A Green Bank Telescope Survey of Large Galactic HII Regions

TL;DR

This survey using the Green Bank Telescope identified 148 new large Galactic HII regions via radio recombination lines, revealing extremely narrow lines and some of the largest known HII regions, thus significantly expanding the Galactic HII region catalog.

Contribution

It reports the discovery of 148 new large HII regions in the Northern sky, including some of the most luminous and largest known, using radio recombination line observations from the Green Bank Telescope.

Findings

Discovery of 148 new large HII regions.

Identification of 7 sources with extremely narrow RRLs <10 km/s.

Detection of some of the largest and most luminous HII regions in the Galaxy.

Abstract

As part of our ongoing HII Region Discovery Survey (HRDS), we report the Green Bank Telescope detection of 148 new angularly-large Galactic HII regions in radio recombination line (RRL) emission. Our targets are located at a declination greater than -45deg., which corresponds to 266deg. > l > -20deg. at b = 0deg. All sources were selected from the WISE Catalog of Galactic HII Regions, and have infrared angular diameters >260''. The Galactic distribution of these "large" HII regions is similar to that of the previously-known sample of Galactic HII regions. The large HII region RRL line width and peak line intensity distributions are skewed toward lower values compared with that of previous HRDS surveys. We discover 7 sources with extremely narrow RRLs <10 km/s. If half the line width is due to turbulence, these 7 sources have thermal plasma temperatures <1100 K. These temperatures are lower than any measured for Galactic HII regions, and the narrow line components may arise instead from partially ionized zones in the HII region photo-dissociation regions. We discover G039.515+00.511, one of the most luminous HII regions in the Galaxy. We also detect the RRL emission from three HII regions with diameters >100 pc, making them some of the physically largest known HII regions in the Galaxy. This survey completes the HRDS HII region census in the Northern sky, where we have discovered 887 HII regions and more than doubled the previously-known census of Galactic HII regions.