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

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.

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KICP Members: Andrey V. Kravtsov
KICP Students: Vadim Semenov

Zoheyr Doctor: "Hearing and Seeing the Universe: Results from Gravitational-Wave and Optical Studies of Merging Neutron Stars and Black Holes"
June 5, 2019 | 2:00 PM | ERC 401
Picture: Zoheyr Doctor: Hearing and Seeing the Universe: Results from Gravitational-Wave and Optical Studies of Merging Neutron Stars and Black Holes
Scientific Advisor: Daniel E. Holz

Ph.D. Committee members: Daniel Holz, Robert Wald, Craig Hogan, David Miller

"Zoheyr's thesis represents a major contribution to multi-messenger gravitational wave astronomy. He has published on a wide range of important topics, including a machine learning analysis of black hole waveforms, the first multi-messenger estimates for kilonova ejecta and r-process element production, and the leading observational constraints on electromagnetic counterparts to black hole mergers. Zoheyr's work is at the forefront of an entirely new and exciting field of science."
- Daniel Holz, Ph.D. advisor

Thesis Abstract: Since the first direct detection of a gravitational wave by the Laser Interferometer Gravitational-Wave Observatory in 2015, gravitational waves have become an indispensable tool for studying extreme astrophysical phenomena. To date, eleven of these "ripples in spacetime" have been detected -- ten from merging pairs of black holes and one from two colliding neutron stars. In principle, electromagnetic and neutrino signals can be combined with gravitational-wave data to form a more complete picture of these compact object mergers. In this thesis, we present four studies of gravitational-wave and optical signatures from black-hole and neutron-star mergers. First we describe a technique for emulating expensive simulations of gravitational waveforms using Gaussian process regression, a method for non-parametrically interpolating functions and their uncertainties. The second topic we address is that of the mass ejected from the collision of the two neutron stars that produced the LIGO-Virgo gravitational wave GW170817. We find that the mass dynamically ejected from the merger should enrich its surroundings with heavy r-process elements, suggesting that neutron-star mergers could have played a significant role in the production of heavy elements we see in our solar system. Next we present the analysis and upper limits from the Dark Energy Survey of an independent optical search for kilonovae, the bright optical transients associated with neutron-star mergers. Finally, we describe Dark Energy Camera optical follow-up of black-hole merger GW170814 and the results of that search.

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KICP Members: Daniel E. Holz
KICP Students: Zoheyr Doctor
Scientific projects: Laser Interferometer Gravitational-wave Observatory (LIGO)