The effects of the environment on the central activity of galaxies as derived using mid-IR tracers

Aims. By exploiting photometry from the Wide-field Infrared Survey Explorer (WISE), we investigate the influence of environment and stellar mass on the prevalence of different excitation types in the interstellar gas of the central regions of galaxies in the Herschel Reference Survey (HRS, z ≈ 0.0047). We extend this analysis to a complementary sample of relatively nearby galaxies that are undergoing ram-pressure stripping (RPS, z ≈ 0.0195). Our goal is to assess whether a connection exists between active galactic nucleus (AGN) activity and either the cluster environment or the ram pressure stripping process. Methods. We computed WISE mid-infrared colour indices from fluxes extracted from central apertures and applied two established mid-infrared diagnostic diagrams to distinguish AGN activity from non-AGN, and star-forming galaxy (SFG) excitation from that associated with ‘retired’ galaxies. The resulting types of excitation were then used in conjunction with stellar mass and environment classifications to construct a stellar mass-excitation fraction relation. Results. The stellar mass-excitation fraction relation reveals that global stellar mass is the primary driver of excitation diversity in galaxy centres. In increasing order of prevalence, excitation types follow a sequence from SFGs to retired galaxies with increasing mass. The number of low-mass galaxies is too small to drive statistical tendencies. In contrast, SFG excitation becomes dominant at intermediate masses. At the highest mass end, the retired-galaxy type clearly prevails. Across the full mass range and in nearly all environments, SFG excitation is the most common, except for H I -gas deficient HRS galaxies, which are mostly retired. The excitation properties of galaxies undergoing RPS resemble those of HRS cluster members, field galaxies, and normal H I -content galaxies, minimising the environmental role. Contrary to previous results, we do not see any increase in AGN activity in H I -deficient cluster galaxies or in those undergoing RPS, since its fractions (∼10% for Seyfert 2 and ∼20% for low-ionisation nuclear emission-line region, LINER) remain largely unaffected along all environments. Conclusions. These findings indicate that while rich environments are associated with certain excitation types, stellar mass remains the primary driver of excitation diversity in galaxy centres.