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Issue A&A
Volume 435, Number 1, May III 2005
Page(s) 131 - 139
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20041767



A&A 435, 131-139 (2005)
DOI: 10.1051/0004-6361:20041767

Large-scale variations of the dust optical properties in the Galaxy

L. Cambrésy1, T. H. Jarrett2 and C. A. Beichman3

1  Observatoire Astronomique de Strasbourg, 67000 Strasbourg, France
    e-mail: cambresy@astro.u-strasbg.fr
2  Infrared Processing and Analysis Center, 100-22, California Institute of Technology, CA 91125, USA
    e-mail: jarrett@ipac.caltech.edu
3  Michelson Science Center, Infrared Processing and Analysis Center, 100-22, California Institute of Technology, CA 91125, USA
    e-mail: chas@ipac.caltech.edu

(Received 2 August 2004 / Accepted 11 January 2005 )

Abstract
We present an analysis of the dust optical properties at large scale, for the whole galactic anticenter hemisphere. We used the 2MASS Extended Source Catalog to obtain the total reddening on each galaxy line of sight and we compared this value to the IRAS 100 $\mu$m surface brightness converted to extinction by Schlegel et al. (1998, ApJ, 500, 525). We performed a careful examination and correction of the possible systematic effects resulting from foreground star contamination, redshift contribution and galaxy selection bias. We also evaluated the contribution of dust temperature variations and interstellar clumpiness to our method. The correlation of the near-infrared extinction to the far-infrared optical depth shows a discrepancy for visual extinction greater than 1 mag with a ratio $A_{\rm V}({\rm FIR}) / A_{\rm V}({\rm gal}) = 1.31$ $\pm$ 0.06. We attribute this result to the presence of fluffy/composite grains characterized by an enhanced far-infrared emissivity. Our analysis, applied to half of the sky, provides new insights on the dust grains nature suggesting fluffy grains are found not only in some very specific regions but in all directions for which the visual extinction reaches about 1 mag.


Key words: ISM: dust, extinction -- galaxies: photometry -- infrared: ISM

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