Total-to-thermal radio flux density ratio as a function of the star formation rate. The evolution of the slope is evaluated from Eqs. (38), (39), assuming a transition at the critical star formation rate given by Eq. (32). We assume a size of the star-forming region of 0.5 kpc, and explore values for the disk height of 0.1,0.25,and0.5 kpc. The normalization of the flux ratios assumes a thermal-to-nonthermal flux ratio of 8% for a star formation rate of 0.1 M_⊙ kpc^-2 yr^-1. The plot assumes a typical radio frequency of ~1 GHz.