A Green Bank Telescope Survey of Large Galactic H ii Regions

As part of our ongoing H ii Region Discovery Survey (HRDS), we report the Green Bank Telescope detection of 148 new angularly large Galactic H ii regions in radio recombination line (RRL) emission. Our targets are located at a declination of , which corresponds to at . All sources were selected from the Wide-field Infrared Survey Explorer Catalog of Galactic H ii Regions, and have infrared angular diameters . The Galactic distribution of these “large” H ii regions is similar to that of the previously known sample of Galactic H ii regions. The large H ii region RRL line width and peak line intensity distributions are skewed toward lower values, compared with that of previous HRDS surveys. We discover seven sources with extremely narrow RRLs . If half the line width is due to turbulence, these seven sources have thermal plasma temperatures . These temperatures are lower than any measured for Galactic H ii regions, and the narrow-line components may arise instead from partially ionized zones in the H ii region photodissociation regions. We discover G039.515+00.511, one of the most luminous H ii regions in the Galaxy. We also detect the RRL emission from three H ii regions with diameters , making them some of the physically largest known H ii regions in the Galaxy. This survey completes the HRDS H ii region census in the Northern sky, where we have discovered 887 H ii regions and more than doubled the size of the previously known census of Galactic H ii regions.