The Structure of a Low-Metallicity Giant Molecular Cloud Complex

TL;DR

This study investigates how low metallicity affects the structure of giant molecular clouds by comparing dust, CO emission, and dynamics in the Small Magellanic Cloud's N83 complex, revealing CO photodissociation and dust shielding effects.

Contribution

It provides the first detailed resolution of H2, dust, and CO structures in a low-metallicity galaxy, demonstrating dust's role in shielding and CO distribution within GMCs.

Findings

CO-to-H2 conversion factor is 20-55 times the Galactic value.

CO emission is confined to regions with A_V >~ 2 mag.

A nested sphere model relates H2 masses from CO and dust.

Abstract

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.