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

Accepted version
Peer-reviewed

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Authors

Murphy, EJ 
Dong, D 
Momjian, E 
Linden, S 
R C Kennicutt, Jr 

Abstract

We present 33 GHz imaging for 112 pointings towards galaxy nuclei and extranuclear star-forming regions at $\approx$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 HII 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 rG≥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 indicate a median extinction value on ≈30−300 pc scales of AHα≈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., AHα≳3.3 mag), suggesting that on average HII regions remain embedded for ≲1 Myr. Finally, we find the median 33 GHz continuum-to-Hα line flux ratio to be statistically larger within rG<250 pc relative the outer-disk regions by a factor of 1.82±0.39, while the ratio of 33 GHz-to-24 μm flux densities are lower by a factor of 0.45±0.08, which may suggest increased extinction in the central regions.

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Keywords

galaxies: nuclei, H II regions, radio continuum: general, stars: formation

Journal Title

The Astrophysical Journal

Conference Name

Journal ISSN

0067-0049
1538-4365

Volume Title

234

Publisher

American Astronomical Society
Sponsorship
Science and Technology Facilities Council (ST/N000927/1)
E.J.M. acknowledges the hospitality of the Aspen Center for Physics, which is supported by National Science Foundation grant No. PHY-1066293. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This research made use of APLpy, an open-source plotting package for Python hosted at http://aplpy.github.com.