ALMA evidence for ram pressure compression and stripping of molecular
  gas in the Virgo cluster galaxy NGC 4402
  High resolution (1'' $\times$ 2'') ALMA CO(2-1) observations of the ram
pressure stripped galaxy NGC 4402 in the Virgo cluster show some of the
clearest evidence yet for the impacts of ram pressure on the molecular ISM of a
galaxy. The eastern side of the galaxy at $r \sim 4.5$ kpc, upon which ram
pressure is incident, has a large (width $\sim$1 kpc, height $\sim$1 kpc above
the disk midplane) extraplanar plume of molecular gas and dust. Molecular gas
in the plume region shows distinct non-circular motions in the direction of the
ram pressure; the kinematic offset of up to 60 km s$^{-1}$ is consistent with
acceleration by ram pressure. We also detect a small amount of gas in clouds
below the plume that are spatially and kinematically distinct from the
surrounding medium, and appear to be decoupled from the stripped ISM. We
propose that diffuse molecular gas is directly stripped but GMC density gas is
not directly stripped, and so decouples from lower density stripped gas.
However, GMCs become effectively stripped on short timescales. We also find
morphological and kinematic signatures of ram pressure compression of molecular
gas in a region of intense star formation on the leading side at $r \sim 3.5$
kpc. We propose that the compressed and stripped zones represent different
evolutionary stages of the ram pressure interaction, and that feedback from
star formation in the compressed zone facilitates the effective stripping of
GMCs by making the gas cycle rapidly to a lower density diffuse state.
