Dust in hot gas: Far-infrared emission from three local elliptical galaxies

We present far-IR Infrared Space Observatory observations of three early-type galaxies in the Virgo Cluster. The data were recorded using the ISOPHOT instrument in both the P32 oversampled maps and the P39/37 sparse maps. The maps reach the limiting sensitivity of the ISOPHOT instrument at 60, 90, and 180 m m. Two of the most prominent elliptical galaxies in Virgo-NGC 4472 and NGC 4649-clearly show no emission at far-IR wavelengths at a level of a few tens of millijanskys, but NGC 4636 is detected at all three wavelengths. We have computed the far-IR emission from dust for NGC 4472 and NGC 4636 under the assumption that dusty outflows from evolving red giant stars are continuously supplying dust to the interstellar medium and that the grains, once diffused into the interstellar medium, are sputtered away by collisions with ions. While the calculated fluxes are consistent with the observed upper limits for NGC 4472, the dust emission detected in NGC 4636 supports the hypothesis that additional dust has been accreted in a very recent (approximately less than a few times 10(8) yr) merger with a dusty, gas-rich galaxy.