ALMA imaging of SDP.81 – II. A pixelated reconstruction of the CO emission lines

Abstract

Abstract We present a sub-100 pc-scale analysis of the CO molecular gas emission and kinematics of the gravitational lens system SDP.81 at redshift 3.042 using Atacama Large Millimetre/submillimetre Array (ALMA) science verification data and a visibility-plane lens reconstruction technique. We find clear evidence for an excitation-dependent structure in the unlensed molecular gas distribution, with emission in CO (5–4) being significantly more diffuse and structured than in CO (8–7). The intrinsic line luminosity ratio is r8–7/5−4 = 0.30 ± 0.04, which is consistent with other low-excitation starbursts at z ∼ 3. An analysis of the velocity fields shows evidence for a star-forming disc with multiple velocity components that is consistent with a merger/post-coalescence merger scenario, and a dynamical mass of M(<1.56 kpc) = 1.6 ± 0.6 × 1010 M⊙. Source reconstructions from ALMA and the Hubble Space Telescope show that the stellar component is offset from the molecular gas and dust components. Together with Karl G. Jansky Very Large Array CO (1–0) data, they provide corroborative evidence for a complex ∼2 kpc-scale starburst that is embedded within a larger ∼15 kpc structure.