The Star Formation in Radio Survey: Adding 90GHz Data to 3–33GHz Observations of Star-forming Regions in Nearby Galaxies

We present new 90 GHz continuum imaging of 119 star-forming regions in 30 nearby galaxies observed with the bolometer array MUSTANG-2 operating on the Robert C. Byrd Green Bank Telescope as part of the Star Formation in Radio Survey. The 90 GHz data were combined with 3, 15, and 33 GHz data taken previously by the Karl G. Jansky Very Large Array to decompose radio spectra on scales ranging from ≈ 0.6 − 2.4 kpc with a median of ≈0.8 kpc into its synchrotron, free-free, and thermal dust emission components. This was done using three different fitting scenarios: (i) a simple power law fit, (ii) Markov Chain Monte Carlo (MCMC) fitting with thermal dust and 90 GHz data, and (iii) MCMC fitting without thermal dust and 90 GHz data. For these three cases, we find a median thermal fraction at 33 GHz of (i) 86 ± 2.7% with a scatter of 20%, (ii) 76 ± 2.8% with a scatter of 25%, and (iii) 84 ± 2.1% with a scatter of 18%. In the case of including 90 GHz data (second scenario), we find an average 10% lower median thermal fraction estimate at 33 GHz. We conclude that a simple power law fit of data from 3 to 33 GHz gives reliable thermal fraction estimates to within 10%. Furthermore, the 33 GHz emission is dominated by free-free emission, not thermal dust, on sub-kiloparsec scales.