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 \sim 7 \times 10 ^ { 3 } O7.5 V stars , and the smallest only have an equivalent of \sim 10 ^ { 2 } 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 ; for a number of galaxies we can conclusively rule out the presence of natal clusters significantly more massive than the Galactic star-forming region W49A ( \sim 5 \times 10 ^ { 4 } M _ { \odot } ) .
The dearth of current massive cluster formation in these galaxies suggests that either their current star formation intensities have fallen to near or below that of the Milky Way and/or that the evolutionary state that gives rise to thermal radio emission is short-lived .