MID-IR ENHANCED GALAXIES in the COMA & VIRGO CLUSTERS: LENTICULARS with A HIGH STAR FORMATION RATE

We explore the properties of early-type galaxies (ETGs), including ellipticals (E) and lenticulars (S0), in rich environments, such as clusters of galaxies (Virgo and Coma). The L24/LK distribution of ETGs in both Virgo and Coma clusters shows that some S0s have a much larger L24/LK ratio (0.5 to ∼2 dex) than the bulk of the ETG population. This could be interpreted as an enhanced star formation rate in these lenticulars. We compare the optical colors of galaxies in these two clusters and investigate the nature of these sources with a large L24/LK ratio by looking at their spatial distribution within the cluster, analyzing their optical spectra, and looking at their optical colors compared to late-types. We obtain 10 Coma and 3 Virgo early-type sources with larger L24/LK ratios than the bulk of their population. We call these sources mid-infrared enhanced galaxies (MIEGs). In Coma, they are mostly located in the southwest part of the cluster where a substructure is falling onto the main cluster. MIEGs present a lower color than the rest of the ETG sample because of a blue continuum. We interpret the excess L24/LK ratio as evidence for enhanced star formation induced as a consequence of their infall into the main cluster.