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

Understand

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 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.