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

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