The Incidence of Highly-Obscured Star-Forming Regions in SINGS Galaxies - AstrophysicsReport as inadecuate

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Abstract: Using the new capabilities of the Spitzer Space Telescope and extensivemultiwavelength data from the Spitzer Infrared Nearby Galaxies Survey SINGS,it is now possible to study the infrared properties of star formation in nearbygalaxies down to scales equivalent to large HII regions. We are therefore ableto determine what fraction of large, infrared-selected star-forming regions innormal galaxies are highly obscured and address how much of the star formationwe miss by relying solely on the optical portion of the spectrum. Employing anew empirical method for deriving attenuations of infrared-selectedstar-forming regions we investigate the statistics of obscured star formationon 500pc scales in a sample of 38 nearby galaxies. We find that the medianattenuation is 1.4 magnitudes in H-alpha and that there is no evidence for asubstantial sub-population of uniformly highly-obscured star-forming regions.The regions in the highly-obscured tail of the attenuation distributionA H-alpha > 3 make up only ~4% of the sample of nearly 1800 regions, thoughvery embedded infrared sources on the much smaller scales and lowerluminosities of compact and ultracompact HII regions are almost certainlypresent in greater numbers. The highly-obscured cases in our sample aregenerally the bright, central regions of galaxies with high overall attenuationbut are not otherwise remarkable. We also find that a majority of the galaxiesshow decreasing radial trends in H-alpha attenuation. The small fraction ofhighly-obscured regions seen in this sample of normal, star-forming galaxiessuggests that on 500pc scales the timescale for significant dispersal or breakup of nearby, optically-thick dust clouds is short relative to the lifetime ofa typical star-forming region.

Author: Moire K. M. Prescott 1, Robert C. Kennicutt Jr. 2 and 1, George J. Bendo 3 and 1, Brent A. Buckalew 4, Daniela Calzetti 6 and 5,


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