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-pc resolution using a
visibility-fitting lens modelling technique. The reconstructed molecular gas
velocity-field is consistent with a rotating disk with a maximum rotational
velocity of 280 km/s. From dynamical model fitting we find an enclosed mass
M(r<5 kpc)=(1.46+/-0.31)*10^11 M_sol. The molecular gas distribution is highly
structured, with clumps that are co-incident with higher gas velocity
dispersion regions 40-50 km/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+/-0.3)*10^10 K km/s pc^2 and the inferred gas mass is
M(H2)=(8.3+/-3.0)*10^10 M_sol, which given its dynamical mass is consistent
with a CO-H2 conversion factor of alpha = 5.5+/-2.0 M_solar(K km/s pc^2)^-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^(+41)_(-25)*(7.3/u_IR)M_sol/yr, which demonstrates the composite
star-forming/AGN nature of this system.
  RX J1131-1231
