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

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1. Introduction

The radio continuum emission of spiral galaxies is a composition of two components: the non-thermal synchrotron emission of relativistic electrons gyrating in the interstellar magnetic field and the thermal free-free emission of electrons in photo-ionized gas surrounding hot stars. Owing to the steep spectrum of the synchrotron emission the thermal emission is expected to become more important in the high-frequency regime. The different spectral behaviour can be used for the separation of the two components, and it is absolutely necessary to get high-quality data in the high-frequency regime.

The spectral index of the synchrotron radiation [FORMULA] ([FORMULA]) is directly connected with the spectral index of the energy distribution of the radiating electrons. If the energy distribution of the cosmic ray electrons is given by [FORMULA], the non-thermal spectral index becomes [FORMULA]. The typical injection spectrum of the electrons is [FORMULA] (e.g. Bogdan & Völk 1983; Völk et al. 1988). Energy dependent diffusion and energy losses due to synchroton emission and inverse Compton scattering vary the energy distribution of the electrons (e.g. Pacholczyk 1970). The result is a steepening of the observed radio spectrum at high frequencies. In principle one should be able to decide if the cosmic ray electrons lose all their energy within their host galaxy or if they can escape from it, by analyzing the radio spectrum of a galaxy. This question is still under discussion. Chi & Wolfendale (1990) expect efficient confinement in very luminous and massive galaxies. Observational evidence that particle retention is less efficient in low mass and dwarf galaxies was presented by Klein et al. (1991). A model for the tight radio-FIR correlation developed by Völk (1989) predicts a totally loss-dominated radio spectrum, corresponding to a non-thermal radio spectral index [FORMULA] in the frequency range of a few GHz. The effect of star formation and the FIR-to-radio ratio on the spectrum of a galaxy was further discussed by Condon et al. (1991).

Previous investigations of the radio spectra of galaxies yielded different results. An analysis of the power-law spectral index of 56 spiral galaxies by Gioia et al. (1982) yielded a narrow distribution of the spectral indices. For a sample of 13 galaxies Klein (1988) separated the thermal and non-thermal emission. His mean value of the non-thermal spectral index is [FORMULA], and he concluded that the radio emission between 1 and 5 GHz is dominated by the non-thermal component. Duric et al. (1988) analyzed the radio spectra of about 30 galaxies and found strong variations in spectral index and thermal fraction from galaxy to galaxy. This was claimed to indicate that the physical conditions in the interstellar medium vary strongly among the galaxies. Hence, it remains unclear whether the non-thermal radio spectrum and the thermal amount are fixed within the variety of galaxies or if there are large variations of these quantities. The main problem with previous analyses of the radio spectra has been the lack of high-quality radio data at high frequencies. With the Shapley-Ames survey carried out at 10.55 GHz with the 100-m radio telescope of the MPIfR Bonn such a data base at high frequencies has now been established. The observational methods, the data reduction and the results are described by Niklas et al. (1995; Paper I).

In this paper we present the integrated radio spectra of a subsample of the Shapley-Ames sample. In Sect. 2 we present the spectral data and introduce the separation method. Then, in Sect. 3 the results of the separation of the thermal and non-thermal radio emission are given. In Sect. 4 we discuss the question if energy losses of the cosmic-ray electrons have significantly affected the radio spectra. The influence of the properties of the galaxies on the radio spectral indices are investigated in Sect. 5. Finally, we summarize the derived results.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998