A VLA Census of the Galactic H II Region Population
William P. Armentrout, Loren D. Anderson, Trey V. Wenger, Dana S., Balser, and Thomas M. Bania

TL;DR
This study uses sensitive VLA radio observations to confirm and estimate the total number of Galactic H II regions, revealing that many remain undiscovered and that most candidates are genuine H II regions.
Contribution
It provides the first large-scale radio survey of infrared-identified H II region candidates, significantly increasing the estimated total Galactic H II region population.
Findings
50% of targeted candidates emit detectable radio continuum
Stars of spectral type B2 or earlier can create observable H II regions
The total H II region population in the Milky Way is at least 7000
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 (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 with infrared diameters less than…
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