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Astron. Astrophys. 322, 19-28 (1997)

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A radio continuum survey of Shapley-Ames galaxies at [FORMULA]  2.8 cm

II. Separation of thermal and non-thermal radio emission

S. Niklas 1, 2, U. Klein 3 and R. Wielebinski 2

1 Max-Planck-Institut für Kernphysik, Postfach 10 39 80, D-69029 Heidelberg, Germany
2 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
3 Radioastronomisches Institut, Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany

Received 28 May 1996 / Accepted 5 July 1996


The radio continuum spectra of 74 galaxies were used to separate thermal and non-thermal radio emission. The thermal fraction [FORMULA] at 1 GHz and the non-thermal spectral index [FORMULA] of the radio continuum emission were determined. The mean values are [FORMULA] and [FORMULA]. Even at 10 GHz the radio continuum emission of most of the galaxies is dominated by the synchrotron component. Using H [FORMULA] 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 [FORMULA]. 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.

Key words: galaxies: spiral – infrared: galaxies – radio continuum: galaxies – ISM: cosmic rays

Send offprint requests to: S.Niklas

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998