Time evolution of the radio continuum of young starbursts: the importance of synchrotron emission

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 SBS 0335-052 and IZw 18, which are the two lowest-metallicity blue compact dwarf galaxies in the nearby universe. The typical radio fluence for SNRs in SBS 0335-052, that is the radio energy emitted per SNR over its radiative lifetime, is estimated to be similar to 6-22 x 10(22) WHz(-1) yr at 5 GHz. On the other hand, the radio fluence in IZw Zw18 is similar to 1-3 x 10(22) WHz(-1) yr at 5 GHz. 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 (SBS 0335-052) and passive (IZw 18) 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.