H i Content of Group Galaxies from the FAST All Sky H i Survey

Abstract We investigate the atomic gas (H i ) content of galaxies in groups using early data from the Five-hundred-meter Aperture Spherical Radio Telescope (FAST) All Sky H i survey. 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 ≤ 0.03. These groups were identified using a halo-based group finder, and they have a median membership of four galaxies. We also derived a matched control sample of isolated systems, and apply censored-data modeling to include both detections and nondetections. At fixed stellar mass and color, we find that the global median H i 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 H i poor (median Δ f H I = −0.12 dex), whereas centrals are not H i deficient (median Δ f H I = 0.13 dex). Group galaxies located within 0.5 R 180 and in denser systems (richness > 10 or local density Σ > 10 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 H i deficiency: with deeper blind data and uniform treatment of upper limits, we show that H i depletion is primarily confined to satellites and compact cores rather than being ubiquitous across groups.