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.

arXiv comment11 pages of text, 146 infrared/radio images of faint HII region candidates

References in corpus (31)

• Astropy: A Community Python Package for Astronomy
• The Astropy Project: Building an inclusive, open-science project and status of the v2.0 core package
• A new calibration of stellar parameters of Galactic O stars
• Trigonometric Parallaxes Of High-Mass Star Forming Regions: Our View Of The Milky Way
• KINGFISH -- Key Insights on Nearby Galaxies: A Far-Infrared Survey with Herschel: Survey Description and Image Atlas
• The VLA Galactic Plane Survey
• The WISE Catalog of Galactic HII Regions
• Realistic Ionizing Fluxes for Young Stellar Populations from 0.05 to twice solar metallicity
• Kinematics in Young Star Clusters and Associations with Gaia DR2
• A spectroscopic survey on the multiplicity of high-mass stars
• Toward a Unification of Star Formation Rate Determinations in the Milky Way and Other Galaxies
• The Green Bank Telescope HII Region Discovery Survey II. The Source Catalog
• The Milky Way Project First Data Release: A Bubblier Galactic Disk
• A Molecular Spiral Arm in the Far Outer Galaxy
• Finding Distant Galactic HII Regions
• The Green Bank Telescope Galactic H II Region Discovery Survey
• Massive Field Stars and the Stellar Clustering Law
• A search for Galactic runaway stars using Gaia Data Release 1 and Hipparcos proper motions
• Metallicity Structure in the Milky Way Disk Revealed by Galactic HII Regions
• Deep Very Large Array Radio Continuum Surveys of GLIMPSE Extended Green Objects (EGOs)
• NGC628 with SITELLE : I. Imaging Spectroscopy of 4285 HII region candidates
• A Green Bank Telescope Survey of Large Galactic HII Regions
• GLOSTAR -- Radio Source Catalog I: $28^{\circ }<\text{l}<36^{\circ }$ and $|b|<1^{\circ }$
• Continuum sources from the THOR survey between 1 and 2 GHz
• Weak and Compact Radio Emission in Early High-Mass Star Forming Regions: II. The Nature of the Radio Sources
• Dynamics of H II regions around exiled O stars
• The Southern HII Region Discovery Survey I: The Bright Catalog
• The Infrared and Radio Fluxes Densities of Galactic HII Regions
• An All-Sky Sample of Intermediate-Mass Star-Forming Regions
• The Milky Way Project: What are Yellowballs?
• High-Mass Star Formation in the Outer Scutum-Centaurus Arm

Cited by in corpus (1)

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