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

Context. 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 practical application in future deep radio surveys, it is crucial to know whether the IRRC persists at high redshift z.
Aims: 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 the stellar mass M_⋆. This should be taken into account for SFR calibrations based on radio luminosity.
Methods: 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 that determine the galaxy properties. To reduce the overall number of model parameters, we also employed observed scaling relations.
Results: 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. Additionally, decreasing the slope of the cosmic ray injection spectrum, α_CR, results in higher radio luminosity, decreasing the absolute values of the q(z, M_⋆) curves. Within the uncertainty range of our model, the observed dependence of the IRRC on M_⋆ and z can be reproduced when the efficiency of supernova-driven turbulence is 5%, 10% of the kinetic energy is converted into magnetic energy, and α_CR ≈ 3.0.
Conclusions: For galaxies with intermediate to high (M_⋆ ≈ 10^9.5 − 10¹¹ M_⊙) stellar masses, our model results in an IRRC that is nearly independent of z. For galaxies with lower masses (M_⋆ ≈ 10^8.5 M_⊙), 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 in 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.