Revealing the complex nature of the strong gravitationally lensed system
  H-ATLAS J090311.6+003906 using ALMA
  We have modelled Atacama Large Millimeter/sub-millimeter Array (ALMA) long
baseline imaging of the strong gravitational lens system H-ATLAS
J090311.6+003906 (SDP.81). We have reconstructed the distribution of band 6 and
7 continuum emission in the z=3.042 source and we have determined its kinematic
properties by reconstructing CO(5-4) and CO(8-7) line emission in bands 4 and
6. The continuum imaging reveals a highly non-uniform distribution of dust with
clumps on scales of 200 pc. In contrast, the CO line emission shows a
relatively smooth, disk-like velocity field which is well fit by a rotating
disk model with an inclination angle of 40+/-5 degrees and an asymptotic
rotation velocity of 320 km/s. The inferred dynamical mass within 1.5kpc is
(3.5+/-0.5)x10^{10} M_sol which is comparable to the total molecular gas masses
of (2.7+/-0.5)x10^{10} M_sol and (3.5+/-0.6)x10^{10} M_sol from the dust
continuum emission and CO emission respectively. Our new reconstruction of the
lensed HST near-infrared emission shows two objects which appear to be
interacting, with the rotating disk of gas and dust revealed by ALMA distinctly
offset from the near-infrared emission. The clumpy nature of the dust and a low
value of the Toomre parameter of Q=0.3 suggest that the disk is in a state of
collapse. We estimate a star formation rate in the disk of 470+/-80 M_sol/yr
with an efficiency 65 times greater than typical low-redshift galaxies. Our
findings add to the growing body of evidence that the most infra-red luminous,
dust obscured galaxies in the high redshift Universe represent a population of
merger induced starbursts.
