ALMA view of RX J1131-1231: Sub-kpc CO (2-1) mapping of a molecular disk in a lensed star-forming quasar host galaxy

We present ALMA 2-mm continuum and CO (2-1) spectral line imaging of the gravitationally lensed z = 0.654 star-forming/quasar composite RX J1131-1231 at 240-400 mas angular resolution. The continuum emission is found to be compact and coincident with the optical emission, whereas the molecular gas forms a complete Einstein ring, which shows strong differential magnification. The de-lensed source structure is determined on 400-parsec-scales resolution using a Bayesian pixelated visibility-fitting lens modelling technique. The reconstructed molecular gas velocity-field is consistent with a large rotating disk with a major-Axis FWHM ~9.4 kpc at an inclination angle of i = 54° and with a maximum rotational velocity of 280 km s^-1. From dynamical model fitting we find an enclosed mass within 5 kpc of M(r < 5 kpc) = (1.46 ± 0.31) × 10¹¹ M_âŠ. The molecular gas distribution is highly structured, with clumps that are co-incident with higher gas velocity dispersion regions (40-50 km s^-1) and with the intensity peaks in the optical emission, which are associated with sites of on-going turbulent star-formation. The peak in the CO (2-1) distribution is not co-incident with the AGN, where there is a paucity of molecular gas emission, possibly due to radiative feedback from the central engine. The intrinsic molecular gas luminosity is L′_CO = 1.2 ± 0.3 × 10¹⁰ K km s^-1 pc² and the inferred gas mass is M_H2 = 8.3 ± 3.0 × 10¹⁰ M_âŠ, which given the dynamical mass of the system is consistent with a CO-H₂ conversion factor of α = 5.5 ± 2.0 M_⊠(K km s^-1 pc²)^-1. This suggests that the star-formation efficiency is dependent on the host galaxy morphology as opposed to the nature of the AGN. The far-infrared continuum spectral energy distribution shows evidence for heated dust, equivalent to an obscured star-formation rate of SFR = 69_-25⁺⁴¹ × (7.3/μ_IR) M_⊠yr^-1, which demonstrates the composite star-forming and AGN nature of this system.