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Two Mass Distributions in the L 1641 Molecular Clouds: The Herschel
connection of Dense Cores and Filaments in Orion A: We present the Herschel Gould Belt survey maps of the L1641 molecular clouds
in Orion A. We extracted both the filaments and dense cores in the region. We
identified which of dense sources are proto- or pre-stellar, and studied their
association with the identified filaments. We find that although most (71%) of
the pre-stellar sources are located on filaments there is still a significant
fraction of sources not associated with such structures. We find that these two
populations (on and off the identified filaments) have distinctly different
mass distributions. The mass distribution of the sources on the filaments is
found to peak at 4 Solar masses and drives the shape of the CMF at higher
masses, which we fit with a power law of the form dN/dlogM \propto
M^{-1.4+/-0.4}. The mass distribution of the sources off the filaments, on the
other hand, peaks at 0.8 Solar masses and leads to a flattening of the CMF at
masses lower than ~4 Solar masses. We postulate that this difference between
the mass distributions is due to the higher proportion of gas that is available
in the filaments, rather than in the diffuse cloud. | astro-ph_GA |
Interstellar and Circumgalactic Properties of an Unseen $z=6.84$ Galaxy:
Abundances, Ionization, and Heating in the Earliest Known Quasar Absorber: We analyze relative abundances and ionization conditions in a strong
absorption system at z=6.84, seen in the spectrum of the z=7.54 background
quasar ULAS J134208.10+092838.61. Singly ionized C, Si, Fe, Mg, and Al
measurements are consistent with a warm neutral medium that is metal-poor but
not chemically pristine. Firm non-detections of C IV and Si IV imply that any
warm ionized phase of the IGM or CGM has not yet been enriched past the
ultra-metal-poor regime (<0.001Z_{solar}), unlike lower redshift DLAs where
these lines are nearly ubiquitous. Relative abundances of the heavy elements
794 Myr after the Big Bang resemble those of metal-poor damped Lyman Alpha
systems at intermediate redshift and Milky Way halo stars, and show no evidence
of enhanced [alpha/Fe], [C/Fe] or other signatures of yields dominated by
massive stars. A detection of the CII* fine structure line reveals local
sources of excitation from heating, beyond the level of photo-excitation
supplied by the CMB. We estimate the total and [CII] cooling rates, balancing
against ISM heating sources to develop an heuristic two-phase model of the
neutral medium. The implied heating requires a surface density of star
formation slightly exceeding that of the Milky Way but not at the level of a
strong starburst. For a typical (assumed) NHI=10^{20.6}, an abundance of
[Fe/H]=-2.2 matches the columns of species in the neutral phase. To remain
undetected in C IV, a warm ionized phase would either need much lower
[C/H]<-4.2 over an absorption path of 1 kpc, or else a very small absorption
path (a few pc). While still speculative, these results suggest a significant
reduction in heavy element enrichment outside of neutral star forming regions
of the ISM, as would be expected in early stages of galactic chemical
evolution. | astro-ph_GA |
Chandra Study of the Massive and Distant Galaxy Cluster SDSS J0150-1005: In this work, we present a high spatial resolution study of a fossil cluster,
SDSS J0150-1005 ($z \simeq 0.364$), based on the imaging spectroscopic analysis
of \chandra\ observation. The \chandra\ X-ray image shows a relax and symmetric
morphology, which indicates SDSS J0150-1005 is a well-developed galaxy cluster
with no sign of recent merger. Its global gas temperature is 5.73 $\pm$ 0.80
keV, and the virial mass is 6.23 $\pm$ 1.34 $\times 10^{14} $ M$_{\odot}$
according to the isothermal model. Compared with polytropic temperature model,
the mass calculated based on isothermal model overestimates 49 $\pm$ 11%. The
central gas entropy, $S_{0.1r_{200}} = 143.9 \pm 18.3$ keV cm$^2$, is
significantly lower than average value of the normal galaxy clusters with
similar temperatures. Our results indicate the early formation epoch of SDSS
J0150-1005. | astro-ph_GA |
Λ CDM is Consistent with SPARC Radial Acceleration Relation: Recent analysis \citep{McGaugh2016} of the SPARC galaxy sample found a
surprisingly tight relation between the radial acceleration inferred from the
rotation curves, and the acceleration due to the baryonic components of the
disc. It has been suggested that this relation may be evidence for new physics,
beyond $\Lambda CDM$. In this letter we show that 32 galaxies from the MUGS2
match the SPARC acceleration relation. These cosmological simulations of star
forming, rotationally supported discs were simulated with a {\sc WMAP3}
$\Lambda CDM$ cosmology, and match the SPARC acceleration relation with less
scatter than the observational data. These results show that this acceleration
relation is a consequence of dissipative collapse of baryons, rather than being
evidence for exotic dark-sector physics or new dynamical laws. | astro-ph_GA |
Estimating the mass of galactic components using machine learning
algorithms: The estimation of the bulge and disk massses, the main baryonic components of
a galaxy, can be performed using various approaches, but their implementation
tend to be challenging as they often rely on strong assumptions about either
the baryon dynamics or the dark matter model. In this work, we present an
alternative method for predicting the masses of galactic components, including
the disk, bulge, stellar and total mass, using a set of machine learning
algorithms: KNN-neighbours (KNN), Linear Regression (LR), Random Forest (RF)
and Neural Network (NN). The rest-frame absolute magnitudes in the
ugriz-photometric system were selected as input features, and the training was
performed using a sample of spiral galaxies hosting a bulge from Guo's mock
catalogue \citep{Guo-Catalog} derived from the Millennium simulation. In
general, all the algorithms provide good predictions for the galaxy's mass
components ranging from $10^9\,M_\odot$ to $10^{11}\,M_\odot$, corresponding to
the central region of the training mass domain; however, the NN give rise to
the most precise predictions in comparison to other methods. Additionally, to
test the performance of the NN architecture, we used a sample of observed
galaxies from the SDSS survey whose mass components are known. We found that
the NN can predict the luminous masses of disk-dominant galaxies within the
same range of magnitudes that for the synthetic sample up to a $99\%$ level of
confidence, while mass components of galaxies hosting larger bulges are well
predicted up to $95\%$ level of confidence. The NN algorithm can also bring up
scaling relations between masses of different components and magnitudes. | astro-ph_GA |
A giant Ly$α$ nebula in the core of an X-ray cluster at $z=1.99$:
implications for early energy injection: We present the discovery of a giant $\gtrsim$100~kpc Ly$\alpha$ nebula
detected in the core of the X-ray emitting cluster CL~J1449+0856 at $z=1.99$
through Keck/LRIS narrow-band imaging. This detection extends the known
relation between Ly$\alpha$ nebulae and overdense regions of the Universe to
the dense core of a $5-7\times10^{13}$ M$_{\odot}$ cluster. The most plausible
candidates to power the nebula are two Chandra-detected AGN host cluster
members, while cooling from the X-ray phase and cosmological cold flows are
disfavored primarily because of the high Ly$\alpha$ to X-ray luminosity ratio
($L_{\mathrm{Ly\alpha}}/L_{\mathrm{X}} \approx0.3$, $\gtrsim10-1000\times$
higher than in local cool-core clusters) and by current modeling. Given the
physical conditions of the Ly$\alpha$-emitting gas and the possible interplay
with the X-ray phase, we argue that the Ly$\alpha$ nebula would be short-lived
($\lesssim10$ Myr) if not continuously replenished with cold gas at a rate of
$\gtrsim1000$ M$_{\odot}$ yr$^{-1}$. We investigate the possibility that
cluster galaxies supply the required gas through outflows and we show that
their total mass outflow rate matches the replenishment necessary to sustain
the nebula. This scenario directly implies the extraction of energy from
galaxies and its deposition in the surrounding intracluster medium, as required
to explain the thermodynamic properties of local clusters. We estimate an
energy injection of the order of $\thickapprox2$ keV per particle in the
intracluster medium over a $2$ Gyr interval. In our baseline calculation AGN
provide up to $85$% of the injected energy and 2/3 of the mass, while the rest
is supplied by supernovae-driven winds. | astro-ph_GA |
Evidence for extended gaseous reservoirs around AGN at cosmic noon from
ALMA CO(3-2) observations: Gaseous outflows are key phenomena in the evolution of galaxies, as they
affect star formation (either positively or negatively), eject gas from the
core or disk, and directly cause mixing of pristine and processed material.
Active outflows may be detected through searches for broad spectral line
emission or high-velocity gas, but it is also possible to determine the
presence of past outflows by searching for extended reservoirs of chemically
enriched molecular gas in the circumgalactic medium (CGM) around galaxies. In
this work, we examine the CO(3-2) emission of a set of seven z~2.0-2.5 AGN host
galaxies, as observed with ALMA. Through a three-dimensional stacking analysis
we find evidence for extended CO emission of radius r~13kpc. We extend this
analysis to the HST/ACS i-band images of the sample galaxies, finding a complex
small-scale (r<10kpc) morphology but no robust evidence for extended emission.
In addition, the dust emission (traced by rest-frame FIR emission) shows no
evidence for significant spatial extension. This indicates that the diffuse CO
emission revealed by ALMA is morphologically distinct from the stellar
component, and thus traces an extended reservoir of enriched gas. The presence
of a diffuse, enriched molecular reservoir around this sample of AGN host
galaxies at cosmic noon hints at a history of AGN-driven outflows that likely
had strong effects on the star formation history of these objects. | astro-ph_GA |
Do halos that form early, have high concentration, are part of a pair,
or contain a central galaxy potential host more pronounced planes of
satellite galaxies?: The Milky Way, the Andromeda galaxy, and Centaurus A host flattened
distributions of satellite galaxies which exhibits coherent velocity trends
indicative of rotation. Comparably extreme satellite structures are very rare
in cosmological LCDM simulations, giving rise to the `satellite plane problem'.
As a possible explanation it has been suggested that earlier-forming, higher
concentration host halos contain more flattened and kinematically coherent
satellite planes. We have tested for such a proposed correlation between the
satellite plane and host halo properties in the ELVIS suite of simulations. We
find evidence neither for a correlation of plane flattening with halo
concentration or formation time, nor for a correlation of kinematic coherence
with concentration. The height of the thinnest sub-halo planes does correlate
with the host virial radius and with the radial extent of the sub-halo system.
This can be understood as an effect of not accounting for differences in the
radial distribution of sub-halos, and selecting them from different volumes
than covered by the actual observations. Being part of a halo pair like the
Local Group does not result in more narrow or more correlated satellite planes
either. Additionally, using the PhatELVIS simulations we show that the presence
of a central galaxy potential does not favor more narrow or more correlated
satellite planes, it rather leads to slightly wider planes. Such a central
potential is a good approximation of the dominant effect baryonic physics in
cosmological simulations has on a sub-halo population. This suggests that, in
contrast to other small-scale problems, the planes of satellite galaxies issue
is made worse by accounting for baryonic effects. | astro-ph_GA |
Simulations of the star-forming molecular gas in an interacting M51-like
galaxy: We present here the first of a series of papers aimed at better understanding
the evolution and properties of giant molecular clouds (GMCs) in a galactic
context. We perform high resolution, three-dimensional {\sc arepo} simulations
of an interacting galaxy inspired by the well-observed M51 galaxy. Our fiducial
simulations include a non-equilibrium, time-dependent, chemical network that
follows the evolution of atomic and molecular hydrogen as well as carbon and
oxygen self-consistently. Our calculations also treat gas self-gravity and
subsequent star formation (described by sink particles), and coupled supernova
feedback. In the densest parts of the simulated interstellar medium (ISM) we
reach sub-parsec resolution, granting us the ability to resolve individual GMCs
and their formation and destruction self-consistently throughout the galaxy. In
this initial work we focus on the general properties of the ISM with a
particular focus on the cold star-forming gas. We discuss the role of the
interaction with the companion galaxy in generating cold molecular gas and
controlling stellar birth. We find that while the interaction drives
large-scale gas flows and induces spiral arms in the galaxy, it is of secondary
importance in determining gas fractions in the different ISM phases and the
overall star-formation rate. The behaviour of the gas on small GMC scales
instead is mostly controlled by the self-regulating property of the ISM driven
by coupled feedback. | astro-ph_GA |
Super-Eddington accretion of dusty gas onto seed black holes:
metallicity-dependent efficiency of mass growth: The super-Eddington accretion onto intermediate seed BHs is a potential
formation mode of supermassive black holes exceeding $10^9~M_\odot$ in the
early universe. We here investigate how such rapid accretion may occur with
finite amounts of heavy elements contained in the gas and dust. In our 1D
radiation-hydrodynamics simulations, the radiative transfer is solved for both
the direct UV lights emitted by an accretion disk and the diffuse IR lights
thermally emitted by dust grains. Our results show that the radiative force by
the IR lights causes a strong feedback to regulate the mass accretion. The
resulting mean accretion rate is lower with the higher metallicity, and there
is the critical metallicity $Z \sim 10^{-2}~Z_\odot$, above which the
super-Eddington accretion is prevented by the radiation pressure of the IR
lights. With this taken into account, we examine if the dusty super-Eddington
accretion occurs in young galaxies using a simple model. We show that a
sufficient number of galaxies at $z \gtrsim 10$ can be such potential sites if
BHs accrete the cold dense gas with $T \sim 10^2$ K, approximately the thermal
equilibrium value at $Z = 10^{-2}~Z_\odot$. We argue that the efficiency of the
BH growth via the rapid accretion depends on the metallicity, and that the
metallicity slightly lower than $10^{-2}~Z_\odot$ provides a chance for the
most efficient growth. | astro-ph_GA |
The MAGIC project. III. Radial and azimuthal Galactic abundance
gradients using classical Cepheids: Radial abundance gradients provide sound constraints for chemo-dynamical
models of galaxies. Azimuthal variations of abundance ratios are solid
diagnostics to understand their chemical enrichment. In this paper we
investigate azimuthal variations of abundances in the Milky Way using Cepheids.
We provide the detailed chemical composition (25 elements) of 105 Classical
Cepheids from high-resolution SALT spectra observed by the MAGIC project.
