A Green Bank Telescope Survey of Large Galactic HII Regions
  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.
