2007ApJ...658.1027L

We present new FIR maps of the SMC at 24, 70, and 160 µm obtained as part of the Spitzer Survey of the Small Magellanic Cloud (S³MC). 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=3x10⁵ M_☉, implying a dust-to-hydrogen ratio over the region studied of log₁₀(D/H)=-2.86, or 1:700, which includes H₂. Assuming the dust to trace the total gas column, we derive H₂surface densities across the SMC. We find a total H₂mass M_H2=3.2x10⁷ M_☉in a distribution similar to that of the CO, but more extended. We compare profiles of CO and H₂around six molecular peaks; on average H₂is more extended than CO by a factor of ∼1.3. The implied CO-to-H₂conversion factor over the whole SMC is X_CO=(13±1)x10²¹/cm2 (K.km/s)^–1. Over the volume occupied by CO the conversion factor is lower, X_CO=(6±1)x10²¹/cm2 (K.km/s)^–1, but still a few times larger than that found using virial mass methods. The molecular peaks have H₂surface densities Σ_H2~180±30 M_☉/pc2, similar to those in Milky Way GMCs, and correspondingly low extinctions, A_V∼1-2 mag. The theory of photoionization-regulated star formation predicts A_V∼6, which would require the GMCs to be ∼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.