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

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