HI Content of Group Galaxies from the FAST All Sky HI Survey

TL;DR

This study uses the FAST All Sky HI survey to analyze the atomic hydrogen content of group galaxies, revealing that HI depletion mainly affects satellites and central regions, with minimal difference from isolated galaxies.

Contribution

First comprehensive analysis of group galaxy HI content using uniform, deep survey data, distinguishing effects on satellites versus centrals and core versus outskirts.

Findings

Satellites are HI-poor compared to controls.

Centrals show no significant HI deficiency.

HI depletion is concentrated in group cores and dense systems.

Abstract

We investigate the atomic gas (HI) content of galaxies in groups using early data from the FAST All Sky HI survey (FASHI). Taking advantage of FAST's blind, wide-area coverage and uniform sensitivity, we assemble a sample of $230$ group galaxies belonging to $182$ groups at $z\leq0.03$. These groups were identified using a halo-based group finder, and they have an median membership of $4$ galaxies. We also derived a matched control sample of isolated systems, and apply censored-data modeling to include both detections and non-detections. At fixed stellar mass and color, we find that the global median HI fraction of group galaxies differs from that of controls by only $-0.04$ dex ($95\%$ CI [$-0.18,\ 0.16$]), indicating at most a mild average offset. The signal is not uniform across populations: satellites are HI-poor (median $\Delta f_{\mathrm{HI}}=-0.12$ dex), whereas centrals are not HI-deficient (median $\Delta f_{\mathrm{HI}}=0.13$ dex). Group galaxies located within $0.5R_{180}$ and in denser systems (richness $>10$ or local density $\Sigma>10\ \mathrm{gal\ Mpc^{-2}}$) show stronger negative offsets, whereas galaxies in the outskirts are statistically indistinguishable from the controls. These results refine earlier reports of global group HI deficiency: with deeper blind data and uniform treatment of upper limits, we show that HI depletion is primarily confined to satellites and compact cores rather than being ubiquitous across groups.