The Spitzer Survey of the Small Magellanic Cloud:: Far-infrared emission and cold gas in the small magellanic cloud

We present new FIR maps of the SMC at 24, 70, and 160 mu m obtained as part of the Spitzer Survey of the Small Magellanic Cloud ((SMC)-M-3). 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-dust = 3; 10(5) M-circle dot, implying a dust-to-hydrogen ratio over the region studied of log(10)(D/H) = -2.86, or 1:700, which includes H-2. Assuming the dust to trace the total gas column, we derive H-2 surface densities across the SMC. We find a total H-2 mass M-H2 = 3.2; 10(7) M-circle dot in a distribution similar to that of the CO, but more extended. We compare profiles of CO and H-2 around six molecular peaks; on average H-2 is more extended than CO by a factor of similar to 1.3. The implied CO-to-H-2 conversion factor over the whole SMC is X-CO (13 +/- 1); 10(21) cm(-2) (K km s(-1))(-1). Over the volume occupied by CO the conversion factor is lower, X-CO = (6 +/- 1); 10(21) cm(-2) (K km s(-1))(-1), but still a few times larger than that found using virial mass methods. The molecular peaks have H-2 surface densities Sigma H-2 approximate to 180 +/- 30 M-circle dot pc(-2), similar to those in Milky Way GMCs, and correspondingly low extinctions, A(V) similar to 1-2 mag. The theory of photoionization-regulated star formation predicts A(V) similar to 6, which would require the GMCs to be similar to 3 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 this galaxy having only 10% of its gas in the molecular phase.