A VLA Census of the Galactic H II Region Population

Abstract

The Milky Way contains thousands of H II region candidates identified by their characteristic mid-infrared morphology, but lacking detections of ionized gas tracers such as radio continuum or radio recombination line emission. These targets thus remain unconfirmed as H II regions. With only $\sim$2500 confirmed H II regions in the Milky Way, Galactic surveys are deficient by several thousand nebulae when compared to external galaxies with similar star formation rates. Using sensitive 9 GHz radio continuum observations with the Karl G. Jansky Very Large Array (VLA), we explore a sample of H II region candidates in order to set observational limits on the actual total population of Galactic H II regions. We target all infrared-identified "radio quiet" sources from the WISE Catalog of Galactic H II regions between $245^{\circ }\ge \ell \ge 90^{\circ }$ with infrared diameters less than 80${}^{\prime \prime }$. We detect radio continuum emission from 50% of the targeted H II region candidates, providing strong evidence that most of the radio quiet candidates are bona fide HII regions. We measure the peak and integrated radio flux densities and compare the inferred Lyman continuum fluxes using models of OB-stars. We conclude that stars of approximately spectral type B2 and earlier are able to create H II regions with similar infrared and radio continuum morphologies as the more luminous H II regions created by O-stars. From our 50% detection rate of "radio quiet" sources, we set a lower limit of $\sim$7000 for the H II region population of the Galaxy. Thus the vast majority of the Milky Way's H II regions remain to be discovered.

11 pages of text, 146 infrared/radio images of faint HII region candidates

References in corpus (7)

• Trigonometric Parallaxes Of High-Mass Star Forming Regions: Our View Of The Milky Way
• The VLA Galactic Plane Survey
• Metallicity Structure in the Milky Way Disk Revealed by Galactic HII Regions
• GLOSTAR -- Radio Source Catalog I: $28^{\circ }<\text{l}<36^{\circ }$ and $|b|<1^{\circ }$
• The Southern HII Region Discovery Survey I: The Bright Catalog
• The Infrared and Radio Fluxes Densities of Galactic HII Regions
• The Milky Way Project: What are Yellowballs?

Cited by in corpus (1)

• The Southern HII Region Discovery Survey. II. The Full Catalog