Negative abundance gradients, with abundances decreasing from the inner to the
outer disc, have been reported both in the Milky Way and in external galaxies,
and our results are in full agreement with literature results. We find
azimuthal variations of the oxygen abundance [O/H]. While a large number of
external spirals show negligible azimuthal variations, the Milky Way seems to
be one of the few galaxies with noticeable [O/H] azimuthal asymmetries. They
reach ~0.2 dex in the inner Galaxy and in the outer disc, where they are the
largest, thus supporting similar findings for nearby spiral galaxies as well as
recent 2D chemo-dynamical models. | astro-ph_GA |
Rotational signature of the Milky Way stellar halo: We measure the rotation of the Milky Way stellar halo on two samples of Blue
Horizontal Branch (BHB) field halo stars from the Sloan Digital Sky Survey
(SDSS) with four different methods. The two samples comprise 1582 and 2563
stars respectively and reach out to ~50 kpc in galactocentric distance. Two of
the methods to measure rotation rely exclusively on line-of-sight velocities,
namely the popular double power-law model and a direct estimate of the
de-projected l.o.s. velocity. The other two techniques use the full 3D motions:
the radial velocity based rotation estimator of Sch\"onrich, Binney & Asplund
(2012) and a simple 3D azimuthal velocity mean. In this context we a) critique
the popular model and b) assess the reliability of the estimators. All four
methods agree on a weakly prograde or non-rotating halo. Further, we observe no
duality in the rotation of sub-samples with different metallicities or at
different radii. We trace the rotation gradient across metallicity measured by
Deason et al. (2011) on a similar sample of BHB stars back to the inclusion of
regions in the apparent magnitude-surface gravity plane known to be
contaminated. In the spectroscopically selected sample of Xue et al. (2011), we
flag ~500 hot metal-poor stars for their peculiar kinematics w.r.t. to both
their cooler metal-poor counter-parts and to the metal-rich stars in the same
sample. They show a seemingly retrograde behaviour in line-of-sight velocities,
which is not confirmed by the 3D estimators. Their anomalous vertical motion
hints at either a pipeline problem or a stream-like component rather than a
smooth retrograde population. | astro-ph_GA |
Massive Quiescent Galaxies at z>3 in The Millennium Simulation Populated
by A Semi-analytic Galaxy Formation Model: We take advantage of the statistical power of the large-volume
dark-matter-only Millennium simulation, combined with a sophisticated
semi-analytic galaxy formation model, to explore whether the recently reported
$z=3.7$ quiescent galaxy ZF-COSMOS-20115 (ZF; Glazebrook et al. 2017) can be
accommodated in current galaxy formation models. In our model, a population of
quiescent galaxies (QGs) with stellar masses and star formation rates
comparable to those of ZF naturally emerges at redshifts $z<4$. There are two
and five ZF analogues at the redshift $3.86$ and $3.58$ in the Millennium
simulation volume, respectively. We demonstrate that, while the $z>3.5$ massive
QGs are rare (about 2\% of the galaxies with the similar stellar masses), the
existing AGN feedback model implemented in the semi-analytic galaxy formation
model can successfully explain the formation of the high-redshift QGs as it
does on their lower redshift counterparts. | astro-ph_GA |
Constraints from Faraday rotation on the magnetic field structure in the
Galactic halo: We examine the constraints imposed by Faraday rotation measures of
extragalactic point sources on the structure of the magnetic field in the halo
of our Galaxy. Guided by radio polarization observations of external spiral
galaxies, we look in particular into the possibility that field lines in the
Galactic halo have an X shape. We employ the analytical models of spiraling,
possibly X-shape magnetic fields derived in a previous paper to generate
synthetic all-sky maps of the Galactic Faraday depth, which we fit to an
observational reference map with the help of Markov Chain Monte Carlo
simulations. We find that the magnetic field in the Galactic halo is slightly
more likely to be bisymmetric (azimuthal wavenumber, $m = 1$) than axisymmetric
($m = 0$). If it is indeed bisymmetric, it must appear as X-shaped in radio
polarization maps of our Galaxy seen edge-on from outside, but if it is
actually axisymmetric, it must instead appear as nearly parallel to the
Galactic plane. | astro-ph_GA |
A high resolution VLT/FLAMES study of individual stars in the centre of
the Fornax dwarf spheroidal galaxy: For the first time we show the detailed late-stage chemical evolution history
of small nearby dwarf spheroidal galaxy in the Local Group. We present the
results of a high resolution (R$\sim$20000) FLAMES/GIRAFFE abundance study at
ESO/VLT of 81 photometrically selected red giant branch stars in the central
25$'$ of the Fornax dwarf spheroidal galaxy. We present abundances of \alfe\
(Mg, Si, Ca and Ti), iron-peak elements (Fe, Ni and Cr) and heavy elements (Y,
Ba, La, Nd and Eu). Our sample was randomly selected, and is clearly dominated
by the younger and more metal rich component of Fornax which represents the
major fraction of stars in the central region. This means that the majority of
our stars are 1$-$4 Gyr old, and thus represent the end phase of chemical
evolution in this system. Our sample of stars has unusually low [$\alpha$/Fe],
[Ni/Fe] and [Na/Fe] compared to the Milky Way stellar populations at the same
[Fe/H]. The particularly important role of stellar winds from low metallicity
AGB stars in the creation of s-process elements is clearly seen from the high
[Ba/Y]. Furthermore, we present evidence for an s-process contribution to Eu. | astro-ph_GA |
New candidates for extremely metal-poor emission-line galaxies in the
SDSS/BOSS DR10: We present a spectroscopic study of eight extremely low-metallicity candidate
emission-line galaxies with oxygen abundances possibly below 12 +log O/H =
7.35. These galaxies were selected from Data Release 10 of the Sloan Digital
Sky Survey/Baryon Oscillation Spectroscopic Survey (SDSS/BOSS DR10). We will
call these extremely metal-deficient galaxies XMD galaxies. The electron
temperature-sensitive emission line [O~{\sc iii}] $\lambda$4363 is detected in
three galaxies and marginally detected in two galaxies, allowing for abundance
determination by a "direct" method. Because of large uncertainties in the [O
{\sc iii}]$\lambda$4363\AA\ line fluxes, we also calculated oxygen abundance in
these galaxies together with the remaining three galaxies using a strong-line
semi-empirical method. This method gives oxygen abundances higher than 7.35 for
three galaxies with detected [O {\sc iii}]$\lambda$4363\AA\ line and lower than
7.35 for the remaining five objects of the sample. The newly-discovered
galaxies represent excellent targets for follow-up spectroscopic observations
with the largest telescopes to improve the oxygen abundance determination and
to increase the number of these very rare low-metallicity objects. The extreme
location of the most massive and luminous XMD galaxies and XMD candidates in
the stellar mass-metallicity diagram implies that these galaxies may be genuine
young objects.
With stellar masses of up to $\sim$ 10$^7$ - 10$^8$$M_{\odot}$, the galaxies
are not chemically enriched and strongly deviate to lower metallicity as
compared to the relation obtained for a large sample of low-redshift,
star-forming galaxies. | astro-ph_GA |
Star Cluster Formation and Survival in the First Galaxies: Using radiation-hydrodynamic cosmological simulations, we present a detailed
($0.1$ pc resolution), physically motivated portrait of a typical-mass dwarf
galaxy before the epoch of reionization, resolving the formation and evolution
of star clusters into individual $10\:\mathrm{M_{\odot}}$ star particles. In
the rest-frame UV, the galaxy has an irregular morphology with no bulge or
galactic disk, dominated by light emitted from numerous, compact, and
gravitationally-bound star clusters. This is especially interesting in light of
recent HST and JWST observations that -- aided by the magnifying power of
gravitational lenses -- have imaged, at parsec-scale resolution, individual
young star clusters in the process of forming in similar galaxies at $z>6$.
Because of their low metallicities and high temperatures, star-forming gas
clouds in this galaxy have densities $\sim 100$ times higher than typical giant
molecular clouds; hence, their star formation efficiencies are high enough
($f_*\sim10-70$ per cent) to produce a sizeable population of potential
globular cluster progenitors but typically smaller (between a few $100\:-\:
2\times10^4\:\mathrm{M_{\odot}}$, sizes of $0.1-3$ pc) and of lower
metallicities ($10^{-3.5}-10^{-2.5}\:\mathrm{Z_{\odot}}$). The initial mass
function of the star-forming clouds is log-normal while the bound star cluster
mass function is a power-law with a slope that depends mainly on $f_*$ but also
on the temporal proximity to a major starburst. We find slopes between $-0.5$
and $-2.5$ depending on the assumed sub-grid $f_*$. Star formation is
self-regulated on galactic scales; however, the multi-modal metallicity
distribution of the star clusters and the fraction of stars locked into
surviving bound star clusters depends on $f_*$. | astro-ph_GA |
The Milky Way Halo in Action Space: We analyse the structure of the local stellar halo of the Milky Way using
$\sim$ 60000 stars with full phase space coordinates extracted from the
SDSS--{\it Gaia} catalogue. We display stars in action space as a function of
metallicity in a realistic axisymmetric potential for the Milky Way Galaxy. The
metal-rich population is more distended towards high radial action $J_R$ as
compared to azimuthal or vertical action, $J_\phi$ or $J_z$. It has a mild
prograde rotation $(\langle v_\phi \rangle \approx 25$ km s$^{-1}$), is
radially anisotropic and highly flattened with axis ratio $q \approx 0.6 -
0.7$. The metal-poor population is more evenly distributed in all three
actions. It has larger prograde rotation $(\langle v_\phi \rangle \approx 50$
km s$^{-1}$), a mild radial anisotropy and a roundish morphology ($q\approx
0.9$). We identify two further components of the halo in action space. There is
a high energy, retrograde component that is only present in the metal-rich
stars. This is suggestive of an origin in a retrograde encounter, possibly the
one that created the stripped dwarf galaxy nucleus, $\omega$Centauri. Also
visible as a distinct entity in action space is a resonant component, which is
flattened and prograde. It extends over a range of metallicities down to [Fe/H]
$\approx -3$. It has a net outward radial velocity $\langle v_R \rangle \approx
12$ km s$^{-1}$ within the Solar circle at $|z| <3.5$ kpc. The existence of
resonant stars at such extremely low metallicities has not been seen before. | astro-ph_GA |
Distinct core and halo stellar populations and the formation history of
the bright Coma cluster early-type galaxy NGC 4889: We study the stellar population far into the halo of one of the two brightest
galaxies in the Coma cluster, NGC 4889, based on deep medium resolution
spectroscopy with FOCAS at the Subaru 8.2m telescope. We fit single stellar
population models to the measured line-strength (Lick) indices (Hbeta, Mgb,
[MgFe]' and <Fe>). Combining with literature data, we construct radial profiles
of metallicity, [alpha/Fe] element abundance ratio and age for NGC 4889, from
the center out to ~60 kpc (~4Re). We find evidence for different chemical and
star formation histories for stars inside and outside 1.2Re = 18 kpc radius.
The inner regions are characterized by a steep [Z/H] gradient and high
[alpha/Fe] at ~2.5 times solar value. In the halo, between 18 and 60 kpc, the
[Z/H] is near-solar with a shallow gradient, while [alpha/Fe] shows a strong
negative gradient, reaching solar values at ~60 kpc. We interpret these data in
terms of different formation histories for both components. The data for the
inner galaxy are consistent with a rapid, quasi-monolithic, dissipative merger
origin at early redshifts, followed by one or at most a few dry mergers. Those
for the halo argue for later accretion of stars from old systems with more
extended star formation histories. The half-light radius of the inner component
alone is estimated as ~6 kpc, suggesting a significantly smaller size of this
galaxy in the past. This may be the local stellar population signature of the
size evolution found for early-type galaxies from high-redshift observations. | astro-ph_GA |
MOSEL Survey: Extremely weak outflows in EoR analogues at z=3-4: This paper presents deep K-band spectroscopic observations of galaxies at
z=3-4 with composite photometric rest-frame Hb+[OIII] equivalent widths
EW_0>600A, comparable to the EW of galaxies observed during the epoch of
reionisation (EoR, z>6). The typical spectroscopic [OIII] EW_0 and stellar mass
of our targets is ~ 700A and log(M_star/M_sun)=8.98. By stacking the [OIII]
emission profiles, we find evidence of a weak broad component with
F_broad/F_narrow ~ 0.2 and velocity width sigma_{broad} ~ 170 km/s. The
strength and velocity width of the broad component does not change
significantly with stellar mass and [OIII] EW_0 of the stacked sample. Assuming
similar broad component profiles for [OIII] and Halpha emission, we estimate a
mass loading factor ~0.2, similar to low stellar mass galaxies at z>1 even if
the star formation rates of our sample is 10 times higher. We hypothesize that
either the multi-phase nature of supernovae driven outflows or the suppression
of winds in the extreme star-forming regime is responsible for the weak
signature of outflows in the EoR analogues. | astro-ph_GA |
ALMA Observations of Multiple-CO and C Lines Toward the Active Galactic
Nucleus of NGC 7469: X-Ray-dominated Region Caught in the Act: We used the Atacama Large Millimeter/submillimeter Array (ALMA) to map
$^{12}$CO($J$ = 1-0), $^{12}$CO($J$ = 2-1), $^{12}$CO($J$ = 3-2), $^{13}$CO($J$
= 2-1), and [CI]($^3P_1$-$^3P_0$) emission lines around the type 1 active
galactic nucleus (AGN) of NGC 7469 ($z = 0.0164$) at $\sim 100$ pc resolutions.
The CO lines are bright both in the circumnuclear disk (central $\sim 300$ pc)
and the surrounding starburst (SB) ring ($\sim 1$ kpc diameter), with two
bright peaks on either side of the AGN. By contrast, the [CI]($^3P_1$-$^3P_0$)
line is strongly peaked on the AGN. Consequently, the brightness temperature
ratio of [CI]($^3P_1$-$^3P_0$) to $^{13}$CO(2-1) is $\sim 20$ at the AGN, as
compared to $\sim 2$ in the SB ring. Our local thermodynamic equilibrium (LTE)
and non-LTE models indicate that the enhanced line ratios (or CI enhancement)
are due to an elevated C$^0$/CO abundance ratio ($\sim 3-10$) and temperature
($\sim 100-500$ K) around the AGN as compared to the SB ring (abundance ratio
$\sim 1$, temperature $\lesssim 100$ K), which accords with the picture of the
X-ray-dominated Region (XDR). Based on dynamical modelings, we also provide
CO(1-0)-to- and [CI]($^3P_1$-$^3P_0$)-to-molecular mass conversion factors at
the central $\sim 100$ pc of this AGN as $\alpha_{\rm CO} = 4.1$ and
$\alpha_{\rm CI} = 4.4~M_\odot$ (K km s$^{-1}$ pc$^2$)$^{-1}$, respectively.
Our results suggest that the CI enhancement is potentially a good marker of
AGNs that could be used in a new submillimeter diagnostic method toward dusty
environments. | astro-ph_GA |
Radiation Hydrodynamical Simulations of the First Quasars: Supermassive black holes (SMBHs) are the central engines of luminous quasars
and are found in most massive galaxies today. But the recent discoveries of
ULAS J1120+0641, a $2 \times 10^9$ M$_{\odot}$ BH at $z =$ 7.1, and ULAS
J1342+0928, a $8.0 \times 10^{8}$ M$_{\odot}$ BH at $z =$ 7.5, now push the era
of quasar formation up to just 690 Myr after the Big Bang. Here we report new
cosmological simulations of SMBHs with X-rays fully coupled to primordial
chemistry and hydrodynamics that show that J1120 and J1342 can form from direct
collapse black holes (DCBHs) if their growth is fed by cold, dense accretion
streams, like those thought to fuel rapid star formation in some galaxies at
later epochs. Our models reproduce all of the observed properties of J1120: its
mass, luminosity, and H II region as well as star formation rates and
metallicities in its host galaxy. They also reproduce the dynamical mass of the
innermost 1.5 kpc of its emission region recently measured by ALMA and J-band
magnitudes that are in good agreement with those found by the VISTA Hemisphere
Survey. | astro-ph_GA |
The Arecibo Galaxy Environment Survey IX: The Isolated Galaxy Sample: We have used the Arecibo L-band Feed Array to map three regions, each of 5
square degrees, around the isolated galaxies NGC 1156, UGC 2082, and NGC 5523.
In the vicinity of these galaxies we have detected two dwarf companions: one
near UGC 2082, previously discovered by ALFALFA, and one near NGC 1156,
discovered by this project and reported in an earlier paper. This is
significantly fewer than the 15.4 $^{+1.7}_{-1.5}$ that would be expected from
the field HI mass function from ALFALFA or the 8.9 $\pm$ 1.2 expected if the HI
mass function from the Local Group applied in these regions. The number of
dwarf companions detected is, however, consistent with a flat or declining HI
mass function as seen by a previous, shallower, HI search for companions to
isolated galaxies.We attribute this difference in Hi mass functions to the
different environments in which they are measured. This agrees with the general
observation that lower ratios of dwarf to giant galaxies are found in lower
density environments. | astro-ph_GA |
The mass--metallicity relation AKARI-FMOS infrared galaxies at
$z\sim0.88$ in the AKARI North Ecliptic Pole Deep Survey Field: Mass, metallicity, and star formation rate (SFR) of a galaxy are crucial
parameters in understanding galaxy formation and evolution. However, the
relation among these is still a matter of debate for luminous infrared
galaxies, which carry a bulk of SFR budget of the universe at $z\sim1$. We have
investigated the relation among stellar mass, gas-phase oxygen abundance, and
SFR of AKARI-detected mid-IR galaxies at $z\sim0.88$ in the AKARI NEP deep
field. We observed about 350 AKARI sources with Subaru/FMOS NIR spectrograph,
and detected secure and expected H$\alpha$ emission lines from 25 and 44
galaxies, respectively. The SFR of our sample is almost constant ($\sim
25M_{\odot}/yr$) over the stellar mass range of our sample. Compared with
main-sequence (MS) galaxies at a similar redshift range, the average SFR of our
detected sample is comparable for massive galaxies
($\sim10^{10.58}~M_{\odot}$), while higher by $\sim$0.6dex for less massive
galaxies ($\sim 10^{10.05}~M_{\odot}$). We measure metallicities from the
[NII]/H$\alpha$ emission line ratio.
We find that the mass-metallicity relation of our individually measured
sources agrees with that for optical-selected star-forming galaxies at
$z\sim0.1$, while metallicities of stacked spectra agree with that of MS
galaxies at $z\sim0.78$. Considering high SFR of individually measured sources,
FMR of the IR galaxies is different from that at $z\sim0.1$. However, on the
mass-metallicity plane, they are consistent with the MS galaxies, highlighting
higher SFR of the IR galaxies. This suggests the evolutionary path of our IR
galaxies is different from that of MS galaxies. A possible physical
interpretation includes that the star-formation activities of IR galaxies at
$z\sim0.88$ in our sample are enhanced by interaction and/or merger of
galaxies, but the inflow of metal-poor gas is not yet induced, keeping the
metallicity intact. | astro-ph_GA |
The SPLASH Survey: A Spectroscopic Analysis of the Metal-Poor,
Low-Luminosity M31 dSph Satellite Andromeda X: Andromeda X (And X) is a newly discovered low-luminosity M31 dwarf spheroidal
galaxy (dSph) found by Zucker et al. (2007) in the Sloan Digital Sky Survey
(SDSS - York et al. 2000). In this paper, we present the first spectroscopic
study of individual red giant branch stars in And X, as a part of the SPLASH
Survey (Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo).
