Cold Dust in Early-Type Galaxies. I. Observations^*

We describe far-infrared observations of early-type galaxies selected from the Infrared Space Observatory (ISO) archive. This rather inhomogeneous sample includes 39 giant elliptical galaxies and 14 S0 (or later) galaxies. These galaxies were observed with the array photometer PHOT on-board the ISO satellite using a variety of different observing modes—sparse maps, mini-maps, oversampled maps, and single pointings—each of which requires different and often rather elaborate photometric reduction procedures. The ISO background data agree well with the COBE-DIRBE results to which we have renormalized our calibrations. As a further check, the ISO fluxes from galaxies at 60 and 100 μm agree very well with those previously observed with IRAS at these wavelengths. The spatial resolution of ISO is several times greater than that of IRAS, and the ISO observations extend out to 200 μm, which views a significantly greater mass of colder dust not assessable to IRAS. Most of the galaxies are essentially point sources at ISO resolution, but a few are clearly extended at FIR wavelengths with image sizes that increase with FIR wavelength. The integrated far-infrared luminosities do not correlate with optical luminosities, suggesting that the dust may have an external, merger-related origin. In general, the far-infrared spectral energy distributions can be modeled with dust at two temperatures, ∼43 and ∼20 K, which probably represent limits of a continuous range of temperatures. The colder dust component dominates the total mass of dust, 10⁶-10⁷ M_⊙, which is typically more than 10 times larger than the dust masses previously estimated for the same galaxies using IRAS observations. For S0 galaxies we find that the optically normalized far-infrared luminosity L_FIR/L_B correlates strongly with the mid-infrared luminosity L_{15 μm}/L_B, but that correlation is weaker for elliptical galaxies.