The Spitzer Survey of the Small Magellanic Cloud: Far-Infrared Emission and Cold Gas in the Small Magellanic Cloud

• Published in: The Astrophysical journal Vol. 658; no. 2; pp. 1027 - 1046
• Main Authors: Leroy, Adam, Bolatto, Alberto, Stanimirovic, Snezana, Mizuno, Norikazu, Israel, Frank, Bot, Caroline
• Format: Journal Article
• Language: English
• Published: Chicago, IL IOP Publishing 01.04.2007; University of Chicago Press
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; Cold gas; dust, extinction; Galaxies; Small Magellanic Cloud; ISM: molecules; galaxies: ISM; Magellanic Clouds; infrared: ISM
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/511150

We present new FIR maps of the SMC at 24, 70, and 160 km obtained as part of the Spitzer Survey of the Small Magellanic Cloud (S super(3)MC). These maps cover most of the star formation in the SMC bar and wing. We combine our maps with literature data to derive the dust mass surface density across the SMC. We find a total dust mass of M sub(dust)=3 x 10 super(5) M sub( ), implying a dust-to-hydrogen ratio over the region studied of super(l) sub(1) super(o) sub(0) super(g) (D/H) = -2.86, or 1: 700, which includes H sub(2). Assuming the dust to trace the total gas column, we derive H sub(2) surface densities across the SMC. We find a total H sub(2) mass M super(Hz) = 3.2 x 10 super(7) M sub( )in a distribution similar to that of the CO, but more extended. We compare profiles of CO and H sub(2) around six molecular peaks; on average H sub(2) is more extended than CO by a factor of 6 1.3. The implied CO-to-H sub(2) conversion factor over the whole SMC is X sub(CO) = (13 c 1) x 10 super(21)cm super(-) super(2) (K km s super(-1)) super(-1). Over the volume occupied by CO the conversion factor is lower, X sub(CO)=(6 c 1) x 10 super(21) cm super(-2) (K km s super(-1)) super(-1), but still a few times larger than that found using virial mass methods. The molecular peaks have H sub(2) surface densities super(Hz) - 180 c 30 M sub( )pc super(-2), similar to those in Milky Way GMCs, and correspondingly low extinctions, Av 6 1--2 mag. The theory of photoionization-regulated star formation predicts Av 6 6, which would require the GMCs to be 63 times smaller than our 46 pc resolution element. For a given hydrostatic gas pressure, the SMC has a 2--3 times lower ratio of molecular to atomic gas than spiral galaxies. Combined with lower mean densities, this results in mis galaxy having only 10% of its gas in the molecular phase.