The preferentially magnified active nucleus in IRAS F10214+4724 - II. Spatially resolved cold molecular gas

We present Jansky Very Large Array observations of the cold (CO (1→0)) molecular gas in IRAS F10214+4724, a lensed ultraluminous infraRed galaxy (ULIRG) at z = 2.3 with an obscured active nucleus. The galaxy is spatially and spectrally well resolved in the CO (1→0) emission line. The total intensity and velocity maps reveal a reasonably ordered system; however, there is some evidence for minor merger activity. A CO (1→0) counter-image is detected at the 3σ level. Five of the 42 km s−1 channels (with >5σ detections) are mapped back into the source plane and their total magnification posterior probability distribution functions are sampled. This reveals a roughly linear arrangement, tentatively a rotating disc. We derive a molecular gas mass of M gas = 1.2 ± 0.2 × 1010 M, assuming a ULIRG L CO-to-M gas conversion ratio of α = 0.8 M (K km s−1 pc2)−1 that agrees well with the derived range of α = 0.3-1.3 M (K km s−1 pc2)−1 for separate dynamical mass estimates at assumed inclinations of i = 90°-30°. The lens modelling and CO (1→0) spectrum asymmetry suggest that there may be substantial (factor ∼2) preferential lensing of certain individual channels; however, the CO (1→0) spatially integrated channel flux uncertainties limit the significance of this result. Based on the AGN and CO (1→0) peak emission positions and the lens model, we predict a distortion of the CO spectral line energy distribution where higher order J lines that may be partially excited by AGN heating will be preferentially lensed owing to their smaller solid angles and closer proximity to the AGN and therefore the cusp of the caustic. Comparison with other lensing inversion results shows that the narrow-line region and AGN radio core in IRAS F10214+4724 are preferentially lensed by a factor of 3 and 11, respectively, relative to the molecular gas emission. This distorts the global continuum emission spectral energy distribution and strongly suggests caution in unsophisticated uses of IRAS F10214+4724 as an archetype high-redshift ULIRG. We explore two large velocity gradient models, incorporating spatial CO (1→0) and CO (3→2) information and present tentative evidence for an extended, low-excitation, cold gas component that implies that the total molecular gas mass in IRAS F10214+4724 is a factor of 2 greater than that calculated using spatially unresolved CO observations.