A VLA Census of the Galactic H II Region Population

Abstract

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 ∼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, 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 Wide-field Infrared Survey Explorer Catalog of Galactic H ii regions between 245° ≥ ℓ ≥ 90° with infrared diameters less than 80′′. 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 H ii 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 ∼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.