The Star Formation in Radio Survey: Jansky Very Large Array 33 GHz Observations of Nearby Galaxy Nuclei and Extranuclear Star-forming Regions

Abstract

We present 33 GHz imaging for 112pointings toward galaxy nuclei and extranuclear star-forming regions at ≈2″ resolution using the KarlG.Jansky VeryLargeArray (VLA) as part of the Star Formation in Radio Survey. A comparison with 33 GHz Robert C.ByrdGreenBankTelescope single-dish observations indicates that the interferometric VLA observations recover 78%±4% of the total flux density over 25″ regions(≈kpc scales) among all fields. On these scales, the emission being resolved out is most likely diffuse non-thermal synchrotron emission. Consequently, on the ≈30-300 pc scales sampled by our VLA observations, the bulk of the 33 GHz emission is recovered and primarily powered by free-free emission from discrete H II regions, making it an excellent tracer of massive star formation. Of the 225 discrete regions used for aperture photometry, 162 are extranuclear (i.e., having galactocentric radii r G 250 pc) and detected at >3σ significance at 33 GHz and in Hα. Assuming a typical 33 GHz thermal fraction of 90%, the ratio of optically-thin 33 GHz to uncorrected Hα star formation rates indicates a median extinction value on ≈30-300 pc scales of A Hα ≈1.26±0.09 mag, with an associated median absolute deviation of 0.87 mag. We find that 10% of these sources are "highly embedded" (i.e., A Hα 3.3 mag), suggesting that on average, H II regions remain embedded for 1 Myr. Finally, we find the median 33 GHz continuum-to-Hα line flux ratio to be statistically larger within r G <250 pc relative to the outer disk regions by a factor of 1.82±0.39, while the ratio of 33 GHz to 24 μm flux densities is lower by a factor of 0.45±0.08, which may suggest increased extinction in the central regions.

The Star Formation in Radio Survey: 3–33 GHz Imaging of Nearby Galaxy Nuclei and Extranuclear Star-forming Regions

“…This demonstrates that we can reliably use the 33 GHz flux density to infer the total free-free emission, and thus current star formation activity, on the scales of individual H II and star-forming regions. While this result had been suggested by our previous GBT and VLA campaigns (e.g., Murphy et al 2011Murphy et al , 2012Murphy et al , 2018a, this is the first measurement of the 33 GHz thermal fraction based on the shape of the radio spectrum at these frequencies and spatial scales in nearby galaxies. Further, by restricting our analysis such that we remove all non-SF regions and all apertures containing multiple smaller individual regions and require r G 250 pc (238 regions), we mitigate contamination from any central AGN and can determine broad population statistics for extragalactic H II regions using our cleanest sample of extranuclear star-forming regions.…”

LeMMINGs – II. The e-MERLIN legacy survey of nearby galaxies. The deepest radio view of the Palomar sample on parsec scale

“…The clearest examples are the well-known galaxies/systems M 82 and Arp 299. Only three sources (NGC 4013,NGC 4102,and NGC 5273) clearly exhibit diffuse low-brightness radio emission, consistent with a stellar disc or ring, similar to what is expected from diffuse extended star forming galaxies (Muxlow et al 2010;Murphy et al 2018;Herrero-Illana et al 2017). Single radio components were detected at the centre of most of Hii galaxies.…”

Paβ, Hα, and Attenuation in NGC 5194 and NGC 6946

“…These values may be biased somewhat high by our selection of bright apertures, but as we saw above that they agree well with past results. This distribution also appears consistent with measurements by Murphy et al (2018). They found a median A(Hα) value of 1.26 ± 0.09 mag with scatter of 0.87 mag for 162 pointings towards star forming regions across 56 nearby galaxies (with measurements on 30-300 pc scales).…”