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Paper: Dust, Atomic, and Molecular Gas in the Nearest Primitive Environment
Volume: 381, The Second Annual Spitzer Science Center Conference: Infrared Diagnostics of Galaxy Evolution
Page: 173
Authors: Leroy, A.; Bolatto, A.D.; Stanimirovic, S.; Sandstrom, K.; Simon, J.D.; Bot, C.; Shah, R.; Jackson, J.M.
Abstract: We present ongoing work studying the dust and gas in the Small Magellanic Cloud (SMC). This work is part of the Spitzer Survey of the Small Magellanic Cloud, S3MC. We combine new IRAC and MIPS observations of the SMC with existing far infrared (FIR) observations from DIRBE, IRAS, and ISO to construct a complete picture of the infrared spectral energy distribution (SED). We compare the FIR SED of the SMC to the SEDs of other nearby galaxies as measured by DIRBE. Although reasonably bright (in a normalized sense) at long wavelengths, the SMC is dimmer than other galaxies at intermediate (∼ 5 – 50 μm) wavelengths, presumably a result of the relative deficiency of small grains in the SMC. We also compare the FIR emission to the distribution of atomic and molecular gas as traced by HI and CO millimeter wave emission. The emissivity of atomic hydrogen in the SMC is extremely low, implying a very low dust to gas ratio. Despite this, there is evidence for a substantial “hidden” reservoir of molecular gas near the CO emission — enough to imply a CO-to-H2 conversion factor ∼ 15 – 35 times the Galactic value. This observation is at odds with CO studies of giant molecular clouds (GMCs) in the SMC and other nearby low metallicity systems, which find that GMC properties (including the virial parameter) are not strong functions of environment. As such, this may be evidence that the excess H2 exists in an envelope of CO-free H2 that lies outside the molecular gas.
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