The Separation of Synchrotron and Bremsstrahlung Radio Emission in Spiral Galaxies

Spectra consisting of both bremsstrahlung and synchrotron components are fit to the observed radio-continuum spectra of 41 spiral galaxies. We report the detection of the bremsstrahlung component in 11 galaxies. Very low upper limits are placed on another 20. For the remaining 11, model spectra cannot be fit with statistical significance. It is found that the thermal fraction (ratio of bremsstrahlung to total radio emission) spans at least two orders of magnitude among the 31 galaxies with good fits. The spectral index of the synchrotron component is found to vary over a larger range than previouslysuggested. A possible trend in the thermal fraction with the van den Bergh luminosity class is reported. The dependence is discussed in the context of cosmic-ray production, star formation, and theories of spiral structure, and it is concluded that, if the correlation is real, a three-way relationship may exist. It is further shown that the existence of such a three-way correlation introduces considerable uncertainty in the interpretation of previously reported two-way correlations between indicators of cosmic-ray production and star formation.