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Astron. Astrophys. 354, 423-430 (2000)

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1. Introduction

The total radio emission of a galaxy consists of different components: The thermal radio emission, [FORMULA], representing bremsstrahlung from [FORMULA] regions and the nonthermal radio emission which is synchrotron emission from cosmic ray electrons (CREs). As far as the nonthermal radio emission, [FORMULA], is concerned, a further division can be made by distinguishing between the diffuse radio emission, [FORMULA], from CREs that are spread over the galactic disks and halos and the emission from discrete supernova remnants (SNRs), [FORMULA]. The three components possess very different spectral indices: Whereas the thermal radio emission has a flat spectral index ([FORMULA]), the nonthermal radio spectrum is steeper. The spectral index of SNRs is on average [FORMULA] (Green 1998) and the diffuse radio emission can have, depending on the dominant process for the CR propagation, spectral indices 1 , [FORMULA], between about 0.5 and 1.1 which can, additionally, vary over the frequency range. Therefore, the total radio spectral index depends very sensitively on the relative contribution of the different components.

The spectrum of the diffuse synchrotron emission is shaped by the physical processes that are characterizing the CR propagation, in particular the type of propagation (diffusion or convection), of energy losses (synchrotron, inverse Compton, adiabatic losses or bremsstrahlung), and the confinement of CREs. Here, we consider CREs confined to a galaxy if they loose their energy through combined synchrotron and inverse Compton losses, down below the energy level corresponding to the observing frequency, before they are able to leave the galactic halo. The question whether CREs are confined to galaxies or can escape from the halos without suffering considerable energy losses is important for the interpretation of the far-infrared (FIR)/radio correlation. Different models have been proposed, explaining this correlation by advocating either the situation that CREs are confined (Völk 1989; Lisenfeld et al. 1996a) or that they can escape more or less freely (Chi & Wolfendale 1990; Helou & Bicay 1993; Niklas & Beck 1997).

The interpretation of the radio spectrum in terms of the above mentioned processes is difficult because they are hard to disentangle without spatially resolved observations. Furthermore, it is necessary to separate the diffuse radio emission from the thermal radio emission and the radio emission from SNRs. The observational separation of the three contributions based on their different spectral indices is difficult, especially for the radio emission of SNRs which has a similar spectral index as the diffuse synchrotron emission. The separation of the thermal radio emission is possible, if high-frequency radio data ([FORMULA] GHz) are available, because its spectral index is very different.

In this paper we are going to examine the influence of these various processes on the radio spectral index. For this, we will start by estimating the contribution of SNRs to the radio emission and to calculate its effect on the spectral index. Then, we discuss within a simple model the influence of energy losses, the type of propagation and escape of CREs on the spectral index.

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© European Southern Observatory (ESO) 2000

Online publication: February 9, 2000