LoTSS Jellyfish Galaxies IV: Enhanced Star Formation on the Leading Half of Cluster Galaxies and Gas Compression in IC3949

TL;DR

This study provides observational evidence that ram pressure stripping in cluster galaxies enhances star formation on the leading half, linked to gas compression, supported by resolved spectroscopy and molecular gas analysis.

Contribution

First observational confirmation that ram pressure induces star formation enhancement on the leading side of jellyfish galaxies, supported by multi-wavelength data and case study of IC3949.

Findings

Enhanced star formation observed on the leading half of galaxies

Ram pressure correlates with star formation enhancement and gas compression

Molecular gas analysis shows localized star formation and gas depletion times

Abstract

With MaNGA integral field spectroscopy, we present a resolved analysis of star formation for 29 jellyfish galaxies in nearby clusters, identified from radio continuum imaging taken by the Low Frequency Array. Simulations predict enhanced star formation on the "leading half" of galaxies undergoing ram pressure stripping, and in this work we report observational evidence for this elevated star formation. The dividing line (through the galaxy center) that maximizes this star formation enhancement is systematically tied to the observed direction of the ram pressure stripped tail, suggesting a physical connection between ram pressure and this star formation enhancement. We also present a case study on the distribution of molecular gas in one jellyfish galaxy from our sample, IC3949, using ALMA CO J=1-0, HCN J=1-0, and HCO$^+$ J=1-0 observations from the ALMaQUEST survey. The $\mathrm{H_2}$ depletion time (as traced by CO) in IC3949 ranges from $\sim\!1\,\mathrm{Gyr}$ in the outskirts of the molecular gas disk to $\sim\!11\,\mathrm{Gyr}$ near the galaxy center. IC3949 shows a clear region of enhanced star formation on the leading half of the galaxy where the average depletion time is $\sim\!2.7\,\mathrm{Gyr}$, in line with the median value for the galaxy on the whole. Dense gas tracers, HCN and HCO$^+$, are only detected at the galaxy center and on the leading half of IC3949. Our results favour a scenario in which ram pressure compresses the interstellar medium, promoting the formation of molecular gas that in turn fuels a localized increase of star formation.