Molecular Gas and Star Formation at Low Metallicity in the Magellanic Clouds
Jameson, Katherine Esther
Bolatto, Alberto D
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The Magellanic Clouds are two interacting, gas-rich, star-forming, low-mass, nearby satellite galaxies of the Milky Way that afford a unique view of low-metallicty star-forming regions, providing the nearest laboratories to study processes relevant to star formation in the early universe. We use the dust emission from HERITAGE Herschel data to map the molecular gas in the Magellanic Clouds, avoiding the known biases of CO emission as a tracer of H2. On small (~ few pc) scales in the Small Magellanic Cloud (SMC), we study the effect of metallicity on the structure of photodissociation regions in the outskirts of molecular clouds using [CII] and [OI] spectroscopy combined with new ALMA 7-m array maps of 12CO and 13CO. We estimate the total amount of molecular gas using [CII] to trace H2 at low-Av and 12CO to trace H2 at high-Av. We find that most of the molecular gas is traced by [CII] emission and that metallicity only affects the relationship between 12CO emission and molecular gas through changes in Av. Using mid-infrared spectroscopy from Spitzer Space Telescope in the SMC, we model the H2 rotational line emission to estimate temperatures, column densities, and fractions of warm H2 gas (T>100 K). The temperatures and column densities of warm H2 gas are similar to nearby galaxies, but the SMC shows somewhat high fractions of warm H2. The properties of the warm H2 gas indicate that it is located in photodissociation regions that are more extended in the low metallicity environment of the SMC. We use dust-based molecular gas maps data to evaluate molecular depletion time scales as a function of spatial scale. We compare galaxy-scale analytic star formation models to our observations and find that successfully predicting the trends in the low metallicity environment needs the inclusion of a diffuse neutral medium. The analytic models, however, do not capture the scatter observed, which computer simulations suggest is driven primarily by the time-averaging effect of star formation rate tracers. The averaging of the scatter in the molecular gas depletion time as a function of scale size suggests that the drivers of the star formation process in these galaxies operate on large scales.