Far‐Infrared Emission From E and E/S0 Galaxies

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Abstract
Early-type galaxies are filled with hot X-ray-emitting gas, but the study of the less plentiful cold gaseous component has been more challenging. Studies of cold material through IRAS 60 and 100 μm observations indicated that half of ordinary E and E/S0 galaxies were detected above the 3 σ level, indicating that cold gas is common, although no correlation was found between the optical and far-infrared fluxes. Most detections were near the instrumental threshold, and given an improved understanding of detection confidence, we reconsider the 60 and 100 μm detection rate. After excluding active galactic nuclei, peculiar 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 (Hα), supporting the presence of gas cooler than 104 K. The 60-100 μm 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∝F, showing that optically bright objects are also brighter in the infrared, although with considerable dispersion. A luminosity correlation is present wth L∝L, but the dust temperature is uncorrelated with luminosity. The dust masses inferred from the far-infrared measurements are 1 order of magnitude greater than those from extinction observations, except for the recent merger candidate NGC 4125, where they are equal. We suggest that the ratio 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 results are compared to the model in which most of the dust comes from stellar mass loss and the heating is primarily by stellar photons. Models that contain large dust grains composed of amorphous carbon plus silicates come close to reproducing the typical 60-100 μm flux ratios, the far-infrared luminosity, and the Lfir-LB relationship.
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