The Structure of a Low-Metallicity Giant Molecular Cloud Complex
  To understand the impact of low metallicities on giant molecular cloud (GMC)
structure, we compare far infrared dust emission, CO emission, and dynamics in
the star-forming complex N83 in the Wing of the Small Magellanic Cloud. Dust
emission (measured by Spitzer as part of the S3MC and SAGE-SMC surveys) probes
the total gas column independent of molecular line emission and traces
shielding from photodissociating radiation. We calibrate a method to estimate
the dust column using only the high-resolution Spitzer data and verify that
dust traces the ISM in the HI-dominated region around N83. This allows us to
resolve the relative structures of H2, dust, and CO within a giant molecular
cloud complex, one of the first times such a measurement has been made in a
low-metallicity galaxy. Our results support the hypothesis that CO is
photodissociated while H2 self-shields in the outer parts of low-metallicity
GMCs, so that dust/self shielding is the primary factor determining the
distribution of CO emission. Four pieces of evidence support this view. First,
the CO-to-H2 conversion factor averaged over the whole cloud is very high 4-11
\times 10^21 cm^-2/(K km/s), or 20-55 times the Galactic value. Second, the
CO-to-H2 conversion factor varies across the complex, with its lowest (most
nearly Galactic) values near the CO peaks. Third, bright CO emission is largely
confined to regions of relatively high line-of-sight extinction, A_V >~ 2 mag,
in agreement with PDR models and Galactic observations. Fourth, a simple model
in which CO emerges from a smaller sphere nested inside a larger cloud can
roughly relate the H2 masses measured from CO kinematics and dust.
