Resolving CO (2-1) in z~1.6 Gas-Rich Cluster Galaxies with ALMA:
  Rotating Molecular Gas Disks with Possible Signatures of Gas Stripping
  We present the first spatially-resolved observations of molecular gas in a
sample of cluster galaxies beyond z>0.1. Using ALMA, we detect CO (2-1) in 8
z~1.6 cluster galaxies, all within a single 70" primary beam, in under 3 hours
of integration time. The cluster, SpARCS-J0225, is replete with gas-rich
galaxies in close proximity. It thus affords an efficient multiplexing strategy
to build up the first sample of resolved CO in distant galaxy clusters. Mapping
out the kinematic structure and morphology of the molecular gas on 3.5 kpc
scales reveals rotating gas disks in the majority of the galaxies, as evidenced
by smooth velocity gradients. Detailed velocity maps also uncover kinematic
peculiarities, including a central gas void, a merger, and a few one-sided gas
tails. We compare the extent of the molecular gas component to that of the
optical stellar component, measured with rest-frame optical HST imaging. We
find that the cluster galaxies, while broadly consistent with a ratio of unity
for stellar-to-gas effective radii, have a moderately larger ratio compared to
the coeval field; this is consistent with the more pronounced trend in the
low-redshift Universe. Thus, at first glance, the z~1.6 cluster galaxies
generally look like galaxies infalling from the field, with typical
main-sequence star formation rates and massive molecular gas reservoirs
situated in rotating disks. However, there are potentially important
differences from their field counterparts, including elevated gas fractions,
slightly smaller CO disks, and possible asymmetric gas tails. Taken in tandem,
these signatures are tentative evidence for gas-stripping in the z~1.6 cluster.
However, the current sample size of spatially-resolved molecular gas in
galaxies at high redshift is small, and verification of these trends will
require much larger samples of both cluster and field galaxies.
