Untangling the Recombination Line Emission from HII Regions with Multiple Velocity Components

Abstract

HII regions are the ionized spheres surrounding high-mass stars. They are ideal targets for tracing Galactic structure because they are predominantly found in spiral arms and have high luminosities at infrared and radio wavelengths. In the Green Bank Telescope HII Region Discovery Survey (GBT HRDS) we found that >30% of first Galactic quadrant HII regions have multiple hydrogen radio recombination line (RRL) velocities, which makes determining their Galactic locations and physical properties impossible. Here we make additional GBT RRL observations to determine the discrete HII region velocity for all 117 multiple-velocity sources within 18deg. < l < 65deg. The multiple-velocity sources are concentrated in the zone 22deg. < l < 32deg., coinciding with the largest regions of massive star formation, which implies that the diffuse emission is caused by leaked ionizing photons. We combine our observations with analyses of the electron temperature, molecular gas, and carbon recombination lines to determine the source velocities for 103 discrete H II regions (88% of the sample). With the source velocities known, we resolve the kinematic distance ambiguity for 47 regions, and thus determine their heliocentric distances.

44 pages, 5 figures, 16 pages of tables; Accepted by ApJS

References in corpus (9)

• The Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey
• The VLA Galactic Plane Survey
• The WISE Catalog of Galactic HII Regions
• Resolution of the Distance Ambiguity for Galactic HII Regions
• The Electron Temperature Gradient in the Galactic Disk
• The HIPASS survey of the Galactic plane in Radio Recombination Lines
• The Chemical Evolution of Helium
• The Molecular Properties of Galactic HII Regions
• Mopra CO Observations of the Bubble HII Region RCW120

Cited by in corpus (14)

• Finding Distant Galactic HII Regions
• Metallicity Structure in the Milky Way Disk Revealed by Galactic HII Regions
• A Green Bank Telescope Survey of Large Galactic HII Regions
• HII Region Ionization of the Interstellar Medium: A Case Study of NGC 7538
• The GBT Diffuse Ionized Gas Survey (GDIGS): Survey Overview and First Data Release
• Ionization Profiles of Galactic HII Regions
• Large-scale Map of Millimeter-wavelength Hydrogen Radio Recombination Lines around a Young Massive Star Cluster
• Peering into the Milky Way by FAST: II. Ionized gas in the inner Galactic disk revealed by the piggyback line observations of the FAST GPPS survey
• 44 GHz methanol masers: Observations toward 95 GHz methanol masers
• A global view on star formation: The GLOSTAR Galactic plane survey X. Galactic HII region catalog using radio recombination lines
• CO observations toward the isolated mid-infrared bubble S44: External triggering of O-star formation by a cloud-cloud collision
• The GBT Diffuse Ionized Gas Survey: Tracing the Diffuse Ionized Gas around the Giant HII Region W43
• Diffuse Ionized Gas in the Milky Way Disk
• Spatially-resolved Galactic HII regions observed by LAMOST medium-Resolution Spectroscopic Survey of Nebulae (LAMOST MRS-N)