Galactic-scale Quasar Pairs from the Sloan Digital Sky Survey and Gaia DR3

Abstract We perform a systematic search for galactic-scale quasar pairs and small-scale (&lt;3″) lenses using the Sloan Digital Sky Survey (SDSS) DR16 quasar catalog and Gaia DR3, and identify 136 double quasar candidates from 1120 Gaia-resolved pairs after a magnitude and redshift cut of G &lt; 20.5 and z &gt; 0.5 (803 double quasars out of 2497 pairs without any cuts) with separations of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>∼</mml:mo> <mml:mn>0</mml:mn> <mml:mover accent="true"> <mml:mrow> <mml:mi>.</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>″</mml:mi> </mml:mrow> </mml:mover> <mml:mn>3</mml:mn> <mml:mspace width="0.1em"/> <mml:mtext>–</mml:mtext> <mml:mspace width="0.1em"/> <mml:mn>3</mml:mn> <mml:mi>″</mml:mi> <mml:mtext/> <mml:mspace width="0.1em"/> <mml:mtext/> <mml:mspace width="0.1em"/> </mml:math> , corresponding to projected physical separations of ∼3–30 kpc at the median redshift z = 1.7 for the sample. Candidate unobscured double quasars are selected as Gaia-resolved pairs around spectroscopically confirmed SDSS quasars ( L bol &gt; 10 44.5 erg s −1 ) at 0.5 &lt; z ≲ 4.5. Gaia astrometry and SDSS spectral decomposition are used to exclude foreground star superpositions, which dominate (≳80% of) the initial Gaia-resolved pair sample. Correcting pair-resolving completeness, we measure an overall double quasar (lens and physical pairs combined) fraction of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>5</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>7</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.3</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>×</mml:mo> <mml:mn>1</mml:mn> <mml:msup> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>4</mml:mn> </mml:mrow> </mml:msup> </mml:math> (bootstrapping errors). The double quasar fraction measured in individual separation bins increases toward smaller separations, consistent with earlier findings. The dual quasar fraction also shows little redshift evolution for the luminous SDSS sample, in agreement with previous observations and simulation. However, the observed fraction is lower than simulation predictions by ∼0.8–1.6 dex, suggesting a significant population of obscured quasar pairs are missed in our search. Future wide-area space missions targeting both unobscured and obscured quasar pairs at subarcsecond resolutions will reveal the population of obscured quasar pairs and extend to much lower luminosities of active galactic nuclei.