Interstellar Extinction in the Vicinity of the Galactic Center

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Abstract
We present J (1.2 μm), H (1.6 μm), K' (2.11 μm), and Brγ (2.166 μm) images from four large regions within the central 40 pc of the Galaxy. Localized variations in the extinction, as determined by observations of the stellar population, are examined using the median H-K' color as a function of position within each region. As the value of the derived extinction from the stars is dependent upon the intrinsic magnitude of the assumed stellar type, the J-H versus H-K' diagrams are first used to investigate the distribution of stellar types in the four regions. We find that there is a distinct OB population, contrary to earlier assumptions, with the ratio of K and M giants and supergiants to OB stars approximately twice that of the solar neighborhood. Although on the scale of ~1' fluctuations in the extinction are on the order of AV ~ 2, throughout the entire region the extinction varies from AV 25 to AV 40. We also examine whether there is any variation in the extinction and stellar number density relative to the unusual radio features in these regions and do not find a significant correlation. Spectral imaging in Brγ 2.166 μm emission shows a strong morphological correspondence between the 6 cm radio images and the diffuse Brγ emission. By comparing the theoretical Brγ flux derived from the radio flux using recombination theory with our measured Brγ flux, we obtain a second, independent, estimate of the extinction. We compare the two data sets and find that the extinction as derived from the stars is consistently greater, sometimes by a factor of 2, than the value of the extinction derived from the Brγ images. The derived extinction in various regions, however, is insufficient for any of these regions to be located behind the molecular clouds as previously observed in the Galactic center, consistent with the theory that the observed radio emission is produced on the foreground surface of these clouds.

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