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

Abstract

We report observations of CO(J = 2 → 1) and {CO}(J=3\to 2) 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 {R}_{CO}∼ 6 kpc and a dynamical mass of {M}_{dyn}∼ 8× {10}¹⁰ 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 M _gas = (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 SFR_FIR = (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 {M}_{BH}/{M}_{bulge} ratio of >0.27{}_-0.08^+0.11% 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.