Spectroscopic follow-up of double quasar candidates

TL;DR

This study presents optical spectroscopic follow-up of four double quasar candidates from SDSS, identifying one as a likely lensed quasar, another as a physical quasar pair, and the remaining as projected active galactic nuclei pairs, providing insights into their nature.

Contribution

First spectroscopic confirmation and classification of four SDSS double quasar candidates, distinguishing between lensing, binary quasars, and projected pairs, with mass constraints on host galaxies.

Findings

SDSS J1617+3827 is a likely lensed quasar at z=2.079.

SDSS J2153+2732 is a physically associated quasar pair at z~2.24.

Two candidates are projected pairs of active galactic nuclei with weak lensing effects.

Abstract

We report the results of an optical spectroscopic follow-up of four double quasar candidates in the Sloan Digital Sky Survey (SDSS) database. SDSS J1617+3827 is most likely a lensed quasar at z = 2.079, consisting of two images with r ~ 19-21 and separated by ~ 2 arcsec. We identify an extended source northeast of the brightest image as an early-type lensing galaxy at z = 0.602, and detect a candidate for the main deflector in the vicinity of the faintest image. SDSS J2153+2732 consists of two distinct but physically associated quasars at z ~ 2.24 (r ~ 19-20, separation of 3.6 arcsec). Although this system might be a binary quasar, there is evidence of a collision or merger within a galaxy cluster at an early stage. The other two candidates are projected pairs of active galactic nuclei: SDSS J1642+3200 (separation of ~ 3 arcsec) comprises a distant quasar (r ~ 18) at z = 2.263 and the active nucleus (r > 20) of a galaxy at z ~ 0.3, while SDSS J0240-0208 (r ~ 18-19, separation of ~ 1 arcsec) is a pair of quasars at z = 1.687 and z = 1.059. In each of these two systems, the background quasar only suffers a weak gravitational lensing effect by the host galaxy of the foreground active nucleus, so the host galaxy mass is constrained to be less than (2.9-3) x 10^{11} solar masses inside 10 kpc.