Search for Quasar Pairs with ${\it Gaia}$ Astrometric Data. I. Method and Candidates

Abstract

Quasar pairs, a special subclass of galaxy pairs, are valuable in the investigation of quasar interaction, clustering, co-evolution between the two quasars' host galaxies, the growth of supermassive black holes, as well as the formation and evolution of galaxies. However, quasar pairs on kpc scales are observationally rare. The scarcity of available samples hindered the deeper exploration and statistics of these objects. In this work, we apply an astrometric method to systematically search for quasar candidates within a transverse distance of 100 kpc to known quasars in the Million Quasar Catalog. These candidates are \textit{Gaia} sources with proper motions and parallaxes that are consistent with zero. Visual inspection of the sample was performed to remove the contamination of crowded stellar fields and nearby galaxies. A total of 4\,112 quasar pair candidates were isolated, with a median member separation of 8.81{\arcsec}, a median \textit{Gaia} $G$-band magnitude of 20.52, and a median redshift of 1.61. Our catalog was compared with three major candidate quasar pair catalogs and identified 3\,984 new quasar pair candidates previously uncataloged in the three catalogs. Several interesting quasar pair candidates are highlighted and discussed. We also briefly discussed our quasar selection and several techniques for improving the success rate of quasar pair selection. Extensive spectroscopic follow-up campaigns are being carried out to validate their astrophysical nature.