For a sample of 74 Shapley-Ames galaxies thermal and non-thermal radio emission was separated. The mean value of the non-thermal spectral index is . On average one third of the radio continuum emission at 10 GHz can be attributed to thermal free-free emission. An independent estimate of the thermal amount using H data yields that energy lossses of the cosmic ray electrons do not significantly steepen the radio continuum spectrum in the GHz-frequency regime. The mean extinction of the H emission by dust was determined to be .
Large and massive galaxies with low SFE tend to have flatter non-thermal spectral indices than low mass-galaxies with high star formation actvitity. The presented correlations are not very tight, but this may be attributed to the uncertainties in the calculation of the mass of the molecular hydrogen. Nevertheless, we found that the star formation efficiency as well as mass and size of a galaxy affects the population of the synchrotron emitting electrons.
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998