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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 $zsim1.5$, 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.
Context. Strong gravitationally lensed quasars are among the most interesting and useful observable extragalactic phenomena. Because their study constitutes a unique tool in various fields of astronomy, they are highly sought, not without difficulty.
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The combination of the final version of the RAVE spectroscopic survey data release 6 with radial velocities and astrometry from Gaia DR2 allows us to identify and create a catalog of single lined binary star candidates (SB1), their inferred orbital p
We construct a supervised classifier based on Gaussian Mixture Models to probabilistically classify objects in Gaia data release 2 (GDR2) using only photometric and astrometric data in that release. The model is trained empirically to classify object
In Ostdiek et al. (2019), we developed a deep neural network classifier that only relies on phase-space information to obtain a catalog of accreted stars based on the second data release of Gaia (DR2). In this paper, we apply two clustering algorithm