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 S ^ { 3 } MC 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 H i-dominated region around N83 .
This allows us to resolve the relative structures of H _ { 2 } , 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 H _ { 2 } 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-H _ { 2 } conversion factor averaged over the whole cloud is very high 4 – 11 \times 10 ^ { 21 } cm ^ { -2 } ( K km s ^ { -1 } ) ^ { -1 } , or 20 – 55 times the Galactic value .
Second , the CO-to-H _ { 2 } 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 } \gtrsim 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 H _ { 2 } masses measured from CO kinematics and dust .