2006A&A...460...67H

We investigate the radio spectral energy distributions (SEDs) of young star-forming galaxies and how they evolve with time. The duration and luminosity of the nonthermal radio emission from supernova remnants (SNRs) are constrained by using the observational radio SEDs of SBS0335-052 and IZw18, which are the two lowest-metallicity blue compact dwarf galaxies in the nearby universe. The typical radio ``fluence'' for SNRs in SBS0335-052, that is the radio energy emitted per SNR over its radiative lifetime, is estimated to be ∼6-22x10<sup>22</sup>(W/Hz)yr at 5GHz. On the other hand, the radio fluence in IZw18 is ∼1-3x10<sup>22</sup>(W/Hz)yr at 5GHz. We discuss the origin of this variation and propose scaling relations between synchrotron luminosity and gas density. We have also predicted the time dependence of the radio spectral index and of the spectrum itself, for both the ``active'' (SBS0335-052) and ``passive'' (IZw18) cases. These models enable us to roughly age date and classify radio spectra of star-forming galaxies into active/passive classes. Implications for high-z galaxy evolution are also discussed.