Using the Keck II telescope and multiobject DEIMOS spectrograph, we target two
spectroscopic masks over the face of the galaxy and measure radial velocities
for ~100 stars with a median accuracy of sigma_v ~ 3 km/s. The velocity
histogram for this field confirms three populations of stars along the sight
line: foreground Milky Way dwarfs at small negative velocities, M31 halo red
giants over a broad range of velocities, and a very cold velocity ``spike''
consisting of 22 stars belonging to And X with v_rad = -163.8 +/- 1.2 km/s. By
carefully considering both the random and systematic velocity errors of these
stars (e.g., through duplicate star measurements), we derive an intrinsic
velocity dispersion of just sigma_v = 3.9 +/- 1.2 km/s for And X, which for its
size, implies a minimum mass-to-light ratio of M/L =37^{+26}_{-19} assuming the
mass traces the light. Based on the clean sample of member stars, we measure
the median metallicity of And X to be [Fe/H] = -1.93 +/- 0.11, with a slight
radial metallicity gradient. The dispersion in metallicity is large,
sigma([Fe/H]) = 0.48, possibly hinting that the galaxy retained much of its
chemical enrichment products. We discuss the potential for better understanding
the formation and evolution mechanisms for M31's system of dSphs through
(current) kinematic and chemical abundance studies, especially in relation to
the Milky Way sample. (abridged version) | astro-ph_GA |
Effect of turbulent velocity on the \HI intensity fluctuation power
spectrum from spiral galaxies: We use numerical simulations to investigate effect of turbulent velocity on
the power spectrum of \HI intensity from external galaxies when (a) all
emission is considered, (b) emission with velocity range smaller than the
turbulent velocity dispersion is considered. We found that for case (a) the
intensity fluctuation depends directly only on the power spectrum of the column
density, whereas for case (b) it depends only on the turbulent velocity
fluctuation. We discuss the implications of this result in real observations of
\HI fluctuations. | astro-ph_GA |
A Newborn AGN in a Starforming Galaxy: We report on the finding of a newborn AGN, i.e. current AGN activity in a
galaxy previously classified as non-active, and characterize its evolution.
Black hole ignition event candidates were selected from a parent sample of
spectrally classified non-active galaxies (2.394.312 objects), that currently
show optical flux variability indicative of a type I AGN, according to the
ALeRCE light curve classifier. A second epoch spectrum for a sample of
candidate newborn AGN were obtained with the SOAR telescope to search for new
AGN features. We present spectral results for the most convincing case of new
AGN activity, for a galaxy with a previous star-forming optical classification,
where the second epoch spectrum shows the appearance of prominent, broad Balmer
lines without significant changes in the narrow line flux ratios. Long term
optical lightcurves show a steady increase in luminosity starting 1.5 years
after the SDSS spectrum was taken and continuing for at least 7 years. MIR
colors from the WISE catalog have also evolved from typical non active galaxy
colors to AGN-like colors and recent X-ray flux detections confirm its AGN
nature. | astro-ph_GA |
Linking High- and Low-Mass Star Formation: Observation-Based Continuum
Modelling and Physical Conditions: Astronomers have yet to establish whether high-mass protostars form from
high-mass prestellar cores, similar to their lower-mass counterparts, or from
lower-mass fragments at the heart of a pre-protostellar cluster undergoing
large-scale collapse. Part of the uncertainty is due to a shortage of envelope
structure data on protostars of a few tens of solar masses, where we expect to
see a transition from intermediate-mass star formation to the high-mass
process. We sought to derive the masses, luminosities, and envelope density
profiles for eight sources in Cygnus-X, whose mass estimates in the literature
placed them in the sampling gap. Combining these sources with similarly evolved
sources in the literature enabled us to perform a meta-analysis of protostellar
envelope parameters over six decades in source luminosity. We performed
spectral energy distribution (SED) fitting on archival broadband photometric
continuum data from 1.2 to 850 $\mu$m, to derive bolometric luminosities for
our eight sources plus initial mass and radius estimates for modelling density
and temperature profiles with the radiative transfer package Transphere. The
envelope masses, densities at 1000 AU, outer envelope radii, and density power
law indices as functions of bolometric luminosity all follow established trends
in the literature spanning six decades in luminosity. Most of our sources
occupy an intermediate to moderately high range of masses and luminosities,
which helps to more firmly establish the continuity between low- and high-mass
star formation mechanisms. Our density power law indices are consistent with
observed values in literature, which show no discernible trends with
luminosity. Finally, we show that the trends in all of the envelope parameters
for high-mass protostars are statistically indistinguishable from trends in the
same variables for low- and intermediate-mass protostars. | astro-ph_GA |
Laboratory Rotational Spectra of Silyl Isocyanide: The rotational spectrum of silyl isocyanide (SiH$_3$NC), an isomer of the
well studied silyl cyanide (SiH$_3$CN), has been detected in the laboratory in
a supersonic molecular beam, and the identification was confirmed by
observations of the corresponding rotational transitions in the rare isotopic
species SiH$_3$$^{15}$NC and SiH$_3$N$^{13}$C. Spectroscopic constants derived
from 19 transitions between $11 - 35$~GHz in the three lowest harmonically
related rotational transitions in the $K = 0 ~{\rm{and}}~1$ ladders of the
normal isotopic species including the nitrogen nuclear quadrupole hyperfine
constant, allow the principal astronomical transitions of SiH$_3$NC to be
calculated to an uncertainty of about 4~km~s$^{-1}$ in equivalent radial
velocity, or within the FWHM of narrow spectral features in the inner region of
IRC+10216 near 200~GHz. The concentration of SiH$_3$NC in our molecular beam is
three times less than SiH$_3$CN, or about the same as the corresponding ratio
of the isomeric pair SiNC and SiCN produced under similar conditions. Silyl
isocyanide is an excellent candidate for astronomical detection, because the
spectroscopic and chemical properties are very similar to SiH$_3$CN which was
recently identified in the circumstellar envelope of IRC+10216 by
\citet{cernicharo_discovery_2017} and of SiNC and SiCN in the same source. | astro-ph_GA |
Signatures of Kinematic Substructure in the Galactic Stellar Halo: Tidal debris from infalling satellites can leave observable structure in the
phase-space distribution of the Galactic halo. Such substructure can be
manifest in the spatial and/or velocity distributions of the stars in the halo.
This paper focuses on a class of substructure that is purely kinematic in
nature, with no accompanying spatial features. To study its properties, we use
a simulated stellar halo created by dynamically populating the Via Lactea II
high-resolution N-body simulation with stars. A significant fraction of the
stars in the inner halo of Via Lactea share a common speed and metallicity,
despite the fact that they are spatially diffuse. We argue that this kinematic
substructure is a generic feature of tidal debris from older mergers and may
explain the detection of radial-velocity substructure in the inner halo made by
the Sloan Extension for Galactic Understanding and Exploration. The GAIA
satellite, which will provide the proper motions of an unprecedented number of
stars, should further characterize the kinematic substructure in the inner
halo. Our study of the Via Lactea simulation suggests that the stellar halo can
be used to map the speed distribution of the local dark-matter halo, which has
important consequences for dark-matter direct-detection experiments. | astro-ph_GA |
Discerning the Form of the Dense Core Mass Function: We investigate the ability to discern between lognormal and powerlaw forms
for the observed mass function of dense cores in star forming regions. After
testing our fitting, goodness-of-fit, and model selection procedures on
simulated data, we apply our analysis to 14 datasets from the literature.
Whether the core mass function has a powerlaw tail or whether it follows a pure
lognormal form cannot be distinguished from current data. From our simulations
it is estimated that datasets from uniform surveys containing more than
approximately 500 cores with a completeness limit below the peak of the mass
distribution are needed to definitively discern between these two functional
forms. We also conclude that the width of the core mass function may be more
reliably estimated than the powerlaw index of the high mass tail and that the
width may also be a more useful parameter in comparing with the stellar initial
mass function to deduce the statistical evolution of dense cores into stars. | astro-ph_GA |
LABOCA mapping of the infrared dark cloud MSXDC G304.74+01.32: Infrared dark clouds (IRDCs) likely represent very early stages of high-mass
star/star cluster formation. In this study, we aim to determine the physical
properties and spatial distribution of dense clumps in the IRDC MSXDC
G304.74+01.32 (G304.74), and bring these characteristics into relation to
theories concerning the origin of IRDCs and their fragmentation into clumps and
star-forming cores. G304.74 was mapped in the 870 $\mu$m dust continuum with
the LABOCA bolometer on APEX. Archival MSX and IRAS infrared data were used to
study the nature and properties of the submillimetre clumps within the cloud.
There are 8 clumps within G304.74 which are not associated with mid-infrared
(MIR) emission. Some of them are candidates for being/harbouring high-mass
starless cores (HMSCs). We compared the clump masses and their spatial
distribution in G304.74 with those in several other recently studied IRDCs.
There is a high likelihood that the clump mass distributions in G304.74 and in
several other IRDCs represent the samples of the same parent distribution. In
most cases the spatial distributions of clumps in IRDCs do not deviate
significantly from random distributions. This is consistent with the idea that
the origin of IRDCs, and their further sub-fragmentation down to scales of
clumps is caused by supersonic turbulence in accordance with results from giant
molecular clouds. | astro-ph_GA |
Planck Early Results: All sky temperature and dust optical depth from
Planck and IRAS: Constraints on the "dark gas" in our galaxy: We construct an all-sky map of the apparent temperature and optical depth of
thermal dust emission using the Planck-HFI and IRAS data. The optical depth
maps are correlated to tracers of the atomic and molecular gas. The correlation
is linear in the lowest column density regions at high galactic latitudes. At
high NH, the correlation is consistent with that of the lowest NH. In the
intermediate NH range, we observe departure from linearity, with the dust
optical depth in excess to the correlation. We attribute this excess emission
to thermal emission by dust associated with a Dark-Gas phase, undetected in the
available HI and CO measurements. We show the 2D spatial distribution of the
Dark-Gas in the solar neighborhood and show that it extends around known
molecular regions traced by CO. The average dust emissivity in the HI phase in
the solar neighborhood follows roughly a power law distribution with beta = 1.8
all the way down to 3 mm, although the SED flattens slightly in the millimetre.
The threshold for the existence of the Dark-Gas is found at NH = (8.0\pm 0.58)
10^{20} Hcm-2. Assuming the same dust emissivity at high frequencies for the
dust in the atomic and molecular phases leads to an average XCO = (2.54\pm0.13)
10^{20} H2cm-2/(K km s-1). The mass of Dark-Gas is found to be 28% of the
atomic gas and 118% of the CO emitting gas in the solar neighborhood. A
possible origin for the Dark-Gas is the existence of a dark molecular phase,
where H2 survives photodissociation but CO does not. The observed transition
for the onset of this phase in the solar neighborhood (AV = 0.4 mag) appears
consistent with recent theoretical predictions. We also discuss the possibility
that up to half of the Dark-Gas could be in atomic form, due to optical depth
effects in the HI measurements. | astro-ph_GA |
The density structure of the L1157 molecular outflow: We present a multiline CS survey towards the brightest bow-shock B1 in the
prototypical chemically active protostellar outflow L1157. We made use of
(sub-)mm data obtained in the framework of the Chemical HErschel Surveys of
Star forming regions (CHESS) and Astrochemical Surveys at IRAM (ASAI) key
science programs. We detected $^{12}$C$^{32}$S, $^{12}$C$^{34}$S,
$^{13}$C$^{32}$S, and $^{12}$C$^{33}$S emissions, for a total of 18
transitions, with $E_{\rm u}$ up to $\sim$ 180 K. The unprecedented sensitivity
of the survey allows us to carefully analyse the line profiles, revealing
high-velocity emission, up to 20 km s$^{-1}$ with respect to the systemic. The
profiles can be well fitted by a combination of two exponential laws that are
remarkably similar to what previously found using CO. These components have
been related to the cavity walls produced by the $\sim$ 2000 yr B1 shock and
the older ($\sim$ 4000 yr) B2 shock, respectively. The combination of low- and
high-excitation CS emission was used to properly sample the different physical
components expected in a shocked region. Our CS observations show that this
molecule is highlighting the dense, $n_{\rm H_2}$ = 1--5 $\times$ 10$^{5}$
cm$^{-3}$, cavity walls produced by the episodic outflow in L1157. In addition,
the highest excitation (E$_u$ $\geq$ 130 K) CS lines provide us with the
signature of denser (1--5 $\times$ 10$^{6}$ cm$^{-3}$) gas, associated with a
molecular reformation zone of a dissociative J-type shock, which is expected to
arise where the precessing jet impacting the molecular cavities. The CS
fractional abundance increases up to $\sim$ 10$^{-7}$ in all the kinematical
components. This value is consistent with what previously found for
prototypical protostars and it is in agreement with the prediction of the
abundances obtained via the chemical code Astrochem. | astro-ph_GA |
Supernova-driven outflows and chemical evolution of dwarf spheroidal
galaxies: We present a general phenomenological model for the metallicity distribution
(MD) in terms of [Fe/H] for dwarf spheroidal galaxies (dSphs). These galaxies
appear to have stopped accreting gas from the intergalactic medium and are
fossilized systems with their stars undergoing slow internal evolution. For a
wide variety of infall histories of unprocessed baryonic matter to feed star
formation, most of the observed MDs can be well described by our model. The key
requirement is that the fraction of the gas mass lost by supernova-driven
outflows is close to unity. This model also predicts a relationship between the
total stellar mass and the mean metallicity for dSphs in accord with properties
of their dark matter halos. The model further predicts as a natural consequence
that the abundance ratios [E/Fe] for elements such as O, Mg, and Si decrease
for stellar populations at the higher end of the [Fe/H] range in a dSph. We
show that for infall rates far below the net rate of gas loss to star formation
and outflows, the MD in our model is very sharply peaked at one [Fe/H] value,
similar to what is observed in most globular clusters. This suggests that
globular clusters may be end members of the same family as dSphs. | astro-ph_GA |
Star Clusters in Tidal Debris: We present results of a Hubble Space Telescope (HST) UBVI-band study of star
clusters in tidal tails, using new WFC3 and ACS imaging to complement existing
WFPC2 data. We survey 12 tidal tails across seven merging systems, deriving
ages and masses for 425 star cluster candidates (SCCs). The stacked mass
distribution across all systems follows a power law of the form $dN/dM \propto
M^{\beta}$, with $\beta = -2.02 \pm 0.15$, consistent with what is seen in
other star forming environments. GALEX and Swift UV imaging provide star
formation rates (SFRs) for our tidal tails, which when compared with ages and
masses of our SCCs, allows for a determination of the cluster formation
efficiency (CFE). We find the CFE increases with increasing SFR surface
density, matching the theoretical model. We confirm this fit down at SFR
densities lower than previously measured (log $\Sigma_\text{SFR} \:
(\text{M}_\odot \: \text{yr}^{-1} \: \text{kpc}^{-2}) \approx -4.2$), as
related to the CFE. We determine the half-light radii for a refined sample of
57 SCCs with our HST WFC3 and ACS imaging, and calculate their dynamical age,
finding the majority of them to be gravitationally bound. We also provide
evidence of only low-mass ($< 10^4 \: \text{M}_\odot$) cluster formation in our
nearest galaxy, NGC 1487, consistent with the theory that this system is a
dwarf merger. | astro-ph_GA |
A dark matter disc in the Milky Way: Predicting the local flux of dark matter particles is vital for dark matter
direct detection experiments. To date, such predictions have been based on
simulations that model the dark matter alone. Here we include the influence of
the baryonic matter for the first time. We use two different approaches.
