Quaia, the Gaia-unWISE Quasar catalog

The catalog draws on the 6,649,162 quasar candidates identified by the Gaia mission that have redshift estimates from the space observatory's low-resolution blue photometer/red photometer spectra. This initial sample is highly homogeneous and complete, but has low purity, and 18% of even the bright (G<20.0) confirmed quasars have discrepant redshift estimates (|{Delta}z/(1+z)|>0.2) compared to those from the Sloan Digital Sky Survey (SDSS).

In this work, we combine the Gaia candidates with unWISE infrared data (based on the Wide-field Infrared Survey Explorer survey) to construct a catalog useful for cosmological and astrophysical quasar studies. We apply cuts based on proper motions and colors, reducing the number of contaminants by approximately four times. We improve the redshifts by training a k-Nearest Neighbor model on SDSS redshifts, and achieve estimates on the G<20.0 sample with only 6% (10%) catastrophic errors with |{Delta}z/(1+z)|>0.2 (0.1), a reduction of approximately three times (approximately two times) compared to the Gaia redshifts.