An HI+radio continuum study of local cluster galaxies

The evolution of galaxies in clusters is driven by their interaction with the environment. This deeply affects and alters the properties of the multiphase interstellar medium. We used MeerKAT observations to study the properties of the neutral and nonthermal interstellar medium, traced by the HI 21 cm line and by the radio continuum emission at 1.4 GHz, respectively, out to about twice the virial radius of three local (z ≃ 0.04) galaxy clusters, namely IIZW108, A4059, and A3558, with M 200 ≃ 2–9 × 10 14 M ⊙ . We assembled a sample of 61 galaxies, including the so-called jellyfish and unwinding galaxies, detected in HI and radio continuum, and derived their optical properties from IR-to-UV photometry using ancillary data. The cluster galaxies in our sample are ~2.7 times more star-forming on average than galaxies with the same stellar and HI mass in the field, indicating that the HI + radio-continuum selection has intercepted galaxies at the very early stages of their environmental processing when the external pressure has not yet removed the gas, but the resulting fast compression has enhanced the star formation and the radio continuum luminosity. Our study also reveals that galaxies in the initial stage can feature a radio luminosity excess because the old relativistic electrons permeate the interstellar medium. We show that unwinding galaxies are characterized by high stellar and HI masses, and we argue that their peculiar morphology might result from their high baryonic mass. Finally, via the stacking analysis of 677 optically selected cluster members, we quantitatively show that cluster galaxies are poorer in HI than in the field and that the deficiency steadily grows toward the cluster center, where the HI mass of galaxies is lower by a factor of about 3 than the HI mass of galaxies in the field.