Probing Star Formation at Low Metallicity: The Radio Emission of Super Star Clusters in SBS0335-052

K. E. Johnson, L. K. Hunt, A. E. Reines

Published 2009-02-04Version 1

We present high-resolution radio continuum observations of the nascent starburst in the metal-poor galaxy SBS 0335-052. These radio data were taken with the Very Large Array and include observations at 0.7cm, 1.3cm, 2cm, 3.6cm, and 6cm. These observations enable us to probe the thermal radio nebulae associated with the extremely young star-forming regions in this galaxy. Two discrete and luminous star-forming regions are detected in the south of the galaxy that appear to be associated with massive star clusters previously identified at optical wavelengths. However, the remaining optically-identified massive star clusters are not clearly associated with radio emission (either thermal or non-thermal) down to the sensitivity limits of these radio data. The spectral energy distributions of the two radio-detected clusters are consistent with being purely thermal, and the entire region has an inferred ionizing flux of ~1.2 x 10^ 53 s^-1, which is equivalent to ~12,000 "typical" O-type stars (type O7.5 V). The observations presented here have resolved out a significant contribution from diffuse non-thermal emission detected previously, implying a previous episode of significant star formation. The current star formation rate (SFR) for this southern region alone is ~1.3 M_sun yr^-1, or ~ 23M_sun yr^-1 kpc^-2. This SFR derived from thermal radio emission also suggests that previous optical recombination line studies are not detecting a significant fraction of the current star formation in SBS 0335-052. From model fits to the radio spectral energy distribution, we infer a global mean density in the two youngest clusters of n_e > 10^3-10^4 cm^-3. In addition, a comparison between the compact and diffuse radio emission indicates that up to ~50% of the ionizing flux could be leaking out of the compact HII regions.