A model for the infrared-radio correlation of main-sequence galaxies at GHz frequencies and its variation on redshift and stellar mass

Abstract: The infrared-radio correlation (IRRC) of star-forming galaxies can be used to estimate their star formation rate (SFR) based on the radio continuum luminosity at MHz-GHz frequencies. For its application in future deep radio surveys, it is crucial to know whether the IRRC persists at high redshift z. Previous works have reported that the 1.4 GHz IRRC correlation of star-forming galaxies is nearly z-invariant up to z=4, but depends strongly on stellar mass M. This should be taken into account for SFR calibrations based on radio luminosity. To understand the physical cause behind the M dependence of the IRRC and its properties at higher z, we constructed a phenomenological model for galactic radio emission. Our model is based on a dynamo-generated magnetic field and a steady-state cosmic ray population. It includes a number of free parameters as well as observed scaling relations. We find that the resulting spread of the infrared-to-radio luminosity ratio, q(z, M), with respect to M is mostly determined by the scaling of the galactic radius with M, while the absolute value of the q(z, M) curves decreases with more efficient conversion of supernova energy to magnetic fields and cosmic rays. Decreasing the slope of the cosmic ray injection spectrum, aCR, results in higher radio luminosity, decreasing the absolute values of the q(z, M) curves. Our model reproduces the observed dependence of the IRRC on M and z when the efficiency of supernova-driven turbulence is 5%, 10% of the kinetic energy is converted into magnetic energy, and aCR = 3. For galaxies with intermediate to high M (10^9.5-10^11 M_sun), our model results in an IRRC that is nearly independent of z. For galaxies with lower M (M=10^8.5 M_sun), we find that the IR-to-radio flux ratio increases with increasing redshift. This matches the observational data in that mass bin which currently, however, only extends to z~1.5.