Bernard, Jean-Philippe and Reach, William T. and Paradis, Deborah and Meixner, Margaret and Paladini, Roberta and Kawamura, Akiko and Onishi, Toshikazu and Vijh, Uma and Gordon, Karl and Indebetouw, Remy and Hora, Joseph L. and Whitney, Barbara and Blum, Robert and Meade, Marilyn and Bable, Brian and Churchwell, Ed B. and Engelbracht, Charles W. and For, Bi-Qing and Misselt, Karl and Leitherer, Claus and Cohen, Martin and Boulanger, François and Frogel, Jay A. and Fukui, Yasuo and Gallagher, Jay and Gorjian, Varoujan and Harris, Jason and Kelly, Douglas and Latter, William B. and Madden, Suzanne and Markwick-Kemper, Ciska and Mizuno, Akira and Mizuno, Norikazu and Mould, Jeremy and Nota, Antonella and Oey, M. S. and Olsen, Knut and Panagia, Nino and Perez-Gonzalez, Pablo and Shibai, Hiroshi and Sato, Shuji and Smith, Linda and Staveley-Smith, Lister and Tielens, A. G. G. M. and Ueta, Toshiya and Van Dyk, Schuyler and Volk, Kevin and Werner, Michael and Zaritsky, Dennis (2008) Spitzer survey of the Large Magellanic Cloud, surveying the agents of a galaxy's evolution (SAGE). IV. Dust properties in the interstellar medium. Astronomical Journal, 136 (3). pp. 919-945. ISSN 0004-6256 http://resolver.caltech.edu/CaltechAUTHORS:20090407-074711707
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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 Hα. 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 ≃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 H_2 phase with no CO emission remains a possible explanation.
|Additional Information:||© 2008 The American Astronomical Society. Received 2007 November 8; accepted 2008 April 28; published 2008 July 21. We are grateful for E. Dwek who provided the MW SED of the diffuse emission at high galactic latitudes.|
|Subject Keywords:||galaxies: ISM; infrared: galaxies; infrared: ISM; ISM: abundances; ISM: clouds magellanic clouds|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Tony Diaz|
|Deposited On:||26 Jun 2009 16:57|
|Last Modified:||26 Dec 2012 10:55|
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