Regulating star formation in diverse galactic environments

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In our own Milky Way and in other galaxies, stars form inside dense clouds of molecular hydrogen gas. How galaxies form and evolve with time is intimately tied to this process of star formation. However, the effect of the larger scale environment on the molecular gas content of galaxies is poorly understood. I will measure the molecular gas content and properties in a sample of nearby spiral galaxies probing environments from isolated galaxies to galaxy groups to the massive Virgo cluster of galaxies. One possibility is that gravitational interactions with neighbouring galaxies are responsible for driving the conversion of atomic hydrogen gas to molecular hydrogen gas through radial inflows. This unique work will determine the role of environment in shaping the molecular gas content of galaxies and place this work in the context of models of galactic star formation and galaxy evolution. The violent mergers of two galaxies produce some of the most extreme star formation environments in the local universe, with stars forming at a rate that is ten to one hundred times faster than in normal galaxies. I will measure how the physical and dynamical properties of the molecular gas change during the merger process to pinpoint the mechanisms responsible for the increase in the rate of star formation in galaxy mergers as well as the similar increase in the rate of gas consumption. I will also measure the rate of cooling in the molecular gas and use radiative transfer models to constrain the heating sources for the gas, such as ultraviolet photons, X-ray photons, or shocks. My work will provide a critical test of the input physics to numerical models of galaxy mergers, as well as models of galaxy formation in the early universe. One of the most important issues in astrophysics today is understanding what sets the distribution of masses with which stars are born. I will complement my work on extragalactic star formation with a detailed study of the properties of dense cores in molecular clouds in our own Milky Way. These dense cores are known to be the sites where individual stars are formed. I will measure the distribution of masses of the cores and determine whether the properties of the cores change with the local environment, from isolated cores to cores along filaments to small clusters of cores. I will also determine whether filaments provide the necessary environment for the formation of dense cores. My work will test the robustness of our understanding of the relationship between the dense cores and the distribution of masses with which stars are born.