ALMA evidence for the direct 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 galaxies. 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$2 kpc) extraplanar plume of gas and dust, first seen in HST observations of dust extinction. Molecular gas in the plume region shows distinct non-circular motions as large as $\sim$80 km s$^{-1}$ in the direction of the ram pressure, a kinematic offset greater than any other feature in the galaxy, indicating 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. We propose that diffuse molecular gas is directly stripped and not GMC density gas, but that gas in GMC's becomes effectively stripped on short timescales because GMC lifetimes are shorter than the stripping timescale. We also find morphological and kinematic signs of ram pressure compression of molecular gas around 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 GMC's by making the gas cycle rapidly to a lower density diffuse state.