Unidentified quasars among stationary objects from Gaia DR2

TL;DR

This paper introduces a new method for identifying quasars by selecting sources with zero proper motion in Gaia DR2 data, achieving high purity at high Galactic latitudes and enabling the discovery of potentially hidden quasar populations.

Contribution

The study presents a novel zero proper motion selection technique for quasars using Gaia DR2, demonstrating its efficiency and potential to uncover previously hidden quasar populations.

Findings

High efficiency of zero proper motion selection at high Galactic latitudes.

Most selected sources have colours consistent with known quasars.

Spectroscopic follow-up is needed to confirm and explore the hidden quasar population.

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.