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

J.-P. Bernard, W. T. Reach, D. Paradis, M. Meixner, R. Paladini, A. Kawamura, T. Onishi, U. Vijh, K. Gordon, R. Indebetouw, J. L. Hora, B. Whitney, R. Blum, M. Meade, B. Babler, E. B. Churchwell, C. W. Engelbracht, B.-Q. For, K. Misselt, C. Leitherer, M. Cohen, F. Boulanger, J. A. Frogel, Y. Fukui, J. Gallagher, V. Gorjian, J. Harris, D. Kelly, W. B. Latter, S. Madden, C. Markwick-Kemper, A. Mizuno, N. Mizuno, J. Mould, A. Nota, M. S. Oey, K. Olsen, N. Panagia, P. Perez-Gonzalez, H. Shibai, S. Sato, L. Smith, L. Staveley-Smith, A. G. G. M. Tielens, T. Ueta, S. Van Dyk, K. Volk, M. Werner, & D. Zaritsky 2008, AJ, 136, 919

The goal of this paper is to present the results of a preliminary analysis of the extended infrared emission by dust in the ISM of the LMC. We combine Spitzer SAGE and IRAS data and correlate the infrared emission with gas tracers of HI, CO, and H-alpha. We present a global analysis of the infrared emission as well as detailed modeling of the 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 micron to 160 micron. The relative abundance of the various dust species appears quite similar to those in the Mily 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 to 34.7 K, with a systematic gradient from the inner to outer regions, tracing the general distribution of massive stars and individual HII regions as well as showing warmer dust in the stellar bar. This map is used to derive the 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 emission near 70 micron, referred to as the 70 micron 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 micron excess increases gradually from the MW to the LMC to the 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 micron 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 approx. 13% of the total dust mass in selected regions. The most likely explanation is that the 70 micron excess is due to the production of large VSGs 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 XCO values derived from the IR emission would then be about 3 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 HI mass. We show it is plausible that the FIR excess is due to cold atomic gas that is optically thick in the 21-cm line, while contribution by a pure H2 phase with no CO emission remains a possible explanation.

[ADS Entry]

ADS Citation Query
# citations = 33
average # citations/year = 11.00 (# years = 3)
# citations vs. year [year=citations]
[2010=16] [2009=15] [2008=2]
[determined from ADS on 10 Sep 2010]