ALMA Observations of Molecular Absorption in the Gravitational Lens PMN
  0134-0931
  We report the detection of molecular absorption lines at z=0.7645 towards the
radio-loud QSO PMN 0134-0931. The CO J=2-1 and HCO+ J=2-1 lines are seen in
absorption along two different lines of sight to lensed images of the
background QSO. The lines of sight are separated by ~0.7", corresponding to 5
kpc in the lens plane. PMN 0134-0931 represents one out of only five known
molecular absorption line systems at cosmologically significant distances.
Moreover, it is also one of three such systems where the absorption occurs in a
galaxy acting as a gravitational lens. The absorption lines through the two
lines of sight are shifted by 215+/-8 km/s, possibly representing rotational
motion in one of the lensing galaxies. The absorption profiles are wide, ~200
km/s, suggesting that the absorption occurs in a highly inclined disk galaxy
with a flat rotation curve and a cloud-cloud velocity dispersion ~30 km/s.
Gravitational lens models require two equal mass galaxies to account for the
observed configuration of lensed images. The presence of two galaxies in close
proximity means that they might be interacting and potentially merging and the
kinematics of the molecular gas may not reflect ordered rotational motion. The
column densities of both CO and HCO+ are normal for diffuse molecular gas
towards one of the lensed images, but significantly higher towards the other.
Also, the abundance ratio N(CO)/N(HCO+) is 2-3 times higher than in typical
diffuse molecular gas. It is plausible that the second line of sight probes
denser molecular gas than what is normally the case for absorption.
