Molecular gas in the Large Magellanic Cloud

This thesis presents new observations and analysis of the molecular gas in the Large Magellanic Cloud (LMC). The observations were conducted at the Mopra Telescope as part of the Magellanic Mopra Assessment (MAGMA) project, which has obtained high resolution (45′′) maps of the 12CO(J = 1 → 0) emission from 70% by mass of the LMC’s molecular cloud population. We show that CO emission in the LMC arises predominantly in spatially compact structures with high surface brightness, and that the total CO luminosity of the LMC is two orders of magnitude lower than would be predicted by the correlations between CO luminosity, stellar mass and 1.4 GHz radio continuum that are observed for nearby late-type galaxies. We present a catalogue of giant molecular clouds (GMCs) in the LMC using the MAGMA CO data, and investigate whether the catalogued clouds are similar to GMCs in the Milky Way and other nearby galaxies. We find that GMCs in the LMC roughly follow the scaling relations between radius, velocity dispersion, mass and CO luminosity that have been determined for Galactic GMCs, but that LMC clouds have narrow linewidths and faint CO luminosities relative to their size. The physical properties of the observed GMCs are mostly insensitive to variations in the local interstellar conditions, but there are significant positive correlations between the atomic gas column density and the GMC velocity dispersion, and the stellar mass surface density and both the peak CO brightness and CO surface brightness of the GMCs. Our results are difficult to reconcile with models that posit molecular clouds as equilibrium structures that are regulated by either the interstellar radiation field or the ambient interstellar pressure. Finally, we consider whether molecular gas is relevant for the correlation between the 1.4 GHz radio continuum and far-infrared (FIR) emission within the LMC. We find robust correlations between the non-thermal radio continuum and the gas and dust emission in regions covering half the LMC’s gas disk, and we identify star formation and the neutral gas surface density as the key parameters that determine the strength of these correlations. In regions where the star-formation activity is low relative to the availability of dense gas, the non-thermal radio continuum is more tightly correlated with the gas and dust emission. We demonstrate that coupling between the magnetic field strength and the gas volume density can account for the exponent of the local radio-FIR correlation that we observe in the LMC, adopting plausible assumptions for the LMC’s UV opacity, dust-to-gas ratio and cosmic ray distribution.