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

TL;DR

This work analyzes how environment in galaxy groups affects atomic gas content (HI) using the blind FASHI survey, cross-matched to a halo-based group catalog. A control sample of isolated galaxies is used to derive a HI-gas-fraction scaling relation from optical properties, incorporating non-detections via censored-data modeling. The main finding is a mild overall HI deficiency in group galaxies, with satellites showing stronger HI depletion and centrals sometimes enhanced, and the strongest effects in dense group cores. This refines the view of group HI content by showing that depletion is not ubiquitous but concentrated in specific POPULATIONS and environments, informing models of gas regulation and star formation in groups.

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 $Δf_{\mathrm{HI}}=-0.12$ dex), whereas centrals are not HI-deficient (median $Δf_{\mathrm{HI}}=0.13$ dex). Group galaxies located within $0.5R_{180}$ and in denser systems (richness $>10$ or local density $Σ>10\ \mathrm{gal\ Mpc^{-2}}$) 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.

Figures (8)

• Figure 1: Distributions of basic properties for isolated galaxies (purple) and group galaxies (black). Panel (a), (b), (c) and (d) correspond to stellar mass, redshift, color $g$-$r$ and the SFR, respectively.
• Figure 2: The number distribution of group richness for our selected groups.
• Figure 3: The posterior distributions for the H i gas fraction prediction: $\log f_{\mathrm{HI}}=m_1\log M_*+m_2(g-r)+b+\sigma_{\mathrm{HI}}$. The blue lines show the mean values for the parameters. The contours in the off-diagonal panels indicate the $68$ percent, $95$ percent, and $98$ percent confidence levels, respectively.
• Figure 4: Predicted versus measured H i gas fraction in control galaxies. The blue points represent galaxies detected in H i, while the orange triangles indicate non-detections. The dashed line shows the one-to-one relation, and the shaded region denotes the $1\sigma$ intrinsic scatter. The censored data points (orange points) only serve a constraining and regularizing role. As a result, they fall below the model.
• Figure 5: Distribution of $\Delta f_{\mathrm{HI}}$ in full sample (a) and H i-detections (b). The red and black columns indicate the control galaxies and the group galaxies, respectively. The red and black dashed lines in each panel indicate the median values of the control and group galaxies, respectively. The region between two dotted lines in each panel is $95\%$ confidence interval, shown in red for the control galaxies and black for the group galaxies.