Ratio of radio to hybrid Σ_SFR. Top panels show histograms of the ratio separately for the three radio spectral index bins at 144 MHz (panel a) and 1365 MHz (panel b). Data points representing young CREs are in red, middle-aged CREs are in green, and old CREs are in blue, respectively. Best-fitting Gaussian distributions are shown as solid lines. Bottom panels show the ratio as function of the radio spectral index between 144 and 1365 MHz for 144 MHz (panel c) and 1365 MHz (panel d). Best-fitting exponential functions (Eq. (11)) shown as solid lines.