Mid-infrared emission from E+A galaxies in the coma cluster

We have used ISO to observe at 12 μm seven E+A galaxies plus an additional emission-line galaxy, all in the Coma Cluster. E+A galaxies lacking narrow emission lines have 2.2-12 μmi flux density ratios or limits similar to old stellar populations (typical of early-type galaxies). Only galaxies with emission lines have enhanced 12 μm flux density. Excess 12 μm emission is therefore correlated with the presence of ongoing star formation or an active galactic nucleus (AGN). From the mid- and far-infrared colors of the brightest galaxy in our sample, which was detected at longer wavelengths with IRAS, we estimate the far-infrared luminosity of these galaxies. By comparing the current star formation rates with previous rates estimated from the Balmer absorption features, we divide the galaxies into two groups: those for which star formation has declined significantly following a dramatic peak ∼1 Gyr ago; and those with a significant level of ongoing star formation or/and an AGN. There is no strong difference in the spatial distribution on the sky between these two groups. However, the first group has systemic velocities above the mean cluster value and the second group has systemic velocities below that value. This suggests that the two groups differ kinematically. Based on surveys of the Coma Cluster in the radio, the IRAS sources, and galaxies detected in Hα emission, we sum the far-infrared luminosity function of galaxies in the cluster. We find that star formation in late-type galaxies is probably the dominant component of the Coma Cluster far-infrared luminosity. The presence of significant emission from intracluster dust is not yet firmly established. The member galaxies also account for most of the far-infrared output from nearby rich clusters in general. We update estimates of the far-infrared luminosities of nearby, rich clusters and show that such clusters are likely to undergo luminosity evolution from z = 0.4 at a rate similar to, or faster than, field galaxies.