Firstly, we use dark matter only simulations to estimate the expected merger
history for a Milky Way mass galaxy, and then add a thin stellar disc to
measure its effect. Secondly, we use three cosmological hydrodynamic
simulations of Milky Way mass galaxies. In both cases, we find that a
stellar/gas disc at high redshift (z~1) causes merging satellites to be
preferentially dragged towards the disc plane. This results in an accreted dark
matter disc that contributes ~0.25 - 1 times the non-rotating halo density at
the solar position. An associated thick stellar disc forms with the dark disc
and shares a similar velocity distribution. If these accreted stars can be
separated from those that formed in situ, future astronomical surveys will be
able to infer the properties of the dark disc from these stars. The dark disc,
unlike dark matter streams, is an equilibrium structure that must exist in disc
galaxies that form in a hierarchical cosmology. Its low rotation lag with
respect to the Earth significantly boosts WIMP capture in the Earth and Sun,
increases the likelihood of direct detection at low recoil energy, boosts the
annual modulation signal, and leads to distinct variations in the flux as a
function of recoil energy that allow the WIMP mass to be determined (see
contribution from T. Bruch this volume). | astro-ph_GA |
NuSTAR J033202-2746.8: direct constraints on the Compton reflection in a
heavily obscured quasar at z~2: We report NuSTAR observations of NuSTAR J033202-2746.8, a heavily obscured,
radio-loud quasar detected in the Extended Chandra Deep Field-South, the
deepest layer of the NuSTAR extragalactic survey (~400 ks, at its deepest).
NuSTAR J033202-2746.8 is reliably detected by NuSTAR only at E>8 keV and has a
very flat spectral slope in the NuSTAR energy band (Gamma=0.55^{+0.62}_{-0.64};
3-30 keV). Combining the NuSTAR data with extremely deep observations by
Chandra and XMM-Newton (4 Ms and 3 Ms, respectively), we constrain the
broad-band X-ray spectrum of NuSTAR J033202-2746.8, indicating that this source
is a heavily obscured quasar (N_H=5.6^{+0.9}_{-0.8}x10^23 cm^-2) with
luminosity L_{10-40 keV}~6.4x10^44 erg s^-1. Although existing optical and
near-infrared (near-IR) data, as well as follow-up spectroscopy with the Keck
and VLT telescopes, failed to provide a secure redshift identification for
NuSTAR J033202-2746.8, we reliably constrain the redshift z=2.00+/-0.04 from
the X-ray spectral features (primarily from the iron K edge). The NuSTAR
spectrum shows a significant reflection component (R=0.55^{+0.44}_{-0.37}),
which was not constrained by previous analyses of Chandra and XMM-Newton data
alone. The measured reflection fraction is higher than the R~0 typically
observed in bright radio-loud quasars such as NuSTAR J033202-2746.8, which has
L_{1.4 GHz}~10^27 W Hz^-1. Constraining the spectral shape of AGN, including
bright quasars, is very important for understanding the AGN population, and can
have a strong impact on the modeling of the X-ray background. Our results show
the importance of NuSTAR in investigating the broad-band spectral properties of
quasars out to high redshift. | astro-ph_GA |
Generation of mock tidal streams: In this paper we discuss a method for the generation of mock tidal streams.
Using an ensemble of simulations in an isochrone potential where the actions
and frequencies are known, we derive an empirical recipe for the evolving
satellite mass and the corresponding mass loss rate, and the ejection
conditions of the stream material. The resulting stream can then be quickly
generated either with direct orbital integration, or by using the action-angle
formalism. The model naturally produces streaky features within the stream.
These are formed due to the radial oscillation of the progenitor and the bursts
of stars emitted near pericenter, rather than clumping at particular
oscillation phases as sometimes suggested. When detectable, these streaky
features are a reliable diagnostic for the stream's direction of motion and
encode other information on the progenitor and its orbit. We show several tests
of the recipe in alternate potentials, including a case with a chaotic
progenitor orbit which displays a marked effect on the width of the stream.
Although the specific ejection recipe may need adjusting when elements such as
the orbit or satellite density profile are changed significantly, our examples
suggest that model tidal streams can be quickly and accurately generated by
models of this general type for use in Bayesian sampling. | astro-ph_GA |
The structural properties of multiple populations in the dynamically
young globular cluster NGC 2419: NGC 2419 is likely the globular cluster (GC) with the lowest dynamical age in
the Galaxy. This makes it an extremely interesting target for studying the
properties of its multiple populations (MPs), as they have been likely affected
only modestly by long-term dynamical evolution effects. Here we present for the
first time a detailed analysis of the structural and morphological properties
of the MPs along the whole extension of this remote and massive GC by combining
high-resolution HST and wide-field ground-based data. In agreement with
formation models predicting that second population (SP) stars form in the inner
regions of the first population (FP) system, we find that the SP is more
centrally concentrated than the FP. This may provide constraints on the
relative concentrations of MPs in the cluster early stages of the evolutionary
phase driven by two-body relaxation. In addition, we find that the fraction of
FP stars is larger than expected from the general trend drawn by Galactic GCs.
If, as suggested by a number of studies, NGC 2419 formed in the Sagittarius
dwarf galaxy and was later accreted by the Milky Way, we show that the observed
FP fraction may be explained as due to the transition of NGC 2419 to a weaker
tidal field (its current Galactocentric distance is d_gc~95 kpc) and
consequently to a reduced loss rate of FP stars. | astro-ph_GA |
High-Fidelity VLA Imaging of the Radio Structure of 3C273: 3C273, the nearest bright quasar, comprises a strong nuclear core and a
bright, one-sided jet extending ~ 23 arcseconds to the SW. The source has been
the subject of imaging campaigns in all wavebands. Extensive observations of
this source have been made with the Very Large Array and other telescopes as
part of a campaign to understand the jet emission mechanisms. Partial results
from the VLA radio campaign have been published, but to date, the complete set
of VLA imaging results has not been made available. We have utilized the VLA to
determine the radio structure of 3C273 in Stokes I, Q, and U, over the widest
possible frequency and resolution range. The VLA observed the source in all
four of its configurations, and with all eight of its frequency bands, spanning
73.8 MHz to 43 GHz. The data were taken in a pseudo-spectral line mode to
minimize the VLA's correlator errors, and were fully calibrated with subsequent
self-calibration techniques to maximise image fidelity. Images in Stokes
parameters I, Q, and U, spanning a resolution range from 6 arcseconds to 88
milliarcseconds are presented. Spectral index images, showing the evolution of
the jet component are shown. Polarimetry demonstrates the direction of the
magnetic fields responsible for the emission, and rotation measure maps show
the RM to be very small with no discernible trend along or across the jet. This
paper presents a small subset of these images to demonstrate the major
characteristics of the source emission. A library of all ~500 images has been
made available for open, free access by interested parties. | astro-ph_GA |
Rings and Halos in the Mid-Infrared: The Planetary Nebulae NGC 7354 and
NGC 3242: We present images of the planetary nebulae (PNe) NGC 7354 and NGC 3242 in
four mid-infrared (MIR) photometric bands centred at 3.6, 4.5, 5.8 and 8.0
microns; the results of observations undertaken using the Spitzer Space
Telescope (SST). The resulting images show the presence of a halo and rings in
NGC 3242, as previously observed through narrow band imaging at visual
wavelengths, as well as evidence for a comparable halo and ring system in NGC
7354. This is the first time that a halo and rings have been observed in the
latter source.
We have analysed the formation of halos as a result of radiatively
accelerated mass loss in the AGB progenitors. Although the models assume that
dust formation occurs in C-rich environments, we note that qualitatively
similar results would be expected for O-rich progenitors as well. The model
fall-offs in halo density are found to result in gradients in halo surface
brightness which are similar to those observed in the visible and MIR. | astro-ph_GA |
Influence of galactic arm scale dynamics on the molecular composition of
the cold and dense ISM II. Molecular oxygen abundance: Molecular oxygen has been the subject of many observational searches as
chemical models predicted it to be a reservoir of oxygen. Although it has been
detected in two regions of the interstellar medium, its rarity is a challenge
for astrochemical models. In this paper, we have combined the physical
conditions computed with smoothed particle hydrodynamics (SPH) simulations with
our full gas-grain chemical model Nautilus, to study the predicted O2 abundance
in interstellar material forming cold cores. We thus follow the chemical
evolution of gas and ices in parcels of material from the diffuse interstellar
conditions to the cold dense cores. Most of our predicted O2 abundances are
below 1e-8 (with respect to the total proton density) and the predicted column
densities in simulated cold cores is at maximum a few 1e14 cm-2, in agreement
with the non detection limits. This low O2 abundance can be explained by the
fact that, in a large fraction of the interstellar material, the atomic oxygen
is depleted onto the grain surface (and hydrogenated to form H2O) before O2 can
be formed in the gas-phase and protected from UV photo-dissociations. We could
achieve this result only because we took into account the full history of the
evolution of the physical conditions from the diffuse medium to the cold cores. | astro-ph_GA |
Supernovae and their host galaxies - IV. The distribution of supernovae
relative to spiral arms: Using a sample of 215 supernovae (SNe), we analyze their positions relative
to the spiral arms of their host galaxies, distinguishing grand-design (GD)
spirals from non-GD (NGD) galaxies. We find that: (1) in GD galaxies, an offset
exists between the positions of Ia and core-collapse (CC) SNe relative to the
peaks of arms, while in NGD galaxies the positions show no such shifts; (2) in
GD galaxies, the positions of CC SNe relative to the peaks of arms are
correlated with the radial distance from the galaxy nucleus. Inside (outside)
the corotation radius, CC SNe are found closer to the inner (outer) edge. No
such correlation is observed for SNe in NGD galaxies nor for SNe Ia in either
galaxy class; (3) in GD galaxies, SNe Ibc occur closer to the leading edges of
the arms than do SNe II, while in NGD galaxies they are more concentrated
towards the peaks of arms. In both samples of hosts, the distributions of SNe
Ia relative to the arms have broader wings. These observations suggest that
shocks in spiral arms of GD galaxies trigger star formation in the leading
edges of arms affecting the distributions of CC SNe (known to have short-lived
progenitors). The closer locations of SNe Ibc vs. SNe II relative to the
leading edges of the arms supports the belief that SNe Ibc have more massive
progenitors. SNe Ia having less massive and older progenitors, have more time
to drift away from the leading edge of the spiral arms. | astro-ph_GA |
Probing Self-interacting Dark Matter with Disk Galaxies in Cluster
Environments: Self-Interacting Dark Matter (SIDM) has long been proposed as a solution to
small scale problems posed by standard Cold Dark Matter (CDM). We use numerical
simulations to study the effect of dark matter interactions on the morphology
of disk galaxies falling into galaxy clusters. The effective drag force on dark
matter leads to offsets of the stellar disk with respect to the surrounding
halo, causing distortions in the disk. For anisotropic scattering
cross-sections of 0.5 and 1.0$\,\textrm{cm}^{2}\textrm{g}^{-1}$, we show that
potentially observable warps, asymmetries, and thickening of the disk occur in
simulations. We discuss observational tests of SIDM with galaxy surveys and
more realistic simulations needed to obtain detailed predictions. | astro-ph_GA |
The Neutron Star Born in the Antlia Supernova Remnant: Among all known young nearby neutron stars, we search for the neutron star
that was born in the same supernova event that formed the Antlia supernova
remnant (SNR). We also look for a runaway star that could have been the former
companion to the neutron star (if it exists) and then got ejected due to the
same supernova. We find the pulsar PSR J0630-2834 to be the best candidate for
a common origin with the Antlia SNR. In that scenario the SNR is ~1.2 Myr old
and is presently located at a distance of ~138 pc. We consider the runaway star
HIP 47155 a former companion candidate to PSR J0630-2834. The encounter time
and place is consistent with both stars being ejected from the Antlia SNR. We
measured the radial velocity of HIP 47155 as 32.42 +/- 0.70km/s. | astro-ph_GA |
Dust in the Torus of the AGN Unified Model: These proceedings are based on an invited review talk at the 7th Meeting on
Cosmic Dust. The scope of the meeting was broad, covering dust-related topics
in areas from comets to debris disks and high-redshift galaxies. This is
therefore intended to be an accessible, introductory overview of the dusty
torus of the AGN unified model, aimed at summarizing our current understanding
of the torus and with some emphasis on the solid-state spectral features
observed. | astro-ph_GA |
Major Substructure in the M31 Outer Halo: Distances and Metallicities
along the Giant Stellar Stream: We present a renewed look at M31's Giant Stellar Stream along with the nearby
structures Stream C and Stream D, exploiting a new algorithm capable of fitting
to the red giant branch (RGB) of a structure in both colour and magnitude
space. Using this algorithm, we are able to generate probability distributions
in distance, metallicity and RGB width for a series of subfields spanning these
structures. Specifically, we confirm a distance gradient of approximately 20
kpc per degree along a 6 degree extension of the Giant Stellar Stream, with the
farthest subfields from M31 lying ~ 120 kpc more distant than the inner-most
subfields. Further, we find a metallicity that steadily increases from
-0.7^{+0.1}_{-0.1} dex to -0.2^{+0.2}_{-0.1} dex along the inner half of the
stream before steadily dropping to a value of -1.0^{+0.2}_{-0.2} dex at the
farthest reaches of our coverage. The RGB width is found to increase rapidly
from 0.4^{+0.1}_{-0.1} dex to 1.1^{+0.2}_{-0.1} dex in the inner portion of the
stream before plateauing and decreasing marginally in the outer subfields of
the stream. In addition, we estimate Stream C to lie at a distance between 794
and 862 kpc and Stream D between 758 kpc and 868 kpc. We estimate the median
metallicity of Stream C to lie in the range -0.7 to -1.6 dex and a metallicity
of -1.1^{+0.3}_{-0.2} dex for Stream D. RGB widths for the two structures are
estimated to lie in the range 0.4 to 1.2 dex and 0.3 to 0.7 dex respectively.
In total, measurements are obtained for 19 subfields along the Giant Stellar
Stream, 4 along Stream C, 5 along Stream D and 3 general M31 spheroid fields
for comparison. We thus provide a higher resolution coverage of the structures
in these parameters than has previously been available in the literature. | astro-ph_GA |
The Intracluster Light and its Link with the Dynamical State of the Host
Group/Cluster: the Role of the Halo Concentration: We investigate on the role of the halo concentration in the formation of the
intra-cluster light (ICL) in galaxy groups and clusters, as predicted by a
state-of-art semi-analytic model of galaxy formation, coupled with a set of
high-resolution dark matter only simulations. The analysis focuses on how the
fraction of ICL correlates with halo mass, concentration and fraction of
early-type galaxies (ETGs) in a large sample of groups and clusters with
$13.0\leq \log M_{halo} \leq 15.0$. The fraction of ICL follows a normal
distribution, a consequence of the stochastic nature of the physical processes
responsible for the formation of the diffuse light. The fractional budget of
ICL depends on both halo mass (very weakly) until group scales, and
concentration (remarkably). More interestingly, the ICL fraction is higher in
more concentrated objects, a result of the stronger tidal forces acting in the
innermost regions of the haloes where the concentration is the quantity playing
the most relevant role. Our model predictions do not show any dependence
between the ICL and ETGs fractions and so, we instead suggest the concentration
rather than the mass, as recently claimed, to be the main driver of the ICL
formation. The diffuse light starts to form in groups via stellar stripping and
mergers and later assembled in more massive objects. However, the formation and
assembly keep going on group/cluster scales at lower redshift through the same
processes, mainly via stellar stripping in the vicinity of the central regions
where tidal forces are stronger. | astro-ph_GA |
Dynamical cloud formation traced by atomic and molecular gas: Context: Atomic and molecular cloud formation is a dynamical process.
However, kinematic signatures of these processes are still observationally
poorly constrained. Methods: Targeting the cloud-scale environment of the
prototypical infrared dark cloud G28.3, we employ spectral line imaging
observations of the two atomic lines HI and [CI] as well as molecular lines
observations in 13CO in the 1--0 and 3--2 transitions. The analysis comprises
investigations of the kinematic properties of the different tracers, estimates
of the mass flow rates, velocity structure functions, a Histogram of Oriented
Gradients (HOG) study as well as comparisons to simulations. Results: The
central IRDC is embedded in a more diffuse envelope of cold neutral medium
(CNM) traced by HI self-absorption (HISA) and molecular gas. The spectral line
data as well as the HOG and structure function analysis indicate a possible
kinematic decoupling of the HI from the other gas compounds. Spectral analysis
and position-velocity diagrams reveal two velocity components that converge at
the position of the IRDC. Estimated mass flow rates appear rather constant from
the cloud edge toward the center. The velocity structure function analysis is
consistent with gas flows being dominated by the formation of hierarchical
structures. Conclusions: The observations and analysis are consistent with a
picture where the IRDC G28 is formed at the center of two converging gas flows.
