We study the spectral energy distribution ( SED ) of the radio continuum emission from the KINGFISH sample of nearby galaxies to understand the energetics and origin of this emission .
Effelsberg multi-wavelength observations at 1.4 GHz , 4.8 GHz , 8.5 GHz , and 10.5 GHz combined with archive data allow us , for the first time , to determine the mid-radio continuum ( 1-10 GHz , MRC ) bolometric luminosities and further present calibration relations vs. the monochromatic radio luminosities .
The 1-10 GHz radio SED is fitted using a Bayesian Markov Chain Monte Carlo ( MCMC ) technique leading to measurements for the nonthermal spectral index ( S _ { \nu } \sim \nu ^ { - \alpha _ { nt } } ) and the thermal fraction ( f _ { th } ) with mean values of \alpha _ { nt } = 0.97 \pm 0.16 ( 0.79 \pm 0.15 for the total spectral index ) and f _ { th } = ( 10 \pm 9 ) % at 1.4 GHz .
The MRC luminosity changes over \sim 3 orders of magnitude in the sample , 4.3 \times 10 ^ { 2 } L _ { \sun } < MRC < 3.9 \times 10 ^ { 5 } L _ { \sun } .
The thermal emission is responsible for \sim 23 % of the MRC on average .
We also compare the extinction-corrected diagnostics of star formation rate with the thermal and nonthermal radio tracers and derive the first star formation calibration relations using the MRC radio luminosity .
The nonthermal spectral index flattens with increasing star formation rate surface density , indicating the effect of the star formation feedback on the cosmic ray electron population in galaxies .
Comparing the radio and IR SEDs , we find that the FIR-to-MRC ratio could decrease with star formation rate , due to the amplification of the magnetic fields in star forming regions .
This particularly implies a decrease in the ratio at high redshifts , where mostly luminous/star forming galaxies are detected .