Mid-infrared emission from E plus A galaxies in the Coma Cluster

We have used ISO to observe at 12 mu m seven E + A galaxies plus an additio nal emission-line galaxy, all in the Coma Cluster. E + A galaxies lacking n arrow emission lines have 2.2-12 mu m flux density ratios or limits similar to old stellar populations (typical of early-type galaxies). Only galaxies with emission lines have enhanced 12 mu m flux density. Excess 12 mu m emi ssion is therefore correlated with the presence of ongoing star formation o r 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 IR IS, we estimate the far- infrared luminosity of these galaxies. By comparing the current star format ion rates with previous rates estimated from the Balmer absorption features , we divide the galaxies into two groups: those for which star formation ha s declined significantly following a dramatic peak similar to 1 Gyr ago; an d those with a significant level of ongoing star formation or/and an AGN. T here is no strong difference in the spatial distribution on the sky between these two groups. However, the first group has systemic velocities above t he mean cluster value and the second group has systemic velocities below th at value. This suggests that the two groups differ kinematically. Based on surveys of the Coma Cluster in the radio, the IRAS sources, and ga laxies detected in Ha emission, we sum the far-infrared luminosity function of galaxies in the cluster. We find that star formation in late-type galax ies is probably the dominant component of the Coma Cluster far-infrared lum inosity. The presence of significant emission from intracluster dust is not yet firmly established. The member galaxies also account for most of the f ar-infrared output from nearby rich clusters in general. We update estimate s of the far-infrared luminosities of nearby, rich clusters and show that s uch clusters are likely to undergo luminosity evolution from z = 0.4 at a r ate similar to, or faster than, field galaxies.