While the approximately constant mass flow rates are consistent with a
self-similar, gravitationally driven collapse of the cloud, external
compression by, e.g., spiral arm shocks or supernovae explosions cannot be
excluded yet. Future investigations should aim at differentiating the origin of
such converging gas flows. | astro-ph_GA |
PSR J1841-0500: a radio pulsar that mostly is not there: In a search for radio pulsations from the magnetar 1E 1841-045, we have
discovered the unrelated pulsar J1841-0500, with rotation period P=0.9 s and
characteristic age 0.4 Myr. One year after discovery with the Parkes telescope
at 3 GHz, radio emission ceased from this bright pulsar. After 580 days,
emission resumed as before. The P-dot during both on states is 250% of the
average in the off state. PSR J1841-0500 is a second example of an extremely
intermittent pulsar, although with a much longer off period and larger ratio of
spin-down rates than PSR B1931+24. The new pulsar is hugely scattered by the
ISM, with a fitted timescale referenced to 1 GHz of tau_1=2 s. Based on
polarimetric observations at 5 GHz with the Green Bank Telescope, the intrinsic
pulse profile has not obviously changed between the two on states observed so
far, although relatively small variations cannot be excluded. The magnitude of
its rotation measure is the largest known, RM=-3000 rad/m^2, and with a
dispersion measure DM=532 pc/cc implies a large electron-weighted average
magnetic field strength along the line of sight, 7 microG. | astro-ph_GA |
The Extension of the Transition Temperature Plasma into the Lower
Galactic Halo: Column densities for H I, Al III, Si IV, C IV, and O VI toward 109 stars and
30 extragalactic objects have been assembled to study the extensions of these
species away from the Galactic plane into the Galactic halo. H I and Al III
mostly trace the warm neutral and warm ionized medium, respectively, while Si
IV, C IV and O VI trace a combination of warm photoionized and collisionally
ionized plasmas. The much larger object sample compared to previous studies
allows us to consider and correct for the effects of the sample bias that has
affected earlier but smaller surveys of the gas distributions. We find Si IV
and C IV have similar exponential scale heights of 3.2(+1.0, -0.6) and
3.6(+1.0, -0.8) kpc. The scale height of O VI is marginally smaller with h =
2.6+/- 0.6 kpc. The transition temperature gas is ~3 times more extended than
the warm ionized medium traced by Al III with h = 0.90(+0.62, -0.33) kpc and
~12 times more extended than the warm neutral medium traced by H I with h =
0.24 +/- 0.06 kpc. There is a factor of 2 decrease in the dispersion of the log
of the column density ratios for transition temperature gas for lines of sight
in the Galactic disk compared to extragalactic lines of sight through the
entire halo. The observations are compared to the predictions of the various
models for the production of the transition temperature gas in the halo. The
appendix presents a revision to the electron scale height proposed by Gaensler
et al. (2008) based on electron dispersion measures. | astro-ph_GA |
MusE GAs FLOw and Wind (MEGAFLOW) VII. A NOEMA pilot program to probe
molecular gas in galaxies with measured circumgalactic gas flows: We present a pilot program using IRAM's NOrthern Extended Millimeter Array
(NOEMA) to probe the molecular gas reservoirs of six $z=0.6-1.1$ star-forming
galaxies whose circumgalactic medium has been observed in absorption along
quasar lines-of-sight as part of the MusE GAs FLOw and Wind (MEGAFLOW) survey
and for which we have estimates of either the accretion or the outflow rate.
This program is motivated by testing the quasi equilibrium model and the
compaction scenario describing the evolution of galaxies along the main
sequence of star formation, which imply tight relations between the gas
content, the star formation activity, and the amount of gas flowing in and out.
We report individual carbon monoxide CO(4-3), CO(3-2) and dust continuum upper
limits, as well as stacked CO detections over the whole sample and the three
galaxies identified with outflows. The resulting molecular gas fractions and
depletion times are compatible with published scaling relations established
within a mass-selected sample, indicating that galaxies selected through their
absorption follow similar relations on average. We further detect the dust
continuum of three of the quasars and a strong emission line in one of them,
which we identify as CO(4-3). Extending the sample to more galaxies and deeper
observations will enable to quantify how the molecular gas fraction and
depletion time depend on the inflow and ouflow rates. | astro-ph_GA |
Preliminary results on SiO v=3 J=1-0 maser emission from AGB stars: We present the results of SiO maser observations at 43GHz toward two AGB
stars using the VLBA. Our preliminary results on the relative positions of the
different J=1-0 SiO masers (v=1,2 and 3) indicate that the current ideas on SiO
maser pumping could be wrong at some fundamental level. A deep revision of the
SiO pumping models could be necessary. | astro-ph_GA |
From the Convergence of Filaments to Disk-Outflow Accretion:
Massive-Star Formation in W33A: Interferometric observations of the W33A massive star-formation region,
performed with the Submillimeter Array (SMA) and the Very Large Array (VLA) at
resolutions from 5 arcsec (0.1 pc) to 0.5 arcsec (0.01 pc) are presented. Our
three main findings are: (1) parsec-scale, filamentary structures of cold
molecular gas are detected. Two filaments at different velocities intersect in
the zone where the star formation is occurring. This is consistent with
triggering of the star-formation activity by the convergence of such filaments,
as predicted by numerical simulations of star formation initiated by converging
flows. (2) The two dusty cores (MM1 and MM2) at the intersection of the
filaments are found to be at different evolutionary stages, and each of them is
resolved into multiple condensations. MM1 and MM2 have markedly different
temperatures, continuum spectral indices, molecular-line spectra, and masses of
both stars and gas. (3) The dynamics of the "hot-core" MM1 indicates the
presence of a rotating disk in its center (MM1-Main) around a faint free-free
source. The stellar mass is estimated to be approximately 10 Msun. A massive
molecular outflow is observed along the rotation axis of the disk. | astro-ph_GA |
Prospects for detecting CII emission during the Epoch of Reionization: We produce simulations of emission of the atomic CII line in large sky fields
in order to determine the current prospects for mapping this line during the
high redshift Epoch of Reionization. We estimate the CII line intensity,
redshift evolution and spatial fluctuations using observational relations
between CII emission and the SFR in a galaxy for the frequency range of 200 GHz
to 300 GHz. We obtained a frequency averaged intensity of CII emission of ${\rm
I_{\rm CII}=(4 \pm 2)\times10^{2}\, Jy\, \rm sr^{-1}}$ in the redshift range
$z\, \sim\, 5.3\, -\, 8.5$. Observations of CII emission in this frequency
range will suffer contamination from emission lines at lower redshifts, in
particular from the CO rotation lines. For the relevant frequency range we
estimated the CO contamination (originated in emission from galaxies at $z\,
<\, 2.5$), using simulations, to be ${\rm I_{\rm CO} \approx 10^{3}\, Jy \,
sr^{-1}}$ and independently confirmed the result based in observational
relations. We generated maps as a function of angle and frequency using
detailed simulations of the CII and CO emission across several redshifts in
order to properly take into account the observational pipeline and light cone
effects. In order to reduce the foreground contamination we found that we
should mask galaxies below redshifts $\sim 2.5$ with a CO flux in one of the
CO(J:2-1) to CO(J:6-5) lines higher than ${\rm 5\times 10^{-22}\, W\ m^{-2}}$
or a AB magnitude lower than ${\rm m_{\rm K}\, =\, 22}$. We estimate that the
additional continuum contamination is of the order of ${\rm 10^{5}\, Jy\,
sr^{-1}}$. It is also considered the possibility of cross correlating
foreground lines with galaxies in order to probe the intensity of the
foregrounds. | astro-ph_GA |
The Lives of Stars: Insights From the TGAS-RAVE-LAMOST Dataset: In this paper we investigate how the chemical and kinematic properties of
stars vary as a function of age. Using data from a variety of photometric,
astrometric and spectroscopic surveys, we calculate the ages, phase space
information and orbits for $\sim$125,000 stars covering a wide range of stellar
parameters.
We find indications that the inner regions of the disk reached high levels of
enrichment early, while the outer regions were more substantially enriched in
intermediate and recent epochs. We consider these enrichment histories through
comparison of the ages of stars, their metallicities, and kinematic properties,
such as their angular momentum in the solar neighborhood (which is a proxy for
orbital radius). We calculate rates at which the velocity dispersions evolve,
investigate the Oort constants for different aged populations (finding a
slightly negative $\partial V_{C} / \partial R$ and $\partial V_{R} / \partial
R$ for all ages, being most negative for the oldest stars), as well as examine
the behavior of the velocity vertex deviation angle as a function of age (which
we find to fall from $\sim$15 degrees for the 2 Gyr aged population to $\sim$6
degrees at around 6.5 Gyr of age, after which it remains unchanged). We find
evidence for stellar churning, and find that the churned stars have a slightly
younger age distribution than the rest of the data. | astro-ph_GA |
An Outer Arm in the Second Galactic Quadrant: Structure: The lack of arm tracers, especially the remote tracers, is one of the most
difficult problems preventing us from studying the structure of the Milky Way.
Fortunately, with its high-sensitivity CO survey, the Milky Way Imaging Scroll
Painting (MWISP) project offers such an opportunity. Since completing about
one-third of its mission, an area of l=[100,150] deg, b=[-3,5] deg has nearly
been covered. The Outer arm of the Milky Way first clearly revealed its shape
in the second galactic quadrant in the form of molecular gas --- this is the
first time that the Outer arm has been reported in such a large-scale mapping
of molecular gas. Using the 115 GHz 12CO(1-0) data of MWISP at the LSR velocity
~= [-100,-60] km s^-1 and in the area mentioned above, we have detected 481
molecular clouds in total, and among them 332 (about 69\%) are newly detected
and 457 probably belong to the Outer arm. The total mass of the detected Outer
arm clouds is ~ 3.1*10^6 M_sun. Assuming that the spiral arm is a logarithmic
spiral, the pitch angle is fitted as ~ 13.1 deg. Besides combining both the CO
data from MWISP and the 21 cm HI data from the Canadian Galactic Plane Survey
(CGPS), the gas distribution, warp, and thickness of the Outer arm are also
studied. | astro-ph_GA |
X-ray galaxies selected from HyperLEDA database: We cross-matched the 4XMM-DR10 catalog with the HyperLEDA database and
obtained the new sample of galaxies that contain X-ray sources. Excluding
duplicate observations and false matches, we present a total of 7759 galaxies
with X-ray sources. In the current work, we present general properties of the
sample: namely the distribution in equatorial coordinates, radial velocity
distribution, morphological type, and X-ray fluxes. The sample includes
morphological classification for 5241 galaxies with X-ray emission, almost half
of which, 42\%, are elliptical (E, E-S0). Most galaxies in the sample have
nuclear X-ray emission (6313 or 81\%), and the remaining 1443 (19\%) present
X-ray emission from the host galaxy. This sample can be used for future deep
studies of multi wavelengths properties of the galaxies with X-ray emission. | astro-ph_GA |
Fragmentation in a Primordial Accretion Flow: Under rapid cooling from molecular hydrogen, the accretion disks around
Population III (PopIII) stars are believed to fragment, resulting in multiple
accreting cores. In this paper, we build a theoretical framework for
calculating the optical depth of H$_2$ ro-vibrational line cooling based on the
vertical structure in these accretion disks. Applying this physically motivated
prescription for the optical depth, we find that cooling in the inner disk with
$r \lesssim 10 {\rm\ AU}$ is attenuated significantly as a result of high
surface density; $PdV$ heating becomes more efficient than cooling, which
prevents fragmentation in the inner disk. Despite this, cooling becomes
dynamically important in the outer disk, favoring fragmentation. We argue that
most of the resultant fragments are initially at the outer disk, and that any
surviving fragment has to migrate slower than the disk-scale photo-evaporation
process. Since type I migration is fast, migration slows down as a result of
gap-opening in the disk structure. Two possible processes for gap-opening are
studied: (1) through a massive, densely-cored ($\rho \gtrsim 10^{-8} {\rm\ g\
cm^{-3}}$) clump able to radiate away the excess gravitational potential
energy, and (2) through a fast-growing central star, with $\dot{M} \gtrsim 2
\times 10^{-3} \, M_\odot {\rm\ yr^{-1}}$, whose gravity dominates the
star-disk system and favors gap opening. | astro-ph_GA |
A Deep Near-Infrared [Fe II]+[Si I] Emission Line Image of the Supernova
Remnant Cassiopeia A: We present a long-exposure (~10 hr) image of the supernova (SN) remnant
Cassiopeia A (Cas A) obtained with the UKIRT 3.8-m telescope using a narrow
band filter centered at 1.644 um emission. The passband contains [Fe II] 1.644
um and [Si I] 1.645 um lines, and our `deep [Fe II]+[Si I] image' provides an
unprecedented panoramic view of Cas A, showing both shocked and unshocked SN
ejecta together with shocked circumstellar medium at subarcsec (~0.7 arcsec or
0.012 pc) resolution. The diffuse emission from the unshocked SN ejecta has a
form of clumps, filaments, and arcs, and their spatial distribution correlates
well with that of the Spitzer [Si II] infrared emission, suggesting that the
emission is likely due to [Si I] line not [Fe II] line as in shocked material.
The structure of the optically-invisible western area of Cas A is clearly seen
for the first time. The area is filled with many Quasi-Stationary Flocculi
(QSFs) and fragments of the disrupted ejecta shell. We suggest that the
anomalous radio properties in this area could be due to the increased number of
such dense clumps. We identified 309 knots in the deep [Fe II]+[Si I] image and
classified them into QSFs and fast-moving knots (FMKs). The total H+He mass of
QSFs is ~0.23 Msun, implying that the mass fraction of dense clumps in the
progenitor's red-supergiant wind is 4--13%. The spatial distribution of QSFs
suggests that there had been a highly asymmetric mass loss $10^4$--$10^5$ yr
before the SN explosion. The mass of the [Fe II] line-emitting, shocked dense
Fe ejecta is ~3x$10^{-5}$ Msun. The comparison with the ionic S-line dominated
Hubble Space Telescope WFC3/IR image suggests that the outermost FMKs in the
southeastern area are Fe-rich. | astro-ph_GA |
Entropy-Conserving Scheme for Modeling Nonthermal Energies in Fluid
Dynamics Simulations: We compare the performance of energy-based and entropy-conserving schemes for
modeling nonthermal energy components, such as unresolved turbulence and cosmic
rays, using idealized fluid dynamics tests and isolated galaxy simulations.
While both methods are aimed to model advection and adiabatic compression or
expansion of different energy components, the energy-based scheme numerically
solves the nonconservative equation for the energy density evolution, while the
entropy-conserving scheme uses a conservative equation for modified entropy.
Using the standard shock tube and Zel'dovich pancake tests, we show that the
energy-based scheme results in a spurious generation of nonthermal energy on
shocks, while the entropy-conserving method evolves the energy adiabatically to
machine precision. We also show that, in simulations of an isolated $L_\star$
galaxy, switching between the schemes results in $\approx 20-30\%$ changes of
the total star formation rate and a significant difference in morphology,
particularly near the galaxy center. We also outline and test a simple method
that can be used in conjunction with the entropy-conserving scheme to model the
injection of nonthermal energies on shocks. Finally, we discuss how the
entropy-conserving scheme can be used to capture the kinetic energy dissipated
by numerical viscosity into the subgrid turbulent energy implicitly, without
explicit source terms that require calibration and can be rather uncertain. Our
results indicate that the entropy-conserving scheme is the preferred choice for
modeling nonthermal energy components, a conclusion that is equally relevant
for Eulerian and moving-mesh fluid dynamics codes. | astro-ph_GA |
A Catalog of Galaxies in the Direction of the Perseus Cluster: We present a catalog of 5437 morphologically classified sources in the
direction of the Perseus galaxy cluster core, among them 496 early-type
low-mass galaxy candidates. The catalog is primarily based on V-band imaging
data acquired with the William Herschel Telescope, which we used to conduct
automated source detection and to derive photometry. We additionally reduced
archival Subaru multiband imaging data in order to measure aperture colors and
to perform a morphological classification, benefiting from 0.5 arcsec seeing
conditions in the r-band data. Based on morphological and color properties, we
extracted a sample of early-type low-mass galaxy candidates with absolute
V-band magnitudes in the range of -10 to -20 mag. In the color-magnitude
diagram the galaxies are located where the red sequence for early-type cluster
galaxies is expected, and they lie on the literature relation between absolute
magnitude and S\'{e}rsic index. We classified the early-type dwarf candidates
into nucleated and nonnucleated galaxies. For the faint candidates, we found a
trend of increasing nucleation fraction toward brighter luminosity or higher
surface brightness, similar to what is observed in other nearby galaxy
clusters. We morphologically classified the remaining sources as likely
background elliptical galaxies, late-type galaxies, edge-on disk galaxies, and
likely merging systems and discussed the expected contamination fraction
through non-early-type cluster galaxies in the magnitude-size surface
brightness parameter space. Our catalog reaches its 50 per cent completeness
limit at an absolute V-band luminosity of -12 mag and a V-band surface
brightness of 26 mag arcsec$^{-2}$. This makes it to the largest and deepest
catalog with coherent coverage compared to previous imaging studies of the
Perseus cluster. | astro-ph_GA |
Turbulent magnetic reconnection in 2D and 3D: Magnetic field embedded in a perfectly conducting fluid preserves its
topology for all time. Although ionized astrophysical objects, like stars and
galactic disks, are almost perfectly conducting, they show indications of
changes in topology, `magnetic reconnection', on dynamical time scales.
