Assembly of Disk Galaxies from the Peak of Cosmic Star-Formation to Today

It was once accepted that galaxies form and maintain thin gas disks at early times. As gas is collisional, its disordered motions should be suppressed quickly. With angular momentum conserved, galaxies should be rotationally-supported within a few galaxy crossing times of their initial formation. The results presented over the chapters of this thesis challenge this picture. We track the evolution of the ionized gas kinematics of star-forming galaxies from z = 2 to the present day, covering 10 Gyrs in cosmic time. First, as a part of a Keck/MOSFIRE spectroscopic survey, we determine that z = 2 (3 Gyr after the Big Bang) is still a period of active disk formation. The majority of massive star-forming galaxies at this time have disk-like characteristics — their kinematics are dominated by rotation, they are consistent with a marginally stable disk model, and they form a Tully-Fisher relation — but with disordered motions much higher than galaxies today. These galaxies are unlike disks today — less than 30% of galaxies at all masses have rotational motions at least 3x higher than their disordered motions. Lower mass galaxies are still in the early stages of disk assembly — their kinematics are often dominated by disordered motions and they fall short of the Tully-Fisher relation. Combining this sample with a similar one at z < 1, we find that all star-forming galaxy populations, on average, increase in rotational-support with time from z = 2 to now. This happens through a dramatic decline in disordered motions, and a mild increase in rotational motions. By the present day, nearly all star-forming galaxies above a stellar mass of 10^9.5 Msun have formed rotationally-supported disks with regular disk-like morphologies, while below this mass a galaxy may or may not form a disk. To better understand potential biases when interpreting these and other observations, we compare mock images and spectra of realistic hydrodynamic simulations against their intrinsic dynamical state. We determine that late-stager mergers are indistinguishable from disks in seeing-limited kinematic data. This implies that the fraction of galaxies that are measured to be “disks” from seeing-limited observations at z = 2 is only an upper-limit on the true disk fraction. Using theoretically-derived close pair fractions at z = 2, we determine that this effect is significant for low mass galaxies — up to a factor of 2 difference between observed and true disk fractions — but insignificant for high mass galaxies