First Results from the Green Bank Telescope HII Region Discovery Survey

Details First Results from the Green Bank Telescope HII Region Discovery Survey First Results from the Green Bank Telescope HII Region Discovery Survey

May 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010aas...21642402a&link_type=abstract

American Astronomical Society, AAS Meeting #216, #424.02; Bulletin of the American Astronomical Society, Vol. 41, p.836

Astronomy and Astrophysics

Astronomy

Scientific paper

The Green Bank Telescope HII Region Discovery Survey has doubled the number of known HII regions in the Galactic zone -16° < l < 67° and |b|<1°. We detected 602 discrete Hydrogen radio recombination line (RRL) components from 448 targets, including almost 50 infrared bubbles found in the Spitzer GLIMPSE survey. Our targets were selected based on spatially coincident 24µm (Spitzer/MIPSGAL) and 20cm (either from the VLA Galactic Plane Survey or the NRAO NVSS) continuum emission. Warm dust absorbs the FUV radiation from the exciting OB star(s) and re-emits in the mid-IR, whereas the HII region plasma ionized by these same stars gives rise to free-free thermal emission at centimeter wavelengths. Sources showing coincident mid-IR and cm-wave continuum emission are almost invariably HII regions: 95% of our sample targets show hydrogen recombination line radiation. The sensitivity of the GBT and the power of its ACS spectrometer together made this survey possible. Because we could tune simultaneously to seven consecutive Hydrogen alpha transitions, H87α to H93α, at two polarizations each, we could average these independent spectra together to search for a RRL signal. Each discovery gives a position and LSR velocity for the nebula. We plan to determine the distance to our sources by using HI emission/absorption experiments to resolve the kinematic distance ambiguity. The vast majority of sources are unresolved with our 85" beam. We are finding that many of these nebulae lie at the far kinematic distance. This suggests that classifying small angular diameter nebulae as "ultra-compact" is an error; they are small because they lie at great distances, not because they are in early evolutionary stages. Here we show some initial results from the survey and compare these new nebulae with the sample of previously known HII regions.

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