THE PHYSICS OF THE FAR-INFRARED-RADIO CORRELATION. II. SYNCHROTRON EMISSION AS A STAR FORMATION TRACER IN HIGH-REDSHIFT GALAXIES

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 cosmic microwave background (CMB) cool CR electrons and positrons rapidly, suppressing their nonthermal radio emission. However, we find that this effect occurs at higher redshifts than previously expected, because escape, bremsstrahlung, ionization, and starlight IC losses act to counter this effect and preserve the radio luminosity of galaxies. The radio dimming of star-forming galaxies at high z is not just a simple competition between magnetic field energy density and the CMB energy density; the CMB must also compete with every other loss process. We predict relations for the critical redshift when radio emission is significantly suppressed compared to the z approximate to 0 FRC as a function of star formation rate per unit area. For example, a MilkyWay like spiral becomes radio faint at z approximate to 2, while an M82-like starburst does not become radio faint until z approximate to 10-20. We show that the "buffering" effect of non-synchrotron losses improves the detectability of star-forming galaxies in synchrotron radio emission with Expanded Very Large Array and Square Kilometer Array. Additionally, we provide a quantitative explanation for the relative radio brightness of some high-z submillimeter galaxies. We show that at fixed star formation rate surface density, galaxies with larger CR scale heights are radio bright with respect to the FRC, because of weaker bremsstrahlung and ionization losses compared to compact starbursts. We predict that these "puffy starbursts" should have steeper radio spectra than compact galaxies with the same star formation rate surface density. We find that radio-bright submillimeter galaxies alone cannot explain the excess radio emission reported by ARCADE2, but they may significantly enhance the diffuse radio background with respect to a naive application of the z approximate to 0 FRC.