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

Abstract

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⊙. 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 MH2 = 8.3 ± 3.0 × 1010 M⊙, 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/μIR) M⊙ yr−1, which demonstrates the composite star-forming and AGN nature of this system.