Dust in Hot Gas: Far-Infrared Emission from Three Local Elliptical Galaxies

We present far-IR ISO observations of three early-type galaxies in the Virgocluster. The data were recorded using the ISOPHOT instrument in both the P32oversampled maps and the P39/37 sparse maps. The maps reach the limitingsensitivity of the ISOPHOT instrument at 60, 90, and 180$\mu$m. Two of the mostprominent elliptical galaxies in Virgo -- NGC 4472 and NGC 4649 -- clearly showno emission at far-IR wavelengths at a level of few tens of mJy, but NGC 4636is detected at all three wavelengths. We have computed the far-IR emission fromdust for NGC4472 and NGC4636 under the assumption that dusty outflows fromevolving red giant stars are continuously supplying dust to the interstellarmedium and that the grains, once diffused into the interstellar medium, aresputtered away by collisions with ions. While the calculated fluxes areconsistent with the observed upper limits for NGC4472, the dust emissiondetected in NGC4636 support the hypothesis that additional dust has beenaccreted in a very recent ($\lta$ few $10^8$ yrs) merger with a dusty, gas-richgalaxy.Comment: Accepted for publication in ApJ