Abstract
We present the first mid-IR study of galaxy groups in the nearby Universe based on Spitzer MIPS observations of a sample of nine redshift-selected groups from the XMM-IMACS (XI) project, at z=0.06. We find that on average the star-forming (SF) galaxy fraction in the groups is about 30% lower than the value in the field and 30% higher than in clusters. The SF fractions do not show any systematic dependence on group velocity dispersion, total stellar mass, or the presence of an X-ray emitting intragroup medium, but a weak anti-correlation is seen between SF fraction and projected galaxy density. However, even in the densest regions, the SF fraction in groups is still higher than that in cluster outskirts, suggesting that preprocessing of galaxies in group environments is not sufficient to explain the much lower SF fraction in clusters. The typical specific star formation rates (SFR/M*) of SF galaxies in groups are similar to those in the field across a wide range of stellar mass (M*>10^9.6 msun), favoring a quickly acting mechanism that suppresses star formation to explain the overall smaller fraction of SF galaxies in groups. If galaxy-galaxy interactions are responsible, then the extremely low starburst galaxy fraction (<1%) implies a short timescale (~0.1 Gyr) for any merger-induced starburst stage. Comparison to two rich clusters shows that clusters contain a population of massive SF galaxies with very low SFR (14% of all the galaxies with M*>10^10 Msun), possibly as a consequence of ram pressure stripping being less efficient in removing gas from more massive galaxies.
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