Dynamical Properties and Assembly of Galaxies in the Epoch of Reionization

In a quest for understanding the nature of the furthest galaxies in the Universe, a large amount of effort has been expended to build large telescopes as well as to perform sophisticated cosmological simulations. Despite these achievements, we still know little about the structural and dynamical properties of star-forming galaxies belonging to the epoch