WALLABY Pilot Survey: HI gas disc truncation and star formation of galaxies falling into the Hydra I cluster

TL;DR

This study uses WALLABY HI observations of the Hydra I cluster to analyze how environment affects gas removal and star formation in galaxies, revealing outskirts gas stripping without full quenching.

Contribution

First wide-field HI survey of Hydra I cluster showing environment-driven gas removal primarily affecting galaxy outskirts without immediate star formation quenching.

Findings

Sharp decrease in HI-detected fraction at 1.5R200 from cluster center.

Evidence of gas removal from galaxy outskirts while inner star-forming regions remain active.

Outer disc star formation is reduced, indicating early stages of quenching.

Abstract

We present results from our analysis of the Hydra I cluster observed in neutral atomic hydrogen (HI) as part of the Widefield ASKAP L-band Legacy All-sky Blind Survey (WALLABY). These WALLABY observations cover a 60-square-degree field of view with uniform sensitivity and a spatial resolution of 30 arcsec. We use these wide-field observations to investigate the effect of galaxy environment on HI gas removal and star formation quenching by comparing the properties of cluster, infall and field galaxies extending up to $\sim5R_{200}$ from the cluster centre. We find a sharp decrease in the HI-detected fraction of infalling galaxies at a projected distance of $\sim1.5R_{200}$ from the cluster centre from $\sim0.85\%$ to $\sim0.35\%$. We see evidence for the environment removing gas from the outskirts of HI-detected cluster and infall galaxies through the decrease in the HI to $r$-band optical disc diameter ratio. These galaxies lie on the star forming main sequence, indicating that gas removal is not yet affecting the inner star-forming discs and is limited to the galaxy outskirts. Although we do not detect galaxies undergoing galaxy-wide quenching, we do observe a reduction in recent star formation in the outer disc of cluster galaxies, which is likely due to the smaller gas reservoirs present beyond the optical radius in these galaxies. Stacking of HI non-detections with HI masses below $M_{\rm{HI}}\lesssim10^{8.4}\,\rm{M}_{\odot}$ will be required to probe the HI of galaxies undergoing quenching at distances $\gtrsim60$ Mpc with WALLABY.