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Abstract

The vast majority of star formation in galaxies begins in cold, dense, fractally-structured reservoirs of molecular hydrogen known as giant molecular clouds. The instantaneous properties of these clouds and the time-scales on which they evolve can therefore be built up into models of the empirical properties of galactic-scale star formation, and so can be used to understand this process. In this thesis, we first propose a simple analytic framework to quantify the expected variation in the physical properties and lifetimes of giant molecular clouds in response to changes in their galactic-dynamical environments, finding that they vary within a fundamental parameter space spanned by the orbital angular velocity of the host galaxy, the degree of galactic shearing, the gravitational stability, and the mid-plane hydrostatic pressure. We then explore this parameter space using a set of high-resolution numerical simulations of Milky Way-like galaxies. Due to their high densities and pressures relative to the galactic mid-plane, we find that giant molecular clouds in Milky Way-like galaxies are self-gravitating and decoupled from galactic dynamics, by contrast to their lower-density progenitor clouds of atomic gas, which display systematic, galactic-dynamical variations. Finally, we analyse the full evolutionary history of each simulated cloud population as a function of the cloud spatial scale. Across all Milky Way-like environments, we find that the lifetimes of self-gravitating clouds decrease with their spatial scale below the scale-height of the thin gas disc of the galaxy, and converge to the disc crossing time at its scale-height.