On the Early Evolution of Young Starbursts

We studied the radio properties of very young massive regions of star formation in HII galaxies, with the aim of detecting episodes of recent star formation in an early phase of evolution where the first supernovae start to appear. The observed radio spectral energy distribution (SED) covers a behaviour range; 1) there are galaxies where the SED is characterized by a synchrotron-type slope, 2) galaxies with a thermal slope, and 3) galaxies with possible free-free absorption at long wavelengths. The latter SED represents a signature of massive star clusters that are still well inside the progenitor molecular cloud. Based on the comparison of the star formation rates (SFR) determined from the recombination lines and those determined from the radio emission we find that SFR(Ha) is on average five times higher than SFR(1.4 GHz). These results suggest that the emission of these galaxies is dominated by a recent and massive star formation event in which the first supernovae (SN) just started to explode. We conclude that the systematic lack of synchrotron emission in those systems with the largest equivalent width of Hb can only be explained if those are young starbursts of less than 3.5Myr of age, i.e. before the first type II SNe emerge.