Molecular gas filaments and fallback in the ram pressure stripped Coma
  spiral NGC 4921
  We investigate the effects of ram pressure on the molecular ISM in the disk
of the Coma cluster galaxy NGC 4921, via high resolution CO observations. We
present 6" resolution CARMA CO(1-0) observations of the full disk, and 0.4"
resolution ALMA CO(2-1) observations of the leading quadrant, where ram
pressure is strongest. We find evidence for compression of the dense
interstellar medium (ISM) on the leading side, spatially correlated with
intense star formation activity in this zone. We also detect molecular gas
along kiloparsec-scale filaments of dust extending into the otherwise gas
stripped zone of the galaxy, seen in HST images. We find the filaments are
connected kinematically as well as spatially to the main gas ridge located
downstream, consistent with cloud decoupling inhibited by magnetic binding, and
inconsistent with a simulated filament formed via simple ablation. Furthermore,
we find several clouds of molecular gas $\sim 1-3$ kpc beyond the main ring of
CO that have velocities which are blueshifted by up to 50 km s$^{-1}$ with
respect to the rotation curve of the galaxy. These are some of the only clouds
we detect that do not have any visible dust extinction associated with them,
suggesting that they are located behind the galaxy disk midplane and are
falling back towards the galaxy. Simulations have long predicted that some gas
removed from the galaxy disk will fall back during ram pressure stripping. This
may be the first clear observational evidence of gas re-accretion in a ram
pressure stripped galaxy.
