4. Conclusions and summary
We have estimated and discussed different processes and components that shape the radio spectral index of a galaxy. In particular, we have considered the contribution of SNRs, the influence of different types of CR propagation (diffusion and convection) and of energy losses (inverse Compton and synchrotron). Our conclusions can be summarized as follows:
(i) We have estimated that the radio emission of SNRs represents about 10% of the nonthermal radio emission of a galaxy. This moderate contribution has a noticeable effect on the nonthermal radio spectral index, lowering it by 0.1 for steep spectra.
(ii) The spatially integrated radio spectral index of a galaxy is difficult to interpret, as it is influenced by various processes. Only for asymptotic cases conclusions can be drawn: In galaxies with very steep spectral indices () energy losses are high and the escape rate of CRE is low. A very flat spectral index (), on the other hand, is a sign for strong convection.
(iii) In order to draw more general conclusions about the CR propagation and the importance of energy losses, spatially resolved radio observations of galactic halos are necessary. A steepening of the spectral index away from the galactic disk is a clear indication that inverse Compton and synchrotron losses are important.
(iv) In all galaxies with multi-frequency radio data for the halo such a steepening has been found and therefore in these galaxies synchrotron and inverse Compton losses take place. Thus, it seems likely that these energy losses are generally important in galactic halos and that escape rates are low. The low overall spectral index that is frequently found in starburst galaxies, at the same time as the steepening of the spectrum away from the disk, can be interpreted as convection.
© European Southern Observatory (ESO) 2000
Online publication: February 9, 2000