Thermal emission from HII galaxies: Discovering the youngest systems

We studied the properties of very young massive star-forming regions in H II galaxies, with the aim of detecting signs of evolution where the first supernovae (SNe) start to appear. Our sample consists of 31 H II galaxies, characterized by strong hydrogen emission lines, for which low-resolution VLA 3.5 and 6 cm observations were obtained. We found that the radio spectral energy distribution ( SED) has a range of behaviors: galaxies where the SED is characterized by a synchrotron-type slope; galaxies with a thermal slope; and galaxies with possible free-free absorption at long wavelengths. The latter represent a signature of heavily embedded massive star clusters. Comparing the different star formation rates (SFRs), we find that SFR(H alpha) is on average significantly higher than SFR( 1.4 GHz). We confirm this tendency by comparing the ratio of the observed flux at 20 cm to the expected one, calculated based on the SFR H alpha, both for the galaxies in our sample and for normal ones. We show that this ratio is a factor of 2 smaller in our galaxies than in normal ones, indicating that they do not follow the FIR/radio correlation (q-parameter). These results suggest that the emission of these galaxies is dominated by a recent star-forming event in which the first SNe started to explode, consistent with the radio emission being dominated by free-free continuum. We propose an evolutionary scenario to explain the observed trends and conclude that the systematic lack of synchrotron emission in those systems with the largest equivalent width of H beta can only be explained if those are young starbursts of less than 3.5 Myr of age, i.e., before the first Type II SNe start to explode.