Neutral Hydrogen in the Shapley Supercluster Core I: Environmental Effects on Gas Content and Galaxy Evolution

Abstract We study the atomic Hydrogen (Hi) content of galaxies in the core of the Shapley Supercluster (SSC) at z ∼0.048, using observations from the MeerKAT Galaxy Cluster Legacy Survey and optical data from the Shapley Supercluster Survey (ShaSS) project. Our sample comprises 169 galaxies with Hi detections in the dynamically active region of Abell 3558 and SC1329-313. Following the literature, we classify galaxies into star-forming main sequence (SFMS), transition (TZ), and red sequence (RS) populations, and examine how the Hi content varies across these populations. Galaxies on the SFMS exhibit an average Hi gas fraction offset of 0.038 dex from the gas fraction main sequence, while TZ and RS populations show depleted Hi fractions of −0.034 and −0.211 dex. Hi depletion timescales span from 6 to 170 Gyr (SFMS-TZ-RS) confirming increasingly inefficient star formation with quenching. Scaling relations between Hi mass and stellar mass in the SSC are generally consistent with field samples. The most direct signature of the dense environment of the SSC is the marked predominance of TZ galaxies, in contrast to what is observed in the field-dominated sample of xGASS, where the population is mostly composed of SFMS galaxies. Moreover, the SFMS and RS populations have similar size, again in contrast with field populations. These results suggest that galaxies in the SSC are undergoing environmental quenching through starvation or strangulation, rather than rapid gas stripping. Despite detectable Hi reservoirs, many galaxies exhibit long depletion times, indicating reduced gas accretion and inefficient star formation.