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 : I _ { CIB } ( nW m ^ { -2 } sr ^ { -1 } ) = 2.7 \times T _ { crb } ( 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 \sim 18 \pm 9 K , about half of the 37 \pm 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 .