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We construct one-zone steady-state models of cosmic ray (CR) injection,cooling, and escape over the entire dynamic range of the FIR-radio correlation(FRC), from normal galaxies to starbursts, over the redshift interval 0 <= z <=10. Normal galaxies with low star-formation rates become radio-faint at high z,because Inverse Compton (IC) losses off the CMB cool CR electrons and positronsrapidly, suppressing their nonthermal radio emission. However, we find thatthis effect occurs at higher redshifts than previously expected, becauseescape, bremsstrahlung, ionization, and starlight IC losses act to counter thiseffect and preserve the radio luminosity of galaxies. The radio dimming ofstar-forming galaxies at high z is not just a simple competition betweenmagnetic field energy density and the CMB energy density; the CMB must alsocompete with every other loss process. We predict relations for the criticalredshift when radio emission is significantly suppressed compared to the z ~ 0FRC as a function of star-formation rate per unit area. Additionally, weprovide a quantitative explanation for the relative radio brightness of somehigh-z submillimeter galaxies. We show that at fixed star formation ratesurface density, galaxies with larger CR scale heights are radio bright withrespect to the FRC, because of weaker bremsstrahlung and ionization lossescompared to compact starbursts. We predict that these "puffy starbursts" shouldhave steeper radio spectra than compact galaxies with the same star-formationrate surface density. We find that radio bright submillimeter galaxies alonecannot explain the excess radio emission reported by ARCADE2, but they maysignificantly enhance the diffuse radio background with respect to a naiveapplication of the z ~ 0 FRC.

The Physics of the FIR-Radio Correlation: II. Synchrotron Emission as a Star-Formation Tracer in High-Redshift Galaxies