Molecular gas in the disks and tails of ram pressure stripped Coma galaxies

Abstract

We present new IRAM 30 m observations of CO(1-0) and CO(2-1) emission in five Coma cluster galaxies undergoing ram pressure stripping. Molecular gas is detected in four galaxies, and CO emission is clearly detected in the stripped tails of three, extending up to $\sim50$ kpc from the stellar disks. The molecular gas masses in the tails range from a few $\times10^8\,M_\odot$ and account for up to 10-20% of the molecular gas remaining in the parent disks. Compared with field scaling relations, several galaxies exhibit elevated molecular-to-stellar gas ratios despite being H I deficient. This enhancement persists toward the lower-mass end of our sample, even in the massive Coma cluster environment, consistent with ram-pressure-induced compression of atomic gas and enhanced H I-to-H$_2$ conversion in the disks. At the same time, the total gas fractions of most galaxies remain close to those of field galaxies with similar stellar masses when the molecular gas in the tails is included, indicating that gas removal and phase transformation act simultaneously. We further investigate the relation between molecular and ionized gas by comparing CO and H$α$ emission in the stripped regions. Although CO is preferentially detected in H$α$-bright regions, the increased scatter compared with previous studies suggests the presence of both star-forming and diffuse CO components in the tails. Our results highlight the dual role of ram pressure stripping in dense environments: it efficiently removes gas from galaxies while also promoting molecular gas formation in both the disks and stripped material, thereby shaping short-term star formation in cluster galaxies.