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Date and Time of the Query: 2019-06-19 T10:20:00 PDT
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For refcode 2001AJ....121.2095Q:
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2001AJ....121.2095Q A COMPARISON BETWEEN Pa{alpha} AND H{alpha} EMISSION: THE RELATION BETWEEN MEAN H II REGION REDDENING, LOCAL GAS DENSITY, AND METALLICITY. ALICE C. QUILLEN AND MIHOKO YUKITA Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065; aquillen@as.arizona.edu Received 2000 August 28; accepted 2000 December 20 ABSTRACT We measure reddenings to H II regions in NGC 1512, 2903, 4449, and 6946, and M51 from Hubble Space Telescope (HST) Pa{alpha} and H{alpha} images. Extinctions range from A_V_ ~ 5 to 0 depending upon the galaxy. For the galaxies with HST images in both lines, NGC 1512, NGC 2903, and M51, the Pa{alpha} and H{alpha} emission are almost identical in morphology, which implies that little emission from bright H II regions is hidden from view by regions of comparatively high extinction. The scatter in the measured extinctions in each galaxy is only +/-0.5 mag. We compare the reddenings we measure in five galaxies using the Pa{alpha}-to-H{alpha} ratios to those measured previously from the Balmer decrement in the Large Magellanic Cloud and as a function of radius in M101 and M51. We find that luminosity- weighted mean extinctions of these ensembles of H II regions are correlated with gas surface density and metallicity. The correlation is consistent with the mean extinction depending on dust density, where the dust-to-gas mass ratio scales with the metallicity. This trend is expected if H II regions tend to be located near the midplane of a gas disk and emerge from their parent molecular clouds soon after birth. In environments with gas densities below a few hundred solar masses per square parsec, star formation rates estimated from integrated line fluxes and mean extinctions are likely to be fairly accurate. Key words: dust, extinction-galaxies: ISM-H II regions -infrared radiation
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