Molecular Gas Kinematics and Star Formation Properties of the Strongly-lensed Quasar Host Galaxy RXS J1131-1231

We report observations of CO(J = 2 → 1) and line emission toward the quadruply-lensed quasar RXS J1131−1231 at z = 0.654 obtained using the Plateau de Bure Interferometer (PdBI) and the Combined Array for Research in Millimeter-wave Astronomy (CARMA). Our lens modeling shows that the asymmetry in the double-horned CO(J = 2 → 1) line profile is mainly a result of differential lensing, where the magnification factor varies from ∼3 to ∼9 across different kinematic components. The intrinsically symmetric line profile and a smooth source-plane velocity gradient suggest that the host galaxy is an extended rotating disk, with a CO size of kpc and a dynamical mass of M . We also find a secondary CO-emitting source near RXS J1131−1231, the location of which is consistent with the optically-faint companion reported in previous studies. The lensing-corrected molecular gas masses are Mgas = (1.4 0.3) × 1010 M and (2.0 0.1) × 109 M for RXS J1131−1231 and the companion, respectively. We find a lensing-corrected stellar mass of M* = (3 1) × 1010 M and a star formation rate of SFRFIR = (120 63) M yr−1, corresponding to a specific SFR and star formation efficiency comparable to z ∼ 1 disk galaxies not hosting quasars. The implied gas mass fraction of ∼18 4% is consistent with the previously observed cosmic decline since z ∼ 2. We thus find no evidence for quenching of star formation in RXS J1131−1231. This agrees with our finding of an elevated ratio of >0.27 % compared to the local value, suggesting that the bulk of its black hole mass is largely in place while its stellar bulge is still assembling.