Very Large Array observations of the H92 alpha line from NGC 5253 and Henize 2-10: Ionized gas around super star clusters

We have detected the H92 alpha radio recombination line from two dwarf star burst galaxies, NGC 5253 and He 2-10, using the Very Large Array. Both the line data as well as the radio continuum data are used to model the propert ies of the ionized gas in the centers of these galaxies. We consider a mult i-density model for radio recombination lines and show why previous models, which were based on the assumption of gas at a single density, are valid i n many situations. The models show that the ionized gas has a density of si milar to 10(4) cm(-3) in both galaxies, with an effective size of 2-10 pc a nd a total mass of similar to 10(4) M-circle dot. The derived production ra te of Lyman continuum photons is similar to2.5 x 10(52) s(-1) in both the g alaxies, and the corresponding mass of stars (assuming a Salpeter initial m ass function) is similar to 10(5) MG, The implied stellar density shows tha t the observed radio recombination lines arise from ionized gas around supe r star clusters (SSCs) in both galaxies (these SSCs have been recently dete cted through their radio continuum emission). The existence of similar to 1 0(4) MD of ionized gas within a few parsecs of an SSC places strict constra ints on dynamical models. Using simple arguments, the parameter space for a few possible models are derived. The well-known radio-far-infrared correla tion also holds for NGC 5253, although the radio emission from this galaxy is almost, completely thermal. It is shown that NGC 5253 is strong evidence that the component of far-infrared emission from warm dust is correlated s eparately with the component of radio emission from thermal bremsstrahlung.