Ph.D. Thesis Defenses
How Galaxies Form Stars: the Connection between Local and Global Star Formation in Galaxies
May 31, 2019 | ERC 576 | 10:30 AM | PhD Advisor: Andrey V. Kravtsov | PhD Thesis Defense
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Vadim Semenov

Ph.D. Committee members: Andrey V. Kravtsov, Nickolay Y. Gnedin, Damiano Caprioli, Richard G. Kron

Thesis Abstract: The fact that observed star-forming galaxies convert their gas into stars inefficiently is a long-standing theoretical puzzle. Available gas in galaxies is depleted on a timescale of several Gyrs which is orders of magnitude longer than any timescale of the processes driving gas evolution in galaxies. Many galaxy simulations can reproduce observed long depletion times but the physical mechanism controlling their values is not well understood. In addition, some of the simulations show a rather counter-intuitive behavior: global depletion times appear to be almost insensitive to the assumptions about local star formation in individual star-forming regions, a phenomenon described as "self-regulation." Yet another part of the puzzle is the observed tight and near-linear correlation between star formation rates and the amount of molecular gas on kiloparsec and larger scales. A linear correlation implies that the depletion time of molecular gas is almost independent of molecular gas density on >kiloparsec scales, while a strong dependence is expected if, e.g., star formation is controlled by molecular gas self-gravity. I will present results from a suite of isolated disk galaxy simulations in which we systematically explored the behavior of depletion times. Using insights from these simulations we formulated an intuitive physical model that explains both the origin of long gas depletion times in observed galaxies and the results of galaxy formation simulations. This model and our simulation results also provide major insights into the origin of the observed linear correlation between star formation rates and molecular gas.