ALMA evidence for ram pressure compression and stripping of molecular gas in the Virgo cluster galaxy NGC 4402

TL;DR

High-resolution ALMA observations of NGC 4402 reveal clear evidence of ram pressure effects on molecular gas, including compression, stripping, and distinct kinematic features, illustrating the impact of environmental interactions on galaxy evolution.

Contribution

This study provides detailed observational evidence of ram pressure effects on molecular gas in a galaxy, highlighting the processes of compression, stripping, and decoupling of GMCs in the Virgo cluster.

Findings

Detection of extraplanar molecular gas and dust plume with non-circular motions.

Identification of decoupled molecular clouds below the plume.

Evidence of ram pressure compression associated with star formation regions.

Abstract

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.