The Cosmic Radio and Infrared Backgrounds Connection

We use the radio–infrared (IR) flux correlation between star–forming galaxies in the local universe to examine the connection between their cumulative contributions to the cosmic infrared and radio backgrounds. The general expression relating the intensities of the two backgrounds is complicated, and depends on details of the evolution of the galaxies’ IR luminosity function with redshift. However, in the specific case when the radio–IR flux correlation is linear, the relation between the intensity of the IR background and the brightness temperature of the radio background reduces to a simple analytical expression which at 178 MHz is: ICIB(nW m −2 sr−1) = 2.7× Tcrb(K), where the numerical coefficient was calculated for a radio spectral index of 0.7. This relation is insensitive to the star formation history of the galaxies that produce the cosmic IR background (CIB). We use the observed CIB intensity to constrain the cosmic star formation history, and the relation between the CIB and the cosmic radio background (CRB) to constrain the relative contribution of star–forming galaxies to the CRB. Current limits on the CIB intensity predict a 178 MHz brightness temperature of ∼ 18±9 K, about half of the 37±8 K inferred for an isotropic radio component. This suggests that star–forming galaxies and AGN contribute about equally to the CRB intensity at that frequency.