IRAS observations of normal galaxies: The UGC redshift sample

Abstract

We have used a large sample of nearly 2000 UGC galaxies with known redshifts and IRAS 60 and 100 µm detections to identify some fundamental correlations between the far infrared (FIR) and optical properties of normal galaxies and to assess the principal dust heating mechanisms that allow a galaxy to be detected by IRAS. We conclude (1) that the 60/100 µm flux density ratio is a useful discriminant of the heating source, (2) that for most normal galaxies the FIR emission is powered by the general diffuse interstellar radiation field (ISRF), and (3) that the overall intensity of the ISRF and the size of the galaxy are the most important parameters driving the FIR luminosity. Thus, the FIR luminosity of a typical spiral is not a good measure of the recent massive star formation rate, but is rather a natural consequence of a constant star formation rate over several Gyr in an optically thin disk of gas and dust.