text: 14,165,1316321913132679168,1140222123006472194,Kasper Elm Heintz,"Happy to announce that our new paper: <LINK> was posted today on ArXiv! In this, we basically ask: can you efficiently identify quasars purely as point-like sources that doesn’t move on the sky? Turns out: Yes! As long as its far away from the Galactic plane This has been a rollercoaster of a project, starting with the release of @ESAGaia-DR2 more than 2 years ago, and with several non-succesful observing runs due to clouds, ice, etc. <LINK> When we *finally* got spectra of our targets, we have had several first-year students at @DAWNCopenhagen/@uni_copenhagen help us classify them as either stars ⭐️ or quasars 💥 (some many billion of lightyears away) — and for many of them, this was their first academic paper! Briefly, what we found was: it is possible to efficiently select quasars (~70% of the total number of targets) purely as sources with zero proper motions and parallaxes as measured by @ESAGaia-DR2. <LINK> This is, however, only feasible to reach at high Galactic latitudes, i.e. far away from the Galactic plane. Moving closer, we would be completely overwhelmed by stars that were apparently non-moving on the sky (at least not in the transverse direction). <LINK> This *complete* sample of quasars allowed us to examine the intrinsic properties of the quasar population, and specifically how large the fraction of “red” quasars is (anyway between 10% and 40% depending on the brightness limit). <LINK> In this survey we also discovered several very interesting quasars — here is one example that shines right through the disk of a nearby (z~0.02) galaxy! This allowed us to study the dust and metals in this disk, that was imposed on the background quasar spectrum. <LINK> Finally, we could also assess how complete other, more commonly used photometric quasar surveys were at similar brightness — turns out, they only manage to find 85-90% of the quasars we do! <LINK> This just cements the fact that this survey is the only way to select quasars, or extragalactic point-sources in general, without assumptions of their intrinsic emission (i.e. radio-loud, blue UV colors, etc) — so basically, quasars that don’t look like typical quasars! @dr_guangtou No apparently not! As far as I understand, extended sources will not trigger a detection on Gaia’s CCD — however, for bright AGN (and this spectacular case) Gaia will be sensitive enough to disentangle and accurately measure the central (or outlying) point-source.",https://arxiv.org/abs/2010.05934,"Here we explore the efficiency and fidelity of a purely astrometric selection of quasars as point sources with zero proper motions in the {\it Gaia} data release 2 (DR2). We have built a complete candidate sample including 104 Gaia-DR2 point sources brighter than $G<20$ mag within one degree of the north Galactic pole (NGP), all with proper motions consistent with zero within 2$\sigma$ uncertainty. In addition to pre-existing spectra, we have secured long-slit spectroscopy of all the remaining candidates and find that all 104 stationary point sources in the field can be classified as either quasars (63) or stars (41). The selection efficiency of the zero-proper-motion criterion at high Galactic latitudes is thus $\approx 60\%$. Based on this complete quasar sample we examine the basic properties of the underlying quasar population within the imposed limiting magnitude. We find that the surface density of quasars is 20 deg$^{-2}$, the redshift distribution peaks at $z\sim1.5$, and that only eight systems ($13^{+5}_{-3}\%$) show significant dust reddening. We then explore the selection efficiency of commonly used optical, near- and mid-infrared quasar identification techniques and find that they are all complete at the $85-90\%$ level compared to the astrometric selection. Finally, we discuss how the astrometric selection can be improved to an efficiency of $\approx70\%$ by including an additional cut requiring parallaxes of the candidates to be consistent with zero within 2$\sigma$. The selection efficiency will further increase with the release of future, more sensitive astrometric measurement from the Gaia mission. This type of selection, purely based on the astrometry of the quasar candidates, is unbiased in terms of colours and emission mechanisms of the quasars and thus provides the most complete census of the quasar population within the limiting magnitude of Gaia. ","Spectroscopic classification of a complete sample of