Reconnection can be observed directly in the solar corona, but can also be
inferred from the existence of large scale dynamo activity inside stellar
interiors. Solar flares and gamma ray busts are usually associated with
magnetic reconnection. Previous work has concentrated on showing how
reconnection can be rapid in plasmas with very small collision rates. Here we
present numerical evidence, based on three dimensional simulations, that
reconnection in a turbulent fluid occurs at a speed comparable to the rms
velocity of the turbulence, regardless of the value of the resistivity. In
particular, this is true for turbulent pressures much weaker than the magnetic
field pressure so that the magnetic field lines are only slightly bent by the
turbulence. These results are consistent with the proposal by Lazarian and
Vishniac (1999) that reconnection is controlled by the stochastic diffusion of
magnetic field lines, which produces a broad outflow of plasma from the
reconnection zone. This work implies that reconnection in a turbulent fluid
typically takes place in approximately a single eddy turnover time, with broad
implications for dynamo activity and particle acceleration throughout the
universe. In contrast, the reconnection in 2D configurations in the presence of
turbulence depends on resistivity, i.e. is slow. | astro-ph_GA |
High abundance ratio of $^{13}$CO to C$^{18}$O toward photon-dominated
regions in the Orion-A giant molecular cloud: Aims. We derive physical properties such as the optical depths and the column
densities of $^{13}$CO and C$^{18}$O to investigate the relationship between
the far ultraviolet (FUV) radiation and the abundance ratios between $^{13}$CO
and C$^{18}$O.
Method. We have carried out wide-field (0.4 deg$^2$) observations with an
angular resolution of 25.8 arcsec ($\sim$ 0.05 pc) in $^{13}$CO ($J$=1--0) and
C$^{18}$O ($J$=1--0) toward the Orion-A giant molecular cloud using the
Nobeyama 45 m telescope in the on-the-fly mode.
Results. Overall distributions and velocity structures of the $^{13}$CO and
C$^{18}$O emissions are similar to those of the $^{12}$CO ($J$=1--0) emission.
The optical depths of the $^{13}$CO and C18O emission lines are estimated to be
0.05 $<$ $\tau_{\rm ^{13}CO}$ $<$ 1.54 and 0.01 $<$ $\tau_{\rm C^{18}O}$ $<$
0.18, respectively. The column densities of the $^{13}$CO and C$^{18}$O
emission lines are estimated to be 0.2 $\times$ 10$^{16}$ $<$ $N_{\rm ^{13}CO}$
$<$ 3.7 $\times$ 10$^{17}$ cm$^{-2}$ and 0.4 $\times$ 10$^{15}$ $<$ $N_{\rm
C^{18}O}$ $<$ 3.5 $\times$ 10$^{16}$ cm$^{-2}$, respectively. The abundance
ratios between $^{13}$CO and C$^{18}$O, $X_{\rm ^{13}CO}$/$X_{\rm C^{18}O}$,
are found to be 5.7 - 33.0. The mean value of $X_{\rm ^{13}CO}$/$X_{\rm
C^{18}O}$ in the nearly edge-on photon-dominated regions is found to be 16.47
$\pm$ 0.10, which is a third larger than that the solar system value of 5.5.
The mean value of $X_{\rm ^{13}CO}$/$X_{\rm C^{18}O}$ in the other regions is
found to be 12.29 $\pm$ 0.02. The difference of the abundance ratio is most
likely due to the selective FUV photodissociation of C$^{18}$O. | astro-ph_GA |
The optical variability of SDSS quasars from multi-epoch spectroscopy.
II. color variation: We investigated the optical/ultraviolet (UV) color variations for a sample of
2169 quasars based on multi-epoch spectroscopy in the Sloan Digital Sky Survey
(SDSS) data release seven (DR7) and data release nine (DR9). To correct the
systematic difference between DR7 and DR9 due to the different instrumental
setup, we produced a correction spectrum by using a sample of F-stars observed
both in DR7 and DR9. The correction spectrum was then applied to quasars when
comparing the spectra of DR7 with DR9. In each object, the color variation was
explored by comparing the spectral index of the continuum power-law fit on the
brightest spectrum with the faintest one, and also by the shape of their
difference spectrum. In 1876 quasars with consistent color variations from two
methods, we found that most sources (1755, $\sim 94\%$) show
bluer-when-brighter (BWB) trend, and the redder-when-brighter (RWB) trend is
only detected in 121 objects ($\sim 6\%$). The common BWB trend is supported by
the bluer composite spectrum constructed from bright spectra than that from
faint spectra, and also by the blue composite difference spectrum. The
correction spectrum is proved to be highly reliable by comparing the composite
spectrum from corrected DR9 and original DR7 spectra. Assuming that the
optical/UV variability is triggered by fluctuations, RWB trend can likely be
explained if the fluctuations occur firstly at outer disk region, and the inner
disk region has not fully responded yet when the fluctuation being propagated
inward. In contrast, the common BWB trend implies that the fluctuations are
likely more often happening firstly in inner disk region. | astro-ph_GA |
JCMT Mapping of CO(3-2) in the Circumnuclear Region of M31: We present a survey of CO(3-2) molecular line emission in the circumnuclear
region of M31 with the James Clerk Maxwell Telescope (JCMT), aiming to explore
the physical conditions of the molecular gas. Significant CO(3-2) lines are
detected primarily along the so-called nuclear spiral, out to a projected
galactocentric radius of 700 pc at a linear resolution of ~50 pc. We find that
the velocity field of the molecular gas is in rough agreement with that of the
ionized gas previously determined from optical observations. Utilizing existed
CO(2-1) and CO(1-0) measurements in selected regions of the nuclear spiral, we
derive characteristic intensity ratios of CO(3-2)/CO(2-1) and CO(3-2)/CO(1-0),
which are both close to unity and are significantly higher than the typical
intensity ratios in the disk. Such line ratios suggest high kinetic
temperatures of the gas, which might be due to strong interstellar shocks
prevalent in the circumnuclear region. | astro-ph_GA |
Seeds of Life in Space SOLIS. IX. Chemical segregation of $\rm SO_2$ and
SO toward the low-mass protostellar shocked region of L1157: We present observations of SO and $\rm SO_2$ lines toward the shocked regions
along the L1157 chemically rich outflow, taken in the context of the Seeds Of
Life In Space IRAM-NOrthern Extended Millimeter Array Large Program, and
supported by data from Submillimeter Array and IRAM-30 m telescope at 1.1--3.6
mm wavelengths. We simultaneously analyze, for the first time, all of the
brightest shocks in the blueshifted lobe, namely, B0, B1, and B2. We found the
following. (1) SO and $\rm SO_2$ may trace different gas, given that the
large(-scale) velocity gradient analysis indicates for $\rm SO_2$ a volume
density ($\rm 10^5\text{--}10^6\,cm^{-3}$) denser than that of the gas emitting
in SO by a factor up to an order of magnitude. (2) Investigating the 0.1 pc
scale field of view, we note a tentative gradient along the path of the
precessing jet. More specifically, $\rm \chi({SO/SO_2})$ decreases from the
B0-B1 shocks to the older B2. (3) At a linear resolution of 500--1400 au, a
tentative spatial displacement between the two emitting molecules is detected,
with the SO peak closer (with respect to $\rm SO_2$) to the position where the
recent jet is impinging on the B1 cavity wall. Our astrochemical modeling shows
that the SO and $\rm SO_2$ abundances evolve on timescales less than about 1000
years. Furthermore, the modeling requires high abundances ($2\times10^{-6}$) of
both $\rm H_2S/H$ and S/H injected in the gas phase due to the shock
occurrence, so pre-frozen OCS only is not enough to reproduce our new
observations. | astro-ph_GA |
Tips learned from panchromatic modeling of AGNs: I will review the tips learned from panchromatic modeling of active galactic
nuclei (AGNs), based on our recent work to study the relationship between AGN
and star formation (SF). Several AGN SED models are compared, and signifficant
AGN contribution is found in the IR luminosities and corresponding star
formation rate (SFR). I will review the AGN-SF relation and how different
parameters and sample selections affect the observed correlation. I will then
report on the constant ratio discovered between the SFR and the black hole mass
accretion rate (BHAR), and their implications on the gas supply and galaxy
formation history of these systems. Caveats and important questions to answer
are summarized at the end. | astro-ph_GA |
Detailed chemical abundances of distant RR Lyrae stars in the Virgo
Stellar Stream: We present the first detailed chemical abundances for distant RR Lyrae stars
members of the Virgo Stellar Stream (VSS), derived from X-Shooter
medium-resolution spectra. Sixteen elements from carbon to barium have been
measured in six VSS RR Lyrae stars, sampling all main nucleosynthetic channels.
For the first time we will be able to compare in detail the chemical evolution
of the VSS progenitor with those of Local Group dwarf spheroidal galaxies (LG
dSph) as well as the one of the smooth halo. | astro-ph_GA |
Testing the universality of the IMF with Bayesian statistics: young
clusters: The universality of the stellar initial mass function (IMF) is tested using
Bayesian statistics with a sample of eight young Galactic stellar clusters (IC
348, ONC, NGC 2024, NGC 6611, NGC 2264, $\rho$ Ophiuchi, Chameleon I, and
Taurus). We infer the posterior probability distribution function (pPDF) of the
IMF parameters when the likelihood function is described by a tapered power law
function, a lognormal distribution at low masses coupled to a power law at
higher masses, and a multi-component power law function. The inter-cluster
comparison of the pPDFs of the IMF parameters for each likelihood function
shows that these distributions do not overlap within the $1\sigma$ uncertainty
level. Furthermore, the most probable values of the IMF parameters for most of
the clusters deviate substantially from their values for the Galactic field
stellar IMF. We also quantify the effects of taking into account the
completeness correction as well as the uncertainties on the measured masses.
The inclusion of the former affects the inferred pPDFs of the slope of the IMF
at the low mass end while considering the latter affects the pPDFs of the slope
of the IMF in the intermediate- to high mass regime. As variations are observed
in all of the IMF parameters at once and for each of the considered likelihood
functions, even for completeness corrected samples, we argue that the observed
variations are real and significant, at least for the sample of eight clusters
considered in this work. The results presented here clearly show that the IMF
is not universal. | astro-ph_GA |
The Milky Way's Shell Structure Reveals the Time of a Radial Collision: We identify shell structures in the Milky Way for the first time. We find 2
shells in the Virgo Overdensity (VOD) region and 2 shells in the Hercules
Aquila Cloud (HAC) region using Sloan Digital Sky Survey, Gaia, and LAMOST
data. These shell stars are a subset of the substructure previously identified
as the Virgo Radial Merger (VRM). Timing arguments for these shells indicate
that their progenitor dwarf galaxy passed through the Galactic center 2.7 +/-
0.2 Gyr ago. Based on the time of collision, it is also possible that the VRM
is related to the phenomenon that created phase-space spirals in the vertical
motion of the disk and/or the Splash, and could have caused a burst of star
formation in the inner disk.
We analyze phase mixing in a collection of radial merger N-body simulations,
and find that shell structure similar to that observed in Milky Way data
disappears by 5 Gyr after collision with the Galactic center. The method used
to calculate the merger time of the VRM was able to reliably recover the
correct merger times for these simulations.
Previous work supports the idea that the VRM and the Gaia
Sausage/Gaia-Enceladus Merger are the same. However, the Gaia Sausage is widely
believed to be 8--11 Gyr old. The disparate ages could be reconciled if the
larger age is associated with an infall time when the progenitor crossed the
virial radius; we do not constrain the time at which the progenitor became
bound to the Milky Way. Alternatively, the Gaia Sausage could be younger than
previously thought. | astro-ph_GA |
The relationship between IGM Lyman-alpha opacity and galaxy density near
the end of reionization: Observed scatter in the Lyman-alpha opacity of quasar sightlines at $z<6$ has
motivated measurements of the correlation between Ly$\alpha$ opacity and galaxy
density, as models that predict this scatter make strong and sometimes opposite
predictions for how they should be related. Our previous work associated two
highly opaque Ly$\alpha$ troughs at $z\sim5.7$ with a deficit of Lyman-$\alpha$
emitting galaxies (LAEs). In this work, we survey two of the most highly
transmissive lines of sight at this redshift, towards the $z=6.02$ quasar SDSS
J1306+0356 and the $z=6.17$ quasar PSO J359-06. We find that both fields are
underdense in LAEs within 10 $h^{-1}$ Mpc of the quasar sightline, somewhat
less extensive than underdensities associated with Ly$\alpha$ troughs. We
combine our observations with three additional fields from the literature, and
find that while fields with extreme opacities are generally underdense,
moderate opacities span a wider density range. The results at high opacities
are consistent with models that invoke UV background fluctuations and/or late
reionization to explain the observed scatter in IGM Ly$\alpha$ opacities. There
is tension at low opacities, however, as the models tend to associate lower IGM
Ly$\alpha$ opacities with higher densities. Although the number of fields
surveyed is still small, the low-opacity results may support a scenario in
which the ionizing background in low-density regions increases more rapidly
than some models suggest after becoming ionized. Elevated gas temperatures from
recent reionization may also be making these regions more transparent. | astro-ph_GA |
Aromatic cycles are widespread in cold clouds: We report the detection of large hydrocarbon cycles toward several cold dense
clouds. We observed four sources (L1495B, Lupus-1A, L483, and L1527) in the Q
band (31-50 GHz) using the Yebes 40m radiotelescope. Using the line stack
technique, we find statistically significant evidence of benzonitrile
(C$_6$H$_5$CN) in L1495B, Lupus-1A, and L483 at levels of 31.8$\,\sigma$,
15.0$\,\sigma$, and 17.2$\,\sigma$, respectively, while there is no hint of
C$_6$H$_5$CN in the fourth source, L1527. The column densities derived are in
the range (1.7-3.8)$\,\times\,10^{11}$ cm$^{-2}$, which is somewhat below the
value derived toward the cold dense cloud TMC-1. When we simultaneously analyze
all the benzonitrile abundances derived toward cold clouds in this study and in
the literature, a clear trend emerges in that the higher the abundance of
HC$_7$N, the more abundant C$_6$H$_5$CN is. This indicates that aromatic cycles
are especially favored in those interstellar clouds where long carbon chains
are abundant, which suggests that the chemical processes that are responsible
for the formation of linear carbon chains are also behind the synthesis of
aromatic rings. We also searched for cycles other than benzonitrile, and found
evidence of indene (C$_9$H$_8$), cyclopentadiene (C$_5$H$_6$), and 1-cyano
cyclopentadiene (1-C$_5$H$_5$CN) at levels of 9.3$\,\sigma$, 7.5$\,\sigma$, and
8.4$\,\sigma$, respectively, toward L1495B, which shows the strongest signal
from C$_6$H$_5$CN. The relative abundances between the various cycles detected
in L1495B are consistent -- within a factor of three -- with those previously
found in TMC-1. It is therefore likely that not only C$_6$H$_5$CN but also
other large aromatic cycles are abundant in clouds rich in carbon chains. | astro-ph_GA |
Gas kinematics, morphology, and angular momentum in the FIRE simulations: We study the z=0 gas kinematics, morphology, and angular momentum content of
isolated galaxies in a suite of cosmological zoom-in simulations from the FIRE
project spanning $M_{\star}=10^{6-11}M_{\odot}$. Gas becomes increasingly
rotationally supported with increasing galaxy mass. In the lowest-mass galaxies
($M_{\star}<10^{8}M_{\odot}$), gas fails to form a morphological disk and is
primarily dispersion and pressure supported. At intermediate masses
($M_{\star}=10^{8-10}M_{\odot}$), galaxies display a wide range of gas
kinematics and morphologies, from thin, rotating disks, to irregular spheroids
with negligible net rotation. All the high-mass
($M_{\star}=10^{10-11}M_{\odot}$) galaxies form rotationally supported gas
disks. Many of the halos whose galaxies fail to form disks harbor high angular
momentum gas in their circumgalactic medium. The ratio of the specific angular
momentum of gas in the central galaxy to that of the dark-matter halo increases
significantly with galaxy mass, from $j_{\rm gas}/j_{\rm DM}\sim0.1$ at
$M_{\star}=10^{6-7}M_{\odot}$ to $j_{\rm gas}/j_{\rm DM}\sim2$ at
$M_{\star}=10^{10-11}M_{\odot}$. The reduced rotational support in the
lowest-mass galaxies owes to (a) stellar feedback and the UV background
suppressing the accretion of high-angular momentum gas at late times, and (b)
stellar feedback driving large non-circular gas motions. We broadly reproduce
the observed scaling relations between galaxy mass, gas rotation velocity,
size, and angular momentum, but may somewhat underpredict the incidence of
disky, high-angular momentum galaxies at the lowest observed masses
($M_{\star}=(10^{6}-2\times10^{7})M_{\odot}$). In our simulations, stars are
uniformly less rotationally supported than gas. The common assumption that
stars follow the same rotation curve as gas thus substantially overestimates
galaxies' stellar angular momentum, particularly at low masses. | astro-ph_GA |
The Initial Conditions of Clustered Star Formation III. The Deuterium
Fractionation of the Ophiuchus B2 Core: We present N2D+ 3-2 (IRAM) and H2D+ 1_11 - 1_10 and N2H+ 4-3 (JCMT) maps of
the small cluster-forming Ophiuchus B2 core in the nearby Ophiuchus molecular
cloud. In conjunction with previously published N2H+ 1-0 observations, the N2D+
data reveal the deuterium fractionation in the high density gas across Oph B2.
