Mid-IR Enhanced Galaxies in the Coma & Virgo Cluster: lenticulars with a high star formation rate

TL;DR

This study identifies Mid-Infrared Enhanced Galaxies (MIEGs) in the Coma and Virgo clusters, showing that some lenticular galaxies exhibit elevated star formation likely triggered by cluster infall, based on IR and optical data.

Contribution

It introduces the concept of MIEGs in galaxy clusters and links their high IR ratios to infall-induced star formation, providing new insights into galaxy evolution in dense environments.

Findings

Identification of 13 MIEGs with high L24/L_K ratios

MIEGs are spatially associated with infall regions in clusters

MIEGs show bluer optical colors indicating active star formation

Abstract

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 L_24/L_K distribution of ETGs in both Virgo and Coma clusters shows that some S0s have a much larger L_24/L_K 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/L_K ratio by looking at their spatial distribution within the cluster, by analyzing their optical spectra and by looking at their optical colors compared to late-types. We obtain 10 Coma and 3 Virgo early-type sources with larger L24/L_K ratios than the bulk of their population. We call these sources Mid-Infrared Enhanced Galaxies (MIEGs). In Coma, they are mostly located in the South-West part of the cluster where a substructure is falling onto the main cluster. MIEGs present lower g-r color than the rest of the ETG sample, because of a blue continuum. We interpret the excess L24/L_K ratio as evidence for an enhanced star-formation induced as a consequence of their infall into the main cluster.