Signatures of the youngest starbursts: Optically thick thermal bremsstrahlung radio sources in Henize 2-10

VLA radio continuum imaging reveals compact (<8 pc) similar to 1 mJy radio sources in the central 5 " starburst region of the blue compact galaxy Henize 2-10. While the global radio continuum spectrum is a power law (S-nu proportional to nu(alpha)) indicative of nonthermal processes (alpha similar or equal to -0.5), the radio sources have positive (alpha > 0.0) spectral indices suggesting an optically thick thermal bremsstrahlung origin. We model the luminosities and spectral energy distributions of these radio knots, finding that they are consistent with unusually dense H II regions having electron densities, 1500 cm(-3) < n(e) < 5000 cm(-3), and sizes 3 pc < R < 8 pc. While the high inferred densities are typical of ultracompact H II regions in the Galaxy, such high optical depth (tau = 0.4-3.0 at 5 GHz) at frequencies as high as 5 GHz is unusual on parsec scales in galaxies. Since these H II regions are not visible in optical images, we propose that the radio data preferentially reveal the youngest, densest, and most highly obscured star-forming events. Energy considerations imply that each of the five H II regions contains similar to 750 O7 V-equivalent stars, greater than the number found in 30 Doradus in the LMC. The high densities imply an overpressure compared to the typical interstellar medium, so that such objects must be short-lived (<0.5 Myr expansion timescales). We conclude that the radio continuum maps reveal the very young (<0.5 Myr) precursors of "super-star clusters" or "proto-globular clusters," which are prominent at optical and UV wavelengths in He 2-10 and elsewhere. The fraction of O stars in these young ultradense H II regions is 15% of the total inferred O star population in Henize 2-10. This body of work leads us to propose that massive extragalactic star clusters with ages less than 10(6) yr, the possible precursors to globular clusters, may be most easily identified by finding compact radio sources with optically thick thermal bremsstrahlung spectral signatures.