CO, H2O, H2O+ line and dust emission in a z = 3.63 strongly lensed
  starburst merger at sub-kiloparsec scales
  Using ALMA, we report high angular-resolution observations of the redshift
z=3.63 galaxy, G09v1.97, one of the most luminous strongly lensed galaxies
discovered by the H-ATLAS survey. We present 0"2-0"4 resolution images of the
rest-frame 188 and 419$\mu$m dust continuum and the CO(6-5), H2O(211-202) and
J=2 H2O+ line emission. We also report the detection of H$_2^{18}$O in this
source. The dust continuum and molecular gas emission are resolved into a
nearly complete ~1"5 diameter Einstein ring plus a weaker image in the center,
which is caused by a special dual deflector lensing configuration. The observed
line profiles of the CO, H2O and H2O+ lines are strikingly similar. In the
source plane, we reconstruct the dust continuum images and the spectral cubes
of the line emission at sub-kpc scales. The reconstructed dust emission in the
source plane is dominated by a compact disk with an effective radius of 0.7kpc
plus an overlapping extended disk with a radius twice as large. While the
average magnification for the dust continuum is $\mu$~10-11, the magnification
of the line emission varies 5 to 22 across different velocity components. The
emission lines have similar spatial and kinematic distributions. The molecular
gas and dust content reveal that G09v1.97 is a gas-rich major merger in its
pre-coalescence phase. Both of the merging companions are intrinsically ULIRGs
with LIR reaching $\gtrsim 4\times10^{12}L_\odot$, and the total LIR of
G09v1.97 is $1.4\times10^{13}L_\odot$. The approaching southern galaxy shows no
obvious kinematic structure with a semi-major half-light radius a_s=0.4kpc,
while the receding galaxy resembles an a_s=1.2kpc rotating disk. The two
galaxies are separated by a projected distance of 1.3kpc, bridged by weak line
emission that is co-spatially located with the cold-dust-emission peak,
suggesting a large amount of cold ISM in the interacting region. (abridged)
