Tom Bania (Department of Astronomy, Boston University)

The GBT H II Region Discovery Survey - Tom Bania Colloquium

ATNF Marsfield Lecture Theatre

Abstract

We discovered hundreds of previously unknown Milky Way HII regions by using the Green Bank Telescope to detect their hydrogen radio recombination line emission. Since the interstellar medium is optically thin at 3 cm wavelength, we can detect HII regions across the entire Galactic disk. Our targets were selected based on spatially coincident mid-infrared and 20 cm radio continuum emission.

Such sources are almost invariably HII regions; we detected hydrogen
RRL emission from 95% of our target sample. These new HRDS nebulae share the same distribution on the sky as does the previously known census of Galactic HII regions. On average, however, the new nebulae have fainter continuum fluxes, smaller continuum angular sizes, fainter RRL intensities and smaller RRL line widths. Though small in angular size, many of our new nebulae show little spatial correlation with tracers associated with extremely young HII regions, implying
that our sample spans a range of evolutionary states.

Because we can detect all nebulae inside the Solar orbit that are ionized by O- stars, the HRDS sources, when combined with existing HII region catalogs, give a more accurate census of Galactic HII regions and their properties. The distribution of HII 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 distri-
bution of HII 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.

The Galactic census of massive star forming regions remains incomplete, however, because the most recent large radio recombination line survey of the Southern Sky was made in 1987 by Caswell and Haynes. The HRDS shows that there are hundreds of nebulae waiting to be discovered.