Thermal Emission from HII Galaxies: Discovering the Youngest Systems

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. Our sample consists of 31 HII galaxies, characterized by strong Hydrogen emission lines, for which low resolution VLA 3.5 cm and 6 cm observations were obtained. We complemented these observations with archival data at 20 cm. We found that the radio spectral energy distribution (SED) has a range of behaviours; 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 SEDs were found in a few galaxies (e.g. UM533, Tololo 1223-388) and represent a signature of heavily embedded massive star clusters closely related to the early stages of massive star formation. Based on the comparison of the star formation rates (SFR) determined from the recombination lines and those determined from the radio emission we find Visiting Fellow, IoA, Cambridge.