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

peer reviewedWe 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[SUP]-1[/SUP]. From dynamical model fitting we find an enclosed mass within 5 kpc of M(r < 5 kpc) = (1.46 ± 0.31) × 10[SUP]11[/SUP] M[SUB]⊙[/SUB]. The molecular gas distribution is highly structured, with clumps that are co-incident with higher gas velocity dispersion regions (40-50 km s[SUP]-1[/SUP]) 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'[SUB]CO[/SUB] = 1.2 ± 0.3 × 10[SUP]10[/SUP] K km s[SUP]-1[/SUP] pc[SUP]2[/SUP] and the inferred gas mass is M[SUB]H[SUB]2[/SUB][/SUB] = 8.3 ± 3.0 × 10[SUP]10[/SUP] M[SUB]⊙[/SUB], which given the dynamical mass of the system is consistent with a CO-H[SUB]2[/SUB] conversion factor of α = 5.5 ± 2.0 M[SUB]⊙[/SUB] (K km s[SUP]-1[/SUP] pc[SUP]2[/SUP])[SUP]-1[/SUP]. 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[SUB]-25[/SUB][SUP]+41[/SUP] × (7.3/μ[SUB]IR[/SUB]) M[SUB]⊙[/SUB] yr[SUP]-1[/SUP], which demonstrates the composite star-forming and AGN nature of this system