Far Infrared Emission from Galaxy Clusters and Early-Type Galaxies

Infrared emission has been detected from normal elliptical galaxies and from clusters of galaxies at 60um and 100um with the IRAS satellite. In both cases, the emission has the characteristics of cool dust with a temperature near 30K. For the elliptical galaxies, there is a correlation between the optical luminosities and the far infrared luminosities. The likely source of the dust in these systems is mass loss from stars, which is heated by starlight and either is distributed throughout the galaxy or falls into a central disk. Analysis of upcoming ISO data will permit us to distinguish between these possibilities. The far infrared emission from clusters of galaxies is of high luminosity but is only detected in 10% of the cases. The heating is either due to electron impact from the hot gas (a suitable explanation for one cluster) or photon absorption. The source of the dust is probably gas stripped from galaxies. For clusters, ISO data will provide good mass determinations and positions, thereby allowing us to determine the source of the heating.