Radio Wavelength Constraints on the Sources of the Far-Infrared Background

The cosmic far infrared background detected recently by the COBE-DIRBE teamis presumably due, in large part, to the far infrared (FIR) emission from allgalaxies. We take the well-established correlation between FIR and radioluminosity for individual galaxies and apply it to the FIR background. We findthat these sources make up about half of the extragalactic radio background,the other half being due to AGN. This is in agreement with other radioobservations, which leads us to conclude that the FIR-radio correlation holdswell for the very faint sources making up the FIR background, and that the FIRbackground is indeed due to star-formation activity (not AGN or other possiblesources). If these star-forming galaxies have a radio spectral index between0.4 and 0.8, and make up 40 to 60% of the extragalactic radio background, wefind that they have redshifts between roughly 1 and 2, in agreement with recentestimates by Madau et al. of the redshift of peak star-formation activity. Wecompare the observed extragalactic radio background to the integral over thelogN-logS curve for star-forming radio sources, and find that the slope of thecurve must change significantly below about 1 microjansky. At 1 microjansky,the faint radio source counts predict about 25 sources per square arcminute,and these will cause SIRTF to be confusion limited at 160micron.Comment: 10 pages including 1 figure, AASTeX, accepted by Ap