The Green Bank Telescope Galactic H II Region Discovery Survey

Abstract: We discovered a large population of previously unknown Galactic H II regions by using the Green Bank Telescope to detect their hydrogen radio recombination line emission. Since recombination lines are optically thin at 3 cm wavelength, we can detect H II regions across the entire Galactic disk. Our targets were selected based on spatially coincident 24 micron and 21 cm continuum emission. For the Galactic zone -16 deg < L_gal < 67 deg and abs(B_gal) < 1 deg, we detected 602 discrete recombination line components from 448 lines of sight, 95% of the sample targets, which more than doubles the number of known H II regions in this part of the Milky Way. We found 25 new first quadrant nebulae with negative LSR velocities, placing them beyond the Solar orbit. Because we can detect all nebulae inside the Solar orbit that are ionized by O-stars, the Discovery Survey targets, when combined with existing H II region catalogs, give a more accurate census of Galactic H II regions and their properties. The distribution of H II regions across the Galactic disk shows strong, narrow (~ 1 kpc wide) peaks at Galactic radii of 4.3 and 6.0 kpc. The longitude-velocity distribution of H II regions now gives unambiguous evidence for Galactic structure, including the kinematic signatures of the radial peaks in the spatial distribution, a concentration of nebulae at the end of the Galactic Bar, and nebulae located on the kinematic locus of the 3 Kpc Arm.