A Radio Search for Natal Super-Star Clusters in Nearby Star-Forming Galaxies 

Super-star clusters (SSCs) are massive star forming regions embedded in
"Ultra-dense HII regions" (UDHIIs) which are in turn enshrouded within
a thick dust cocoon. Characterized by high pressures and densities,
SSCs are the most extreme star formation environments known. Since they are
also very young objects, they could be precursors to globular clusters.
SSCs' large visible extinctions (A_V >> 30 mag) necessitate infrared or
radio observations for their detection.

To search for evidence of natal SSCs, we observed in K- and X-band a total of
28 star-forming galaxies with the Australia Telescope Compact Array (ATCA)
and the Very Large Array (VLA). By measuring K- and X-band flux densities,
we computed spectral indices (S_\nu \propto \nu^\alpha) for each
potential SSC. Those sources with \alpha > -0.1 represent thermal
bremsstrahlung radiation, purportedly powered by a central, hot SSC engine.
Based on the radio emission flux densities we can infer the ionizing flux
of the SSC and determine how many hot O7.5 V stars are necessary to produce
the observed radio flux densities.

Within the 1 sigma errors of our spectral index measurements, we detected
8 galaxies with natal star formation out of the 28 selected for our sample.
This is surprising because we selected all the galaxies in our sample
based upon their likelihood of harboring young star formation.  The largest
clusters we detected have ~104 O7.5 V stars (definitely "super" star
clusters) and the smallest only have ~102 stars.