the galaxies. We perform mock
spectroscopic simulations mimicking our observed galaxies to quantify the
impact of the underlying disc on our bulge kinematic measurements ($\lambda$
and $v/\sigma$). We compare our bulge corrected kinematic measurements with the
results from Schwarzschild dynamical modelling. The good agreement confirms the
robustness of our results and allows us to use bulge reprojected values of
$\lambda$ and $v/\sigma$. We find that the photometric ($n$ and $B/T$) and
kinematic ($v/\sigma$ and $\lambda$) properties of our field S0 bulges are not
correlated. We demonstrate that this morpho-kinematic decoupling is intrinsic
to the bulges and it is not due to projection effects. We conclude that
photometric diagnostics to separate different types of bulges (disc-like vs
classical) might not be useful for S0 galaxies. The morpho-kinematics
properties of S0 bulges derived in this paper suggest that they are mainly
formed by dissipation processes happening at high redshift, but dedicated
high-resolution simulations are necessary to better identify their origin. | astro-ph_GA |
Evidence for gravitational quadrupole moment variations in the companion
of PSR J2051-0827: We have conducted radio timing observations of the eclipsing millisecond
binary pulsar J2051-0827 with the European Pulsar Timing Array network of
telescopes and the Parkes radio telescope, spanning over 13 years. The
increased data span allows significant measurements of the orbital
eccentricity, e = (6.2 {\pm} 1.3) {\times} 10^{-5} and composite proper motion,
{\mu}_t = 7.3 {\pm} 0.4 mas/yr. Our timing observations have revealed secular
variations of the projected semi-major axis of the pulsar orbit which are much
more extreme than those previously published; and of the orbital period of the
system. Investigation of the physical mechanisms producing such variations
confirm that the variations of the semi-major axis are most probably caused by
classical spin-orbit coupling in the binary system, while the variations in
orbital period are most likely caused by tidal dissipation leading to changes
in the gravitational quadrupole moment of the companion. | astro-ph_GA |
Simulating radio synchrotron emission in star-forming galaxies:
small-scale magnetic dynamo and the origin of the far infrared-radio
correlation: In star-forming galaxies, the far-infrared (FIR) and radio-continuum
luminosities obey a tight empirical relation over a large range of
star-formation rates (SFR). We examine magneto-hydrodynamic galaxy simulations
with cosmic rays (CRs), accounting for their advective and anisotropic
diffusive transport. We show that gravitational collapse of the proto-galaxy
generates a corrugated accretion shock, which injects turbulence and drives a
small-scale magnetic dynamo. As the shock propagates outwards and the
associated turbulence decays, the large velocity shear between the
supersonically rotating cool disc with respect to the (partially)
pressure-supported hot circumgalactic medium excites Kelvin-Helmholtz surface
and body modes. Those inject turbulence and drive multiple small-scale dynamos,
which exponentially amplify magnetic fields. They grow in scale to reach
equipartition with thermal and CR energies in Milky Way-mass galaxies. In small
galaxies, the magnetic energy saturates at the turbulent energy while it fails
to reach equipartition with thermal and CR energies. We solve for steady-state
spectra of CR protons, secondary electrons/positrons from hadronic CR-proton
interactions with the interstellar medium, and primary shock-accelerated
electrons at supernovae. The radio-synchrotron emission is dominated by primary
electrons, irradiates the magnetised disc, bulge, and bubble-shaped
magnetically-loaded outflows of our simulated Milky Way-mass galaxy. Our
star-forming and star-bursting galaxies with saturated magnetic fields match
the global FIR-radio correlation (FRC) across four orders of magnitude. Its
intrinsic scatter arises due to (i) different magnetic saturation levels that
result from different seed magnetic fields, (ii) different radio synchrotron
luminosities for different specific SFRs at fixed SFR and (iii) a varying radio
intensity with galactic inclination. (abridged) | astro-ph_GA |
The effect of the deforming dark matter haloes of the Milky Way and the
Large Magellanic Cloud on the Orphan-Chenab stream: It has recently been shown that the Large Magellanic Cloud (LMC) has a
substantial effect on the Milky Way's stellar halo and stellar streams. Here,
we explore how deformations of the Milky Way and LMC's dark matter haloes
affect stellar streams, and whether these effects are observable. In
particular, we focus on the Orphan-Chenab (OC) stream which passes particularly
close to the LMC, and spans a large portion of the Milky Way's halo. We
represent the Milky Way--LMC system using basis function expansions that
capture their evolution in an $N$-body simulation. We present the properties of
this system, such as the evolution of the densities and force fields of each
galaxy. The OC stream is evolved in this time-dependent, deforming potential,
and we investigate the effects of the various moments of the Milky Way and the
LMC. We find that the simulated OC stream is strongly influenced by the
deformations of both the Milky Way and the LMC, and that this effect is much
larger than current observational errors. In particular, the Milky Way dipole
has the biggest impact on the stream, followed by the evolution of the LMC's
monopole, and the LMC's quadrupole. Detecting these effects would confirm a key
prediction of collisionless, cold dark matter, and would be a powerful test of
alternative dark matter and alternative gravity models. | astro-ph_GA |
Note on fundamental physics tests from black hole imaging: Comment on
"Hunting for extra dimensions in the shadow of Sagittarius A$^*$": Several works over the past years have discussed the possibility of testing
fundamental physics using Very Long Baseline Interferometry horizon-scale black
hole (BH) images, such as the Event Horizon Telescope (EHT) images of M87$^*$
and Sagittarius A$^*$ (Sgr A$^*$), using the size $r_{\rm sh}$ and deviation
from circularity $\Delta \mathcal{C}$ of the BH shadow. For the case of the EHT
image of Sgr A$^*$, limits on $\Delta \mathcal{C}$ are not available due to the
sparse interferometric coverage of the 2017 observations, alongside the short
variability timescale of Sgr A$^*$ compared to M87$^*$. Concerning this point,
we comment on the results of a recent preprint which purports to have derived
new limits on extra dimensions using the deviation from circularity of Sgr
A$^*$'s shadow. The latter is quoted to be $\lesssim 10\%$ as with M87$^*$,
based on the "similarity" of the two shadows: however, this is an incorrect
assumption, invalidating the subsequent results. In the immediate future, the
simplest tests of fundamental physics from Sgr A$^*$'s image will therefore
mostly have to rely on $r_{\rm sh}$, whereas additional observables such as the
photon ring and azimuthal angle lapse should soon be available and allow for
novel tests. | astro-ph_GA |
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