FAR-INFRARED EMISSION FROM E-GALAXIES AND E SO-GALAXIES/

Early-type galaxies are filled with hot X-ray-emitting gas, but the st udy of the less plentiful cold gaseous component has been more challen ging. Studies of cold material through IRAS 60 and 100 mu m observatio ns indicated that half of ordinary E and E/S0 galaxies were detected a bove the 3 sigma level, indicating that cold gas is common, although n o correlation was found between the optical and far-infrared fluxes. M ost detections were near the instrumental threshold, and given an impr oved understanding of detection confidence, we reconsider the 60 and 1 00 mu m detection rate. After excluding active galactic nuclei, peculi ar systems, and background contamination, only 15 nonpeculiar E and E/ S0 galaxies from the RSA catalog are detected above the 98% confidence level, about 12% of the sample. An unusually high percentage of these 15 galaxies possess cold gas (H I CO) and optical emission lines (H a lpha), supporting the presence of gas cooler than 10(4) K. The 60-100 mu m flux ratios imply a median dust temperature for the sample of 30 K, with a range of 23-28 K. These detections define the upper envelope of the optical to far-infrared relationship, F(fir)proportional to F- B(0.24+/-0.08) showing that optically bright objects are also brighter in the infrared, although with considerable dispersion. A luminosity correlation is present wth L-fir proportional to L-B(1.65+/-0.28) but the dust temperature is uncorrelated with luminosity. The dust masses inferred from the far-infrared measurements are 1 order of magnitude g reater than those from extinction observations, except for the recent merger candidate NGC 4125, where they are equal. We suggest that the r atio of the far-infrared dust mass to the extinction dust mass may be an indicator of the time since the last spiral-spiral merger. These re sults are compared to the model in which most of the dust comes from s tellar mass loss and the heating is primarily by stellar photons. Mode ls that contain large dust grains composed of amorphous carbon plus si licates come close to reproducing the typical 60-100 mu m flux ratios, the far-infrared luminosity, and the L-fir-L-B relationship.