Unidentified quasars among stationary objects from Gaia DR2

Abstract

We here apply a novel technique selecting quasar candidates purely as sources with zero proper motions in the Gaia data release 2 (DR2). We demonstrate that this approach is highly efficient toward high Galactic latitudes with < 25% contamination from stellar sources. Such a selection technique offers a very pure sample completeness, since all cosmological point sources are selected regardless of their intrinsic spectral properties within the limiting magnitude of Gaia. We carry out a pilot-study by defining a sample compiled by including all Gaia-DR2 sources within one degree of the North Galactic Pole (NGP) selected to have proper motions consistent with zero within 2-sigma uncertainty. By cross-matching the sample to the optical Sloan Digital Sky Survey (SDSS) and the mid-infrared AllWISE photometric catalogues we investigate the colours of each of our sources. Together with already spectroscopically confirmed quasars we are therefore able to determine the efficiency of our selection. The majority of the zero proper motion sources have optical to mid-infrared colours consistent with known quasars. The remaining population may be contaminating stellar sources, but some may also be quasars with colours similar to stars. Spectroscopic follow-up of the zero proper motion sources is needed to unveil such a hitherto hidden quasar population. This approach has the potential to allow substantial progress on many important questions concerning quasars such as determining the fraction of dust-obscured quasars, the fraction of broad absorption line (BAL) quasars, and the metallicity distribution of damped Lyman-$\alpha$ absorbers. The technique could also potentially reveal new types of quasars or even new classes of cosmological point sources.

Submitted to A&A, comments welcome

References in corpus (8)

• Gaia Data Release 2. Summary of the contents and survey properties
• Gaia Data Release 2: The astrometric solution
• The Sloan Digital Sky Survey Quasar Catalog: Fourteenth Data Release
• Mining for Dust in Type 1 Quasars
• Optical/near-infrared selection of red QSOs: Evidence for steep extinction curves towards galactic centers?
• The High A(V) Quasar Survey: Reddened quasi-stellar objects selected from optical/near-infrared photometry - II
• The extended High A(V) Quasar Survey: Searching for dusty absorbers toward mid-infrared selected quasars
• A study of purely astrometric selection of extragalactic point sources with Gaia

Cited by in corpus (15)

• The Cosmic Baryon and Metal Cycles
• Gravitationally lensed quasars in Gaia: III. 22 new lensed quasars from Gaia Data Release 2
• Gaia Data Release 3: The extragalactic content
• Quaia, the Gaia-unWISE Quasar Catalog: An All-Sky Spectroscopic Quasar Sample
• Quasar and galaxy classification in Gaia Data Release 2
• On the discovery of stars, quasars, and galaxies in the Southern Hemisphere with S-PLUS DR2
• The effect of dust bias on the census of neutral gas and metals in the high-redshift Universe due to SDSS-II quasar colour selection
• Gaia-assisted selection of a quasar reddened by dust in an extremely-strong Damped Lyman-α Absorber at z=2.226
• Spectroscopic classification of a complete sample of astrometrically-selected quasar candidates using Gaia DR2
• Gaia-assisted discovery of a detached low-ionisation BAL quasar with very large ejection velocities
• Serendipitous discovery of a physical binary quasar at z=1.76
• Search for Quasar Pairs with $Gaia$ Astrometric Data. I. Method and Candidates
• Identifying Radio Active Galactic Nuclei with Machine Learning and Large-Area Surveys
• On the Dusty Proximate Damped Lyman-α System toward Q2310-3358 at z = 2.40
• 4MOST ChANGES: Catalog of high-redshift quasar candidates (4.5 < $z$ < 7) selected with SED fitting