Spitzer Survey of the Large Magellanic Cloud, Surveying the Agents of a Galaxy's Evolution (SAGE). IV. Dust Properties in the Interstellar Medium

doi:10.1088/0004-6256/136/3/919

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Jean-Philippe Bernard et al. 2008 The Astronomical Journal 136 919 doi:10.1088/0004-6256/136/3/919

SPITZER SURVEY OF THE LARGE MAGELLANIC CLOUD, SURVEYING THE AGENTS OF A GALAXY'S EVOLUTION (SAGE). IV. DUST PROPERTIES IN THE INTERSTELLAR MEDIUM

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Jean-Philippe Bernard¹, William T. Reach², Deborah Paradis¹, Margaret Meixner³, Roberta Paladini², Akiko Kawamura⁴, Toshikazu Onishi⁴, Uma Vijh³, Karl Gordon⁵, Remy Indebetouw⁶, Joseph L. Hora⁷, Barbara Whitney⁸, Robert Blum⁹, Marilyn Meade¹⁰, Brian Babler¹⁰, Ed B. Churchwell¹⁰, Charles W. Engelbracht¹¹, Bi-Qing For¹¹, Karl Misselt¹¹, Claus Leitherer³, Martin Cohen¹², François Boulanger¹³, Jay A. Frogel¹⁴, Yasuo Fukui⁴, Jay Gallagher⁸, Varoujan Gorjian¹⁵, Jason Harris¹¹, Douglas Kelly¹¹, William B. Latter², Suzanne Madden¹⁶, Ciska Markwick-Kemper⁶, Akira Mizuno⁴, Norikazu Mizuno⁴, Jeremy Mould¹⁷, Antonella Nota³, M. S. Oey¹⁸, Knut Olsen¹¹, Nino Panagia³, Pablo Perez-Gonzalez¹¹, Hiroshi Shibai⁴, Shuji Sato⁴, Linda Smith¹⁹, Lister Staveley-Smith²⁰, A. G. G. M. Tielens²¹, Toshiya Ueta²¹, Schuyler Van Dyk², Kevin Volk²², Michael Werner¹⁵ and Dennis Zaritsky¹¹

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¹ CESR, Toulouse, France
² Spitzer Science Center, Caltech, MS 220-6, Pasadena, CA 91125, USA
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁴ Department of Astrophysics, Nagoya University, Chikusa-Ku, Nagoya 464-01, Japan
⁵ Steward Observatory, University of Arizona, 933 North Cherry Ave., Tucson, AZ 85721, USA
⁶ Department of Astronomy, University of Virginia, P.O. Box 3818, Charlottesville, VA 22903-0818, USA
⁷ Harvard-Smithsonian, CfA, 60 Garden St., MS 65, Cambridge, MA 02138-1516, USA
⁸ Space Science Institute, 4750 Walnut St., Suite 205, Boulder, CO 80301, USA
⁹ Cerro Tololo Interamerican Observatory, Casilla 603, la Serena, Chile
¹⁰ Department of Astronomy, University of Wisconsin-Madison, 475 N. Charter St., Madison, WI 53706, USA
¹¹ Steward Observatory, University of Arizona, 933 North Cherry Ave., Tucson, AZ 85721, USA
¹² University of California, Berkeley, Radio Astronomy Lab., 601 Campbell Hall, Berkeley, CA 94720-3411, USA
¹³ Institut d'Astrophsique Spatiale, Université Paris XI, Orsay, France
¹⁴ AURA, Inc., 1200 New York Avenue, NW, Suite 350, Washington, DC, 20005, USA
¹⁵ Jet Propulsion Laboratory, 4800 Oak Grove Blvd., MS 169-327, Pasadena, CA 91109, USA
¹⁶ Service d'Astrophysique, CEA, Saclay, 91191 Gif-Sur-Yvette Cedex, France
¹⁷ National Optical Astronomy Observatory, 950 N. Cherry Ave., Tucson, AZ 85726-6732, USA
¹⁸ Department of Astronomy, University of Michigan, 830 Dennison Bldg., Ann Arbor, MI 48109, USA
¹⁹ Department of Physics and Astronomy, University College London, Gower St., London WC1E 6BT, UK
²⁰ Commonwealth Science and Industrial Research Organization (CSIRO), Head Office, GPO Box 4908, Melbourne, VIC 3001 Australia
²¹ NASA Ames Research Center, Mail Stop 211-3, SOFIA, Moffett Field, CA 94035, USA
²² Gemini Observatory, Northern Operations Center, 670 N. Aohuku Place, Hilo, HI 96720, USA

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Abstract References Cited By

The goal of this paper is to present the results of a preliminary analysis of the extended infrared (IR) emission by dust in the interstellar medium (ISM) of the Large Magellanic Cloud (LMC). We combine Spitzer Surveying the Agents of Galaxy Evolution (SAGE) and Infrared Astronomical Satellite (IRAS) data and correlate the infrared emission with gas tracers of H I, CO, and H_α. We present a global analysis of the infrared emission as well as detailed modeling of the spectral energy distribution (SED) of a few selected regions. Extended emission by dust associated with the neutral, molecular, and diffuse ionized phases of the ISM is detected at all IR bands from 3.6 μm to 160 μm. The relative abundance of the various dust species appears quite similar to that in the Milky Way (MW) in all the regions we have modeled. We construct maps of the temperature of large dust grains. The temperature map shows variations in the range 12.1-34.7 K, with a systematic gradient from the inner to outer regions, tracing the general distribution of massive stars and individual H II regions as well as showing warmer dust in the stellar bar. This map is used to derive the far-infrared (FIR) optical depth of large dust grains. We find two main departures in the LMC with respect to expectations based on the MW: (1) excess mid-infrared (MIR) emission near 70 μm, referred to as the 70 μm excess, and (2) departures from linear correlation between the FIR optical depth and the gas column density, which we refer to as FIR excess emission. The 70 μm excess increases gradually from the MW to the LMC to the Small Magellanic Cloud (SMC), suggesting evolution with decreasing metallicity. The excess is associated with the neutral and diffuse ionized gas, with the strongest excess region located in a loop structure next to 30 Dor. We show that the 70 μm excess can be explained by a modification of the size distribution of very small grains with respect to that in the MW, and a corresponding mass increase of sime 13% of the total dust mass in selected regions. The most likely explanation is that the 70 μm excess is due to the production of large very small grains (VSG) through erosion of larger grains in the diffuse medium. This FIR excess could be due to intrinsic variations of the dust/gas ratio, which would then vary from 4.6 to 2.3 times lower than the MW values across the LMC, but X _CO values derived from the IR emission would then be about three times lower than those derived from the Virial analysis of the CO data. We also investigate the possibility that the FIR excess is associated with an additional gas component undetected in the available gas tracers. Assuming a constant dust abundance in all ISM phases, the additional gas component would have twice the known H I mass. We show that it is plausible that the FIR excess is due to cold atomic gas that is optically thick in the 21 cm line, while the contribution by a pure H₂ phase with no CO emission remains a possible explanation.

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SPITZER SURVEY OF THE LARGE MAGELLANIC CLOUD, SURVEYING THE AGENTS OF A GALAXY'S EVOLUTION (SAGE). IV. DUST PROPERTIES IN THE INTERSTELLAR MEDIUM

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