A radio continuum survey of Shapley - Ames galaxies at λ 2 . 8 cm II . Separation of thermal and non - thermal radio emission

The radio continuum spectra of 74 galaxies were used to separate thermal and non-thermal radio emission. The thermal fraction f1GHz th at 1 GHz and the non-thermal spectral index αnth of the radio continuum emission were determined. The mean values are 〈f1GHz th 〉 = 0.08± 0.01 and 〈αnth〉 = 0.83± 0.02. Even at 10 GHz the radio continuum emission ofmost of the galaxies is dominated by the synchrotron component. UsingHα data the thermal radio flux densities were independently estimated and the non-thermal spectral indices were recalculated. Good agreement was found between the results of the two methods and the mean internal extinction was determined to be AHα ' 0.8m. Early-type spirals and irregulars tend to have flatter non-thermal spectra as the thermal fraction is independent of morphological type. Galaxies with strong star formation activity are found to have flatter non-thermal spectral indices.