THE JOINT FAR-INFRARED OPTICAL LUMINOSITY FUNCTION FOR SPIRAL GALAXIES AND DATA FOR THE ABELL-400 AND CANCER CLUSTERS

The joint far-infrared-optical luminosity function for late-type galaxies, PSI, is examined using an optically selected sample of 183 galaxies from the Revised Shapley-Ames Catalog and from the Virgo Cluster compilation of Helou and coworkers, including peculiar spirals but excluding Sa's. For each of these galaxies we have the far-infrared (FIR) flux at 60-mu-m, f60, and at 100-mu-m, f100. We show that the distribution of the ratio of FIR to blue luminosity, r = L(FIR)/L(B), depends weakly on L(B), so that PSI can be approximated by a function of a single variable, psi(r'), where r' = r(L(B)/L*)-delta with delta congruent-to 0.08 (congruent-to 0 for large r) and L* a constant. The function psi(r') is well fitted by a lognormal curve which peaks at r' = 0.35 and has a dispersion of 0.28. While an excess of galaxies, with respect to the lognormal curve, is visible for large r', our optically selected sample of nearby galaxies shows that a population of galaxies with very small values of r' is absent. This implies that spiral galaxies with a very low abundance of interstellar dust are rare. We also suggest an explanation for the positive correlation of both r and the FIR color c(c = f100/f60) with L(B). By studying the FIR visibility function, we argue that the FIR luminosity function is proportional to psi for large r and that they both decline with a power law of index almost-equal-to -2. Using the Minnesota Automated Plate Scanner, we have an optical list of galaxies as faint as 17th magnitude in the blue in two more distant clusters: the Cancer Cluster and Abell 400. For groups of galaxies, optically similar but individually undetected in the FIR, we build up co-added "class average scans" which can give meaningful class average fluxes as small as 50 mJy at 60-mu-m. Using these, we are able to check the distribution function of r' at the faint FIR end, even in these more distant clusters.