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

TL;DR

This study uses radio observations to identify discrete velocities of HII regions with multiple components, resolving their distances and improving understanding of Galactic structure and star formation zones.

Contribution

It provides a comprehensive method to determine velocities and distances for complex HII regions with multiple velocity components, enhancing Galactic mapping accuracy.

Findings

Identified source velocities for 103 HII regions.

Resolved distance ambiguities for 47 regions.

Found concentration of multiple-velocity sources in star-forming zones.

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.