THE RELATIVISTIC ISM IN M33 - ROLE OF THE SUPERNOVA-REMNANTS

The role of supernova remnants in producing and maintaining the relativistic interstellar medium is investigated for the case of the nearby galaxy M33. Analysis of a radio continuum sample of supernova remnants (SNRs) has led to the following results. (1) The SNRs use roughly 1%-10% of their blast energy to produce relativistic particles. (2) The currently observed SNR population contains between 0.1% and 1% of the relativistic particle energy of the entire interstellar medium of M33, which leads to reasonable values of the particle residence time in the disk. (3) The distribution of synchrotron spectral indices indicates that the particle populations of the observed SNRs have energy spectra with power-law indices of 2.2 +/- 0.4, consistent with values predicted by diffusive shock acceleration theory. Taken together, the three results favor the hypothesis that SNRs account for the bulk of M33's relativistic medium. It is further shown that, as a consequence of these results, the predicted SN rate is 1 per 140-250 yr, in general agreement with independent estimates of the SN rate and the absence of historical supernovae.