Radio Continuum Emission from Evolving Star-forming Galaxies. I. Correlations Involving the Total Synchrotron Luminosity

Synchrotron radiation dominates the continuum emission of star-forming galaxies in the frequency range from a few MHz to about 30 GHz. We model the total synchrotron emission of a large population of evolving star-forming galaxies using the semianalytic galaxy formation model galform combined with the dynamo simulation code magnetizer . Assuming local energy equipartition between cosmic rays and magnetic fields, we calculate the specific synchrotron luminosity L ν for each simulated galaxy at various frequencies and find strong positive correlations between L ν and both the star formation rate (SFR) and characteristic galaxy rotation speed V rot for redshifts up to z ≃ 3. At low redshifts, the turbulent magnetic field is found to dominate in the synchrotron luminosity, but the contribution of the large-scale magnetic field increases with redshift and becomes important for z ≳ 1. The correlation between L ν and SFR arises from the tight correlation between the disk gas mass M gas and SFR, and the correlation between L ν and V rot is additionally a consequence of the stellar mass Tully–Fisher relation for main-sequence galaxies. At low redshifts, the model predictions and observational data compiled for this work show remarkable agreement, but a discrepancy arises at higher redshifts, where modelled SFR values are systematically smaller than those previously inferred from observations. These theoretical models will aid the interpretation of next-generation radio surveys with the Square Kilometre Array and other telescopes.