VLA and ATCA Search for Natal Star Clusters in Nearby Star-Forming
  Galaxies
  In order to investigate the relationship between the local environment and
the properties of natal star clusters, we obtained radio observations of 25
star-forming galaxies within 20 Mpc using the Very Large Array (VLA) and the
Australia Telescope Compact Array (ATCA). Natal star-forming regions can be
identified by their characteristic thermal radio emission, which is manifest in
their spectral index at centimeter wavelengths. The host galaxies in our sample
were selected based upon their likelihood of harboring young star formation. In
star-forming regions, the ionizing flux of massive embedded stars powers the
dominant thermal free-free emission of those sources, resulting in a spectral
index of {\alpha} {\gtrsim} -0.2 (where S{\nu} {\propto} {\nu}{\alpha}), which
we compute. With the current sensitivity, we find that of the 25 galaxies in
this sample only five have radio sources with spectral indices that are only
consistent with a thermal origin; four have radio sources that are only
consistent with a non-thermal origin; six have radio sources whose nature is
ambiguous due to uncertainties in the spectral index; and sixteen have no
detected radio sources. For those sources that appear to be dominated by
thermal emission, we infer the ionizing flux of the star clusters and the
number of equivalent O7.5 V stars that are required to produce the observed
radio flux densities. The most radio-luminous clusters that we detect have an
equivalent of ~7x103 O7.5 V stars, and the smallest only have an equivalent of
~102 O7.5 V stars; thus these star-forming regions span the range of large
OB-associations to moderate "super star clusters" (SSCs). With the current
detection limits, we also place upper limits on the masses of clusters that
could have recently formed...