The average deuterium fractionation R_D = N(N2D+)/N(N2H+) ~ 0.03 over Oph B2,
with several small scale R_D peaks and a maximum R_D = 0.1. The mean R_D is
consistent with previous results in isolated starless and protostellar cores.
The column density distributions of both H2D+ and N2D+ show no correlation with
total H2 column density. We find, however, an anticorrelation in deuterium
fractionation with proximity to the embedded protostars in Oph B2 to distances
>= 0.04 pc. Destruction mechanisms for deuterated molecules require gas
temperatures greater than those previously determined through NH3 observations
of Oph B2 to proceed. We present temperatures calculated for the dense core gas
through the equating of non-thermal line widths for molecules (i.e., N2D+ and
H2D+) expected to trace the same core regions, but the observed complex line
structures in B2 preclude finding a reasonable result in many locations. This
method may, however, work well in isolated cores with less complicated velocity
structures. Finally, we use R_D and the H2D+ column density across Oph B2 to
set a lower limit on the ionization fraction across the core, finding a mean
x_e, lim >= few x 10^{-8}. Our results show that care must be taken when using
deuterated species as a probe of the physical conditions of dense gas in
star-forming regions. | astro-ph_GA |
Cepheid Kinematics and the Galactic Warp: The space velocities of 200 long-period ($P>5$ days) classical Cepheids with
known proper motions and line-of-sight velocities whose distances were
estimated from the period--luminosity relation have been analyzed. The linear
Ogorodnikov-Milne model has been applied, with the Galactic rotation having
been excluded from the observed velocities in advance. Two significant
gradients have been found in the Cepheid velocities,
$\partial W/\partial Y=-2.1\pm0.7$ km s$^{-1}$ kpc$^{-1}$ and
$\partial V/\partial Z= 27\pm10 $ km s$^{-1}$ kpc$^{-1}$.
In such a case, the angular velocity of solid-body rotation around the
Galactic $X$ axis directed to the Galactic center is $-15\pm5$ km s$^{-1}$
kpc$^{-1}$. | astro-ph_GA |
The interaction of a large-scale nuclear wind with the high velocity HII
region G0.17+0.15: We investigate the nature of a Galactic center source, G0.17+0.15, lying
along the northern extension of the Radio Arc near l~0.2deg. G0.17+0.15 is an
HII region located toward the eastern edge of the radio bubble, embedded within
the highly polarized Galactic center eastern Lobe where a number of radio
filaments appear to cross through the HII region. We report the detection of
hydrogen and helium recombination lines with a radial velocity exceeding 140
km/s based on GBT and VLA observations. The morphology of G0.17+0.15, aided by
kinematics, and spectral index characteristics, suggests the presence of an
external pressure dragging and shredding the ionized gas. We argue that this
ionized cloud is interacting with a bundle of radio filaments and is entrained
by the ram pressure of the radio bubble, which itself is thought to be produced
by cosmic-ray driven outflows at the Galactic center. In this interpretation,
the gas streamers on the western side of G0.17+0.15 are stripped, accelerated
from 0 to deltav~35 km/s, over a time scale roughly 8x10^4 years, implying that
ablating ram pressure is ~700 eV cm-3, comparable to the ~10^3 eV cm-3
cosmic-ray driven wind pressure in the Galactic center region. | astro-ph_GA |
NGC 3628-UCD1: A possible $ω$ Cen Analog Embedded in a Stellar
Stream: Using Subaru/Suprime-Cam wide-field imaging and both Keck/ESI and LBT/MODS
spectroscopy, we identify and characterize a compact star cluster, which we
term NGC 3628-UCD1, embedded in a stellar stream around the spiral galaxy NGC
3628. The size and luminosity of UCD1 are similar to $\omega$ Cen, the most
luminous Milky Way globular cluster, which has long been suspected to be the
stripped remnant of an accreted dwarf galaxy. The object has a magnitude of
$i=19.3$ mag (${\rm L}_{\rm i}=1.4\times10^{6}~{\rm L}_{\odot}$). UCD1 is
marginally resolved in our ground-based imaging, with a half-light radius of
$\sim10$ pc. We measure an integrated brightness for the stellar stream of
$i=13.1$ mag, with $(g-i)=1.0$. This would correspond to an accreted dwarf
galaxy with an approximate luminosity of ${\rm L}_i\sim4.1\times10^{8}~{\rm
L}_{\odot}$. Spectral analysis reveals that UCD1 has an age of $6.6$ Gyr ,
$[\rm{Z}/\rm{H}]=-0.75$, an $[{\alpha}/\rm{Fe}]=-0.10$. We propose that UCD1 is
an example of an $\omega$ Cen-like star cluster possibly forming from the
nucleus of an infalling dwarf galaxy, demonstrating that at least some of the
massive star cluster population may be created through tidal stripping. | astro-ph_GA |
Persistent Non-Gaussian Structure in the Image of Sagittarius A* at 86
GHz: Observations of the Galactic Center supermassive black hole Sagittarius A*
(Sgr A*) with very long baseline interferometry (VLBI) are affected by
interstellar scattering along our line of sight. At long radio observing
wavelengths ($\gtrsim1\,$cm), the scattering heavily dominates image
morphology. At 3.5 mm (86 GHz), the intrinsic source structure is no longer
sub-dominant to scattering, and thus the intrinsic emission from Sgr A* is
resolvable with the Global Millimeter VLBI Array (GMVA). Long-baseline
detections to the phased Atacama Large Millimeter/submillimeter Array (ALMA) in
2017 provided new constraints on the intrinsic and scattering properties of Sgr
A*, but the stochastic nature of the scattering requires multiple observing
epochs to reliably estimate its statistical properties. We present new
observations with the GMVA+ALMA, taken in 2018, which confirm non-Gaussian
structure in the scattered image seen in 2017. In particular, the ALMA-GBT
baseline shows more flux density than expected for an anistropic Gaussian
model, providing a tight constraint on the source size and an upper limit on
the dissipation scale of interstellar turbulence. We find an intrinsic source
extent along the minor axis of $\sim100\,\mu$as both via extrapolation of
longer wavelength scattering constraints and direct modeling of the 3.5 mm
observations. Simultaneously fitting for the scattering parameters, we find an
at-most modestly asymmetrical (major-to-minor axis ratio of $1.5\pm 0.2$)
intrinsic source morphology for Sgr A*. | astro-ph_GA |
A Magnified Compact Galaxy at Redshift 9.51 with Strong Nebular Emission
Lines: Ultraviolet light from early galaxies is thought to have ionized gas in the
intergalactic medium. However, there are few observational constraints on this
epoch because of the faintness of those galaxies and the redshift of their
optical light into the infrared. We report the observation, in JWST imaging, of
a distant galaxy that is magnified by gravitational lensing. JWST spectroscopy
of the galaxy, at rest-frame optical wavelengths, detects strong nebular
emission lines that are attributable to oxygen and hydrogen. The measured
redshift is z = 9.51 +- 0.01, corresponding to 510 million years after the Big
Bang. The galaxy has a radius of 16.2+4.6-7.2 parsecs, which is substantially
more compact than galaxies with equivalent luminosity at z = 6 to 8, leading to
a high star formation rate surface density. | astro-ph_GA |
An HST/COS survey of molecular hydrogen in DLAs & sub-DLAs at z < 1:
Molecular fraction and excitation temperature: We present the results of a systematic search for molecular hydrogen (H2) in
low redshift ($ 0.05 \lesssim z \lesssim 0.7$) DLAs and sub-DLAs with $N(HI)
\gtrsim 10^{19.0}$ cm$^{-2}$, in the archival HST/COS spectra. Our core sample
is comprised of 27 systems with a median $\log N(HI) = 19.6$. On the average,
our survey is sensitive down to $\log N(H2) = 14.4$ corresponding to a
molecular fraction of $\log f_{H2} = -4.9$ at the median $N(HI)$. H2 is
detected in 10 cases (3/5 DLAs and 7/22 sub-DLAs) down to this $f_{H2}$ limit.
The H2 detection rate of $50^{+25}_{-12}$ percent seen in our sample, is a
factor of $\gtrsim 2$ higher than that of the high-$z$ sample of Noterdaeme et
al. (2008), for systems with $N(H2) > 10^{14.4}$ cm$^{-2}$. In spite of having
$N(HI)$ values typically lower by a factor of 10, low-$z$ H2 systems show
molecular fractions ($\log f_{H2}=-1.93\pm0.63$) that are comparable to the
high-$z$ sample. The rotational excitation temperatures ($T_{01} = 133\pm55$
K), as measured in our low-$z$ sample, are typically consistent with high-$z$
measurements. Simple photoionization models favour a radiation field much
weaker than the mean Galactic ISM field for a particle density in the range 10
- 100 cm$^{-3}$. The impact parameters of the identified host-galaxy candidates
are in the range $10 \lesssim \rho$ (kpc) $\lesssim 80$. We, therefore,
conjecture that the low-$z$ H2 bearing gas is not related to star-forming disks
but stems from self-shielded, tidally stripped or ejected disk-material in the
extended halo. | astro-ph_GA |
Globular Cluster Intrinsic Iron Abundance Spreads: II. Protocluster
Metallicities and the Age-Metallicity Relations of Milky Way Progenitors: Intrinsic iron abundance spreads in globular clusters, although usually
small, are very common, and are signatures of self enrichment: some stars
within the cluster have been enriched by supernova ejecta from other stars
within the same cluster. We use the Bailin (2018) self enrichment model to
predict the relationship between properties of the protocluster -- its mass and
the metallicity of the protocluster gas cloud -- and the final observable
properties today -- its current metallicity and the internal iron abundance
spread. We apply this model to an updated catalog of Milky Way globular
clusters where the initial mass and/or the iron abundance spread is known to
reconstruct their initial metallicities. We find that with the exception of the
known anomalous bulge cluster Terzan 5 and three clusters strongly suspected to
be nuclear star clusters from stripped dwarf galaxies, the model provides a
good lens for understanding their iron spreads and initial metallicities. We
then use these initial metallicities to construct age-metallicity relations for
kinematically-identified major accretion events in the Milky Way's history. We
find that using the initial metallicity instead of the current metallicity does
not alter the overall picture of the Milky Way's history, since the difference
is usually small, but does provide information that can help distinguish which
accretion event some individual globular clusters with ambiguous kinematics
should be associated with, and points to potential complexity within the
accretion events themselves. | astro-ph_GA |
The Lifecycle of Clusters in Galaxies: We review many of the basic properties of star cluster systems, and focus in
particular on how they relate to their host galaxy properties and ambient
environment. The cluster mass and luminosity functions are well approximated by
power-laws of the form $Ndm \propto M^{\alpha}dm$, with $\alpha\sim-2$ over
most of the observable range. However, there is now clear evidence that both
become steeper at high masses/luminosities, with the value of the downward turn
dependent on environment. The host galaxy properties also appear to affect the
cluster formation efficiency ($\Gamma$ - i.e., the fraction of stars that form
in bound clusters), with higher star-formation rate density galaxies having
higher $\Gamma$ values. Within individual galaxies, there is evidence for
$\Gamma$ to vary by a factor of 3-4, likely following the molecular gas surface
density, in agreement with recent predictions. Finally, we discuss cluster
disruption and its effect on the observed properties of a population, focussing
on the age distribution of clusters. We briefly discuss the expectations of
theoretical and numerical studies, and also the observed distributions in a
number of galaxies. Most observational studies now find agreement with
theoretical expectations, namely nearly a constant cluster age distribution for
ages up to ~100 Myr (i.e. little disruption), and a drastic steepening above
this value caused by a combination of cluster disruption and incompleteness.
Rapid cluster disruption for clusters with ages < 100 Myr is ruled out for most
galaxies. | astro-ph_GA |
Bar-driven evolution and quenching of spiral galaxies in cosmological
simulations: We analyse the output of the hi-res cosmological zoom-in simulation ErisBH to
study self-consistently the formation of a strong stellar bar in a Milky
Way-type galaxy and its effect on the galactic structure, on the central gas
distribution and on star formation. The simulation includes radiative cooling,
star formation, SN feedback and a central massive black hole which is
undergoing gas accretion and is heating the surroundings via thermal AGN
feedback. A large central region in the ErisBH disk becomes bar-unstable after
z~1.4, but a clear bar-like structure starts to grow significantly only after
z~0.4, possibly triggered by the interaction with a massive satellite. At z~0.1
the bar reaches its maximum radial extent of l~2.2 kpc. As the bar grows, it
becomes prone to buckling instability, which we quantify based on the
anisotropy of the stellar velocity dispersion. The actual buckling event is
observable at z~0.1, resulting in the formation of a boxy-peanut bulge clearly
discernible in the edge-on view of the galaxy at z=0. The bar in ErisBH does
not dissolve during the formation of the bulge but remains strongly
non-axisymmetric down to the resolution limit of ~100 pc at z=0. During its
early growth, the bar exerts a strong torque on the gas within its extent and
drives gas inflows that enhance the nuclear star formation on sub-kpc scales.
Later on the infalling gas is nearly all consumed into stars and, to a lesser
extent, accreted onto the central black hole, leaving behind a gas-depleted
region within the central ~2 kpc. Observations would more likely identify a
prominent, large-scale bar at the stage when the galactic central region has
already been quenched. Bar-driven quenching may play an important role in
disk-dominated galaxies at all redshift. [Abridged] | astro-ph_GA |
Stirring up an embedded star cluster with a moving gas filament: We perform simulations to test the effects of a moving gas filament on a
young star cluster (i.e. the "Slingshot" Model). We model Orion Nebula
Cluster-like clusters as Plummer spheres and the Integral Shaped Filament gas
as a cylindrical potential. We observe that in a static filament, an initially
spherical cluster evolves naturally into an elongated distribution of stars.
For sinusoidal moving filaments, we observe different remnants, and classify
them into 4 categories.%: 3 different objects and one transition object.
