Thermal Emission from HiiGalaxies: Discovering the Youngest Systems

We studied the radio properties of very young massive regions of starformation in HII galaxies, with the aim of detecting episodes of recent starformation in an early phase of evolution where the first supernovae start toappear. Our sample consists of 31 HII galaxies, characterized by strongHydrogen emission lines, for which low resolution VLA 3.5cm and 6cmobservations were obtained. The radio spectral energy distribution has a rangeof behaviours; 1) there are galaxies where the SED is characterized by asynchrotron-type slope, 2) galaxies with a thermal slope, and, 3) galaxies withpossible free-free absorption at long wavelengths. The latter SEDs were foundin a few galaxies and represent a signature of heavily embedded massive starclusters closely related to the early stages of massive star formation. Basedon the comparison of the star formation rates determined from the recombinationlines and those determined from the radio emission we find that SFR(Ha) is onaverage five times higher than SFR(1.4GHz). We confirm this tendency bycomparing the ratio between the observed flux at 20 cm and the expected one,calculated based on the Ha star formation rates, both for the galaxies in oursample and for normal ones. This analysis shows that this ratio is a factor of2 smaller in our galaxies than in normal ones, indicating that they fall belowthe FIR/radio correlation. These results suggest that the emission of thesegalaxies is dominated by a recent and massive star formation event in which thefirst supernovae (SN) just started to explode. We conclude that the systematiclack of synchrotron emission in those systems with the largest equivalent widthof Hb can only be explained if those are young starbursts of less than 3.5Myrof age.Comment: Accepted for publication in Ap