Mid‐Infrared Emission from E+A Galaxies in the Coma Cluster

We have used ISO to observe at 12$\mu$m seven E+A galaxies plus an additionalemission line galaxy, all in the Coma cluster. E+A galaxies lacking narrowemission lines have 2.2$\mu$m to 12$\mu$m flux density ratios or limits similarto old stellar populations (typical of early-type galaxies). Only galaxies withemission lines have enhanced 12$\mu$m flux density. Excess 12$\mu$m emission istherefore correlated with the presence of on-going star formation or an activegalactic nucleus (AGN). By comparing the current star formation rates with previous rates estimatedfrom the Balmer absorption features, we divide the galaxies into two groups:those for which star formation has declined significantly following a dramaticpeak $\sim$ 1 Gyr ago; and those with a significant level of ongoing starformation or/and an AGN. There is no strong difference in the spatialdistribution on the sky between these two groups. However, the first group hassystemic velocities above the mean cluster value and the second group belowthat value. This suggests that the two groups differ kinematically. Based on surveys of the Coma cluster in the radio, the IRAS sources, andgalaxies detected in H$\alpha$ emission, we sum the far infrared luminosityfunction of galaxies in the cluster. We find that star formation in late typegalaxies is probably the dominant component of the Coma cluster far infraredluminosity. The presence of significant emission from intracluster dust is notyet firmly established. The member galaxies also account for most of the farinfrared output from nearby rich clusters in general.Comment: AAS Latex, accepted for publication in Ap