"Healthy" clusters, where almost all the stars stay inside the filament and the
cluster; "destroyed" clusters are the opposite case, with almost no particles
in the filament or near the centre of density of the clusters; "ejected"
clusters, where a large fraction of stars are close to the centre of density of
the stars , but almost none of them in the filament; and "transition" clusters,
where roughly the same number of particles is ejected from the cluster and from
the filament. An {{Orion Nebula Cluster-like}} cluster might stay inside the
filament or be ejected, but it will not be destroyed. | astro-ph_GA |
Possible routes for the Formation of Prebiotic Molecules in the
Horsehead Nebula: This article presents the results of a study concerning interstellar
molecules which are useful for the bookkeeping of the organic content of the
universe and for providing a glimpse into prebiotic conditions on Earth and in
other environments in the universe. We explored production channels for
astrobiological relevant nitrogen-bearing cyclic molecules (N-heterocycles), e.
g. pyrrole and pyridine. The present simulations demonstrate how the
exploration of a few possible routes of production of N-heterocycles resulted
in significant abundances for these species. One particularly efficient class
of channels for the production of N-heterocycles incorporates polycyclic
aromatic hydrocarbons (PAHs) as catalysts. Thereby, an exploration of a variety
of production paths should reveal more species to be target of astrophysical
observations. | astro-ph_GA |
Medium-resolution Spectroscopy of Red Giant Branch Stars in $ω$
Centauri: We present [Fe/H] and [Ca/Fe] of $\sim600$ red giant branch (RGB) members of
the globular cluster $\omega$ Centauri. We collect medium-resolution
($R\sim2000$) spectra using the Blanco 4 m telescope at the Cerro Tololo
Inter-American Observatory equipped with Hydra, the fiber-fed multi-object
spectrograph. We demonstrate that blending of stellar light in optical fibers
severely limits the accuracy of spectroscopic parameters in the crowded central
region of the cluster. When photometric temperatures are taken in the
spectroscopic analysis, our kinematically selected cluster members, excluding
those that are strongly affected by flux from neighboring stars, include
relatively fewer stars at intermediate metallicity ([Fe/H]$\sim-1.5$) than seen
in the previous high-resolution survey for brighter giants in Johnson &
Pilachowski. As opposed to the trend of increasing [Ca/Fe] with [Fe/H] found by
those authors, our [Ca/Fe] estimates, based on Ca II H & K measurements, show
essentially the same mean [Ca/Fe] for most of the metal-poor and
metal-intermediate populations in this cluster, suggesting that mass- or
metallicity-dependent SN II yields may not be necessary in their proposed
chemical evolution scenario. Metal-rich cluster members in our sample show a
large spread in [Ca/Fe], and do not exhibit a clear bimodal distribution in
[Ca/Fe]. We also do not find convincing evidence for a radial metallicity
gradient among RGB stars in $\omega$ Centauri. | astro-ph_GA |
Exemplary Merging Clusters: Weak-lensing and X-ray Analysis of the
Double Radio Relic Merging Galaxy Clusters MACS 1752.0+4440 and ZWCL
1856.8+6616: The investigation of merging galaxy clusters that exhibit radio relics is
strengthening our understanding of the formation and evolution of galaxy
clusters, the nature of dark matter, the intracluster medium, and astrophysical
particle acceleration. Each merging cluster provides only a single view of the
cluster formation process and the variety of merging clusters is vast. Clusters
hosting double radio relics are rare and extremely important because they allow
tight constraints on the merger scenario. We present a weak-lensing and X-ray
analysis of MACSJ1752.0+4440 ($z$=0.365) and ZWCL1856.8+6616 ($z$=0.304), two
double radio relic clusters. Our weak-lensing mass estimates show that each
cluster is a major merger with approximately 1:1 mass ratio. The total mass of
MACSJ1752.0+4440 (ZWCL1856.8+6616) is $M_{200}=14.7^{+3.8}_{-3.3}\times10^{14}\
$M$_\odot$ ($M_{200}=2.4^{+0.9}_{-0.7}\times10^{14}\ $M$_\odot$). We find that
these two clusters have comparable features in their weak-lensing and gas
distributions, even though the systems have vastly different total masses. From
the likeness of the X-ray morphologies and the remarkable symmetry of the radio
relics, we propose that both systems underwent nearly head-on collisions.
However, revelations from the hot-gas features and our multiwavelength data
analysis suggest that ZWCL1856.8+6618 is likely at a later merger phase than
MACSJ1752.0+4440. We postulate that the SW radio relic in MACSJ1752.0+4440 is a
result of particle re-acceleration. | astro-ph_GA |
Low-frequency radio spectra of submillimetre galaxies in the Lockman
Hole: We investigate the radio properties of a sample of 53 sources selected at 850
$\mu$m from the SCUBA-2 Cosmology Legacy Survey using new deep, low-frequency
radio imaging of the Lockman Hole field from the Low Frequency Array. Combining
these data with additional radio observations from the GMRT and the JVLA, we
find a variety of radio spectral shapes and luminosities within our sample
despite their similarly bright submillimetre flux densities. We characterise
their spectral shapes in terms of multi-band radio spectral indices. Finding
strong spectral flattening at low frequencies in ~20% of sources, we
investigate the differences between sources with extremely flat low-frequency
spectra and those with `normal' radio spectral indices. As there are no other
statistically significant differences between the two subgroups of our sample
as split by the radio spectral index, we suggest that any differences are
undetectable in galaxy-averaged properties that we can observe with our
unresolved images, and likely relate to galaxy properties that we cannot
resolve, on scales $\lesssim$ 1 kpc. We attribute the observed spectral
flattening in the radio to free-free absorption, proposing that those sources
with significant low-frequency spectral flattening have a clumpy distribution
of star-forming gas. We estimate an average spatial extent of absorbing
material of at most several hundred parsecs to produce the levels of absorption
observed in the radio spectra. This estimate is consistent with the
highest-resolution observations of submillimetre galaxies in the literature,
which find examples of non-uniform dust distributions on scales of ~100 pc,
with evidence for clumps and knots in the interstellar medium. Additionally, we
find two bright (> 6 mJy) submm sources undetected at all other wavelengths. We
speculate that these objects may be very high redshift sources, likely residing
at z > 4. | astro-ph_GA |
Zooming in on the circumgalactic medium: resolving small-scale gas
structure with the GIBLE cosmological simulations: We introduce Project GIBLE (Gas Is Better resoLved around galaxiEs), a suite
of cosmological zoom-in simulations where gas in the circumgalactic medium
(CGM) is preferentially simulated at ultra-high numerical resolution. Our
initial sample consists of eight galaxies, all selected as Milky Way-like
galaxies at $z=0$ from the TNG50 simulation. Using the same galaxy formation
model as IllustrisTNG, and the moving-mesh code AREPO, we re-simulate each of
these eight galaxies maintaining a resolution equivalent to TNG50-2
($m_{\rm{gas}}$ $\sim$ $8 \times 10^5 {\rm M}_{\odot}$). However, we use our
super-Lagrangian refinement scheme to more finely resolve gas in the CGM around
these galaxies. Our highest resolution runs achieve 512 times better mass
resolution ($\sim$ $10^3 {\rm M}_{\odot}$). This corresponds to a median
spatial resolution of $\sim$ $75$ pc at $0.15~R_{\rm{200,c}}$, which coarsens
with increasing distance to $\sim$ $700$ pc at the virial radius. We make
predictions for the covering fractions of several observational tracers of
multi-phase CGM gas: HI, MgII, CIV and OVII. We then study the impact of
improved resolution on small scale structure. While the abundance of the
smallest cold, dense gas clouds continues to increase with improving
resolution, the number of massive clouds is well converged. We conclude by
quantifying small scale structure with the velocity structure function and the
auto-correlation function of the density field, assessing their resolution
dependence. The GIBLE cosmological hydrodynamical simulations enable us to
improve resolution in a computationally efficient manner, thereby achieving
numerical convergence of a subset of key CGM gas properties and observables. | astro-ph_GA |
Unfolding the Laws of Star Formation: The Density Distribution of
Molecular Clouds: The formation of stars shapes the structure and evolution of entire galaxies.
The rate and efficiency of this process are affected substantially by the
density structure of the individual molecular clouds in which stars form. The
most fundamental measure of this structure is the probability density function
of volume densities (rho-PDF), which determines the star formation rates
predicted with analytical models. This function has remained unconstrained by
observations. We have developed an approach to quantify rho-PDFs and establish
their relation to star formation. The rho-PDFs instigate a density threshold of
star formation and allow us to quantify the star formation efficiency above it.
The rho-PDFs provide new constraints for star formation theories and correctly
predict several key properties of the star-forming interstellar medium. | astro-ph_GA |
The Effects of Stellar Population and Gas Covering Fraction on the
Emergent Lyman Alpha Emission of High-Redshift Galaxies: We perform joint modeling of the composite rest-frame far-UV (FUV) and
optical spectra of redshift 1.85<z<3.49 star-forming galaxies to deduce key
properties of the massive stars, ionized ISM, and neutral ISM, with the aim of
investigating the principal factors affecting the production and escape of
Ly-alpha (Lya) photons. Our sample consists of 136 galaxies with deep Keck/LRIS
and MOSFIRE spectra covering, respectively, Ly-beta through CIII] 1907, 1909;
and [OII], [NeIII], H-beta, [OIII], H-alpha, [NII], and [SII]. Spectral and
photoionization modeling indicate that the galaxies are uniformly consistent
with stellar population synthesis models that include the effects of stellar
binarity. Over the dynamic range of our sample, there is little variation in
stellar and nebular abundance with Lya equivalent width, W(Lya), and only a
marginal anti-correlation between age and W(Lya). The inferred range of
ionizing spectral shapes is insufficient to solely account for the variation in
W(Lya). Rather, the covering fraction of optically-thick HI appears to be the
principal factor modulating the escape of Lya, with most of the Lya photons in
down-the-barrel observations of galaxies escaping through low-column-density or
ionized channels in the ISM. Our analysis shows that a high star-formation-rate
surface density, Sigma_SFR, particularly when coupled with a low galaxy
potential (i.e., low stellar mass), can aid in reducing the covering fraction
and ease the escape of Lya photons. We conclude with a discussion of the
implications of our results for the escape of ionizing radiation at high
redshift. | astro-ph_GA |
The HST/ACS star formation history of the Tucana dwarf spheroidal
galaxy: clues from the horizontal branch: We report a new star formation history for the Tucana dwarf spheroidal
galaxy, obtained from a new look at a deep HST/ACS colour-magnitude diagram. We
combined information from the main sequence turn-off and the horizontal branch
to resolve the ancient star formation rates on a finer temporal scale than
previously possible. We show that Tucana experienced three major phases of star
formation, two very close together at ancient times and the last one ending
between 6 and 8 Gyr ago. We show that the three discrete clumps of stars on the
horizontal branch are linked to the distinct episodes of star formation in
Tucana. The spatial distribution of the clumps reveals that each generation of
stars presents a higher concentration than the previous one. The simultaneous
modelling of the horizontal branch and the main sequence turn-off also allows
us to measure the amount of mass lost by red giant branch stars in Tucana with
unprecedented precision, confirming dwarf spheroidals to be excellent
laboratories to study the advanced evolution of low-mass stars. | astro-ph_GA |
The building blocks of the Milky Way halo using APOGEE and Gaia -- or --
Is the Galaxy a typical galaxy?: We summarise recent results from analysis of APOGEE/Gaia data for stellar
populations in the Galactic halo, disk, and bulge, leading to constraints on
the contribution of dwarf galaxies and globular clusters to the stellar content
of the Milky Way halo. Interpretation of the extant data in light of
cosmological numerical simulations suggests that the Milky Way has been subject
to an unusually intense accretion history at z >~ 1.5. | astro-ph_GA |
High resolution elemental abundance analysis of the Hyades Supercluster: The existence of a kinematically defined moving group of stars centred at U =
-40, V = -17 km/s referred to as the Hyades Supercluster, has been suggested as
the debris of an originally large star forming event, with its core being the
present day Hyades open cluster. Using high-resolution UVES spectra, we present
elemental abundances for a range of alpha, Fe-peak and neutron-capture elements
for 26 proposed supercluster stars. Our results show that the sample stars
display a heterogeneous abundance distribution, with a clump around [Fe/H] =
+0.15. We also calculate stellar radial velocities and U,V,W space velocities.
Enforcing a strict chemical and kinematical membership criteria, we find 4
supercluster stars share the Hyades open cluster abundances and kinematics,
while many of the remaining stars fit the disc field kinematics and abundance
range. We discuss our findings in the context of the Hyades supercluster being
a dispersed star-forming remnant, a stellar stream of purely dynamical origin
or a result of several processes. | astro-ph_GA |
Gaseous-phase metallicities and stellar populations in the centres of
barred galaxies: Numerical simulations predict that bars represent a very important agent for
triggering gas inflows, which in turn could lead to central star formation.
Bars thus are thought to contribute to the formation of the bulge.This changes
both, the gaseous-phase and stellar-phase metallicities in the centres of
galaxies. With the aim of quantifying the importance of this process we present
a comparative study of the gaseous-phase and stellar-phase metallicities in the
centres of members of a sample of barred and unbarred galaxies from SDSS. We do
not find a significant difference in the metallicity (neither gaseous nor
stellar) of barred and unbarred galaxies, but we find different trends in the
metallicities of early- and late- type galaxies, with larger differences in the
metallicity in the early-type subsample. Our results contradict some previous
research in this field, but we find a possible origin of the discrepancies
between previous works and our results. | astro-ph_GA |
Enhanced tidal disruption rates from massive black hole binaries: "Hard" massive black hole (MBH) binaries embedded in steep stellar cusps can
shrink via three-body slingshot interactions. We show that this process will
inevitably be accompanied by a burst of stellar tidal disruptions, at a rate
that can be several orders of magnitude larger than that appropriate for a
single MBH. Our numerical scattering experiments reveal that: 1) a significant
fraction of stars initially bound to the primary hole are scattered into its
tidal disruption loss cone by gravitational interactions with the secondary
hole, an enhancement effect that is more pronounced for very unequal-mass
binaries; 2) about 25% (40%) of all strongly interacting stars are tidally
disrupted by a MBH binary of mass ratio q=1/81 (q=1/243) and eccentricity 0.1;
and 3) two mechanisms dominate the fueling of the tidal disruption loss cone, a
Kozai non-resonant interaction that causes the secular evolution of the stellar
angular momentum in the field of the binary, and the effect of close encounters
with the secondary hole that change the stellar orbital parameters in a chaotic
way. For a hard MBH binary of 10^7 solar masses and mass ratio 0.01, embedded
in an isothermal stellar cusp of velocity dispersion sigma*=100 km/s, the tidal
disruption rate can be as large as 1/yr. This is 4 orders of magnitude higher
than estimated for a single MBH fed by two-body relaxation. When applied to the
case of a putative intermediate-mass black hole inspiraling onto Sgr A*, our
results predict tidal disruption rates ~0.05-0.1/yr. | astro-ph_GA |
A highly magnified gravitationally lensed red quasar at z = 2.5 with
significant flux anomaly: Uncovering a missing population: We present the discovery of a gravitationally lensed dust-reddened QSO at
$z=2.517$ discovered in a survey for red QSOs by infrared selection.
$Hubble~Space~Telescope$ imaging in the WFC3/IR F160W and F125W filters reveals
a quadruply lensed system in a cusp configuration. We find that compared to the
central image of the cusp, the nearby, brightest image is anomalous by a factor
of $\sim7-11$. Although the source is extremely bright in the mid-infrared, a
magnification by a factor of $\sim50-120$ places an upper limit of 1.35 mJy on
the intrinsic mid-infrared brightness, well below the $WISE~W4$ detection limit
of 6 mJy. We find that this QSO is moderately reddened, with $E(B-V)=0.7$ and
that $\sim1\%$ of the intrinsic spectrum is leaked back into the line of sight
resulting in an upturn in its UV spectrum. We conclude that the QSO's reddening
is intrinsic and not due to the lens. Consistent with previous red quasar
samples, this source exhibits outflows in its spectrum as well as morphological
properties suggestive of it being in a merger-driven transitional phase.
Depending on how $L_{\rm bol}$ is computed, the quasar's accretion rate may be
as high as $0.26~L_{\rm Edd}$. We detect two Lyman limit systems, at $z=2.102$
and $z=2.431$, with absorption by metal lines likely at small impact parameter
to the QSO, and a putative lens redshift of $z=0.599$. Given the rarity of quad
lenses, the discovery of this source allows detailed study of a less luminous,
more typical infrared-selected quasar at high redshift. | astro-ph_GA |
The Green Bank Ammonia Survey (GAS): First Results of NH3 mapping the
Gould Belt: We present an overview of the first data release (DR1) and first-look science
from the Green Bank Ammonia Survey (GAS). GAS is a Large Program at the Green
Bank Telescope to map all Gould Belt star-forming regions with $A_V \gtrsim 7$
mag visible from the northern hemisphere in emission from NH$_3$ and other key
molecular tracers. This first release includes the data for four regions in
Gould Belt clouds: B18 in Taurus, NGC 1333 in Perseus, L1688 in Ophiuchus, and
Orion A North in Orion. We compare the NH$_3$ emission to dust continuum
emission from Herschel, and find that the two tracers correspond closely.
NH$_3$ is present in over 60\% of lines-of-sight with $A_V \gtrsim 7$ mag in
three of the four DR1 regions, in agreement with expectations from previous
observations. The sole exception is B18, where NH$_3$ is detected toward ~ 40\%
of lines-of-sight with $A_V \gtrsim 7$ mag. Moreover, we find that the NH$_3$
emission is generally extended beyond the typical 0.1 pc length scales of dense
cores. We produce maps of the gas kinematics, temperature, and NH$_3$ column
densities through forward modeling of the hyperfine structure of the NH$_3$
(1,1) and (2,2) lines. We show that the NH$_3$ velocity dispersion,
${\sigma}_v$, and gas kinetic temperature, $T_K$, vary systematically between
the regions included in this release, with an increase in both the mean value
and spread of ${\sigma}_v$ and $T_K$ with increasing star formation activity.
The data presented in this paper are publicly available. | astro-ph_GA |
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