2018A&A...613A..34P

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∼9.4kpc at an inclination angle of i=54° and with a maximum rotational velocity of 280km/s. From dynamical model fitting we find an enclosed mass within 5kpc of M(r<5kpc)=(1.46±0.31)x10¹¹M_☉. The molecular gas distribution is highly structured, with clumps that are co-incident with higher gas velocity dispersion regions (40-50km/s) 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±0.3x10¹⁰K.(km/s).pc² and the inferred gas mass is M_H2=8.3±3.0x10¹⁰M_☉, which given the dynamical mass of the system is consistent with a CO-H₂ conversion factor of α=5.5±2.0M_☉/(K.(km/s).pc²). 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⁺⁴¹x(7.3/µ_IR)M_☉/yr, which demonstrates the composite star-forming and AGN nature of this system.