Signatures of the Youngest Starbursts: Optically Thick Thermal Bremsstrahlung Radio Sources in Henize 2‐10

Abstract

VLA radio continuum imaging reveals compact (<8 pc)~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 (Sν~να) indicative of nonthermal processes (α~=-0.5), the radio sources have positive (α>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 <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 106 yr, the possible precursors to globular clusters, may be most easily identified by finding compact radio sources with optically thick thermal bremsstrahlung spectral signatures.