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

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 application in future deep radio surveys, it is crucial to know whether the IRRC persists at high redshift z. Delvecchio et al. (2020) observed that the 1.4 GHz IRRC correlation of star-forming galaxies is nearly z-invariant up to z~4, but depends strongly on the stellar mass M_star. This should be taken into account for SFR calibrations based on radio luminosity. To understand the physical cause of the M_star-dependence of the IRRC and its properties at higher z, we construct a phenomenological model for galactic radio emission involving magnetic fields generated by a small-scale dynamo, a steady-state cosmic ray population, as well as observed scaling relations that reduce the number of free parameters. The best agreement between the model and the characteristics of the IRRC observed by Delvecchio et al. (2020) is found when the efficiency of the SN-driven turbulence is 5 % and when saturation of the small-scale dynamo occurs once 10 % of the kinetic energy is converted into magnetic energy. The observed dependence of the IRRC on M_star and z can be reproduced with our model. For galaxies with intermediate to high (M_star ~ 10^9.5 - 10^11 M_sun) stellar masses, our model results in a IRRC which is nearly independent of z. For galaxies with lower masses (M_star ~ 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, however, only extends to z~1.5. The increase of the IR-to-radio flux ratio for low-mass galaxies at z>1.5 that is predicted by our model could be tested with future deep radio observations.
20 pages, 18 figures, submitted to A&A