Far Infrared Emission from Elliptical Galaxies: NGC 4649, NGC 4472, and NGC 4636

We present ISOPHOT P32 oversampled maps and P37/39 sparse maps, of three bright elliptical galaxies in the Virgo Cluster. The maps reach the limiting sensitivity of the ISOPHOT instrument at 60, 100, 170 and 200 microns. Two elliptical galaxies show no emission at all far-IR ISOPHOT wavelengths at a level of few tens of mJy. The null detection provides a test of the evolution of dust in elliptical galaxies and its size distribution and composition. As previous studies have shown, in many elliptical galaxies both IRAS and ISO have detected mid-IR excess 6-15 micron emission relative to the stellar continuum indicating emission from circumstellar dust. Under the assumption that these dusty outflows from evolving red giant stars and planetary nebulae are continuously supplying dust to the interstellar medium, we have computed the infrared luminosity at the ISOPHOT bands appropriate for NGC4472. The null far-IR ISOPHOT observations exceed the far-IR flux expected from dust expelled from a normal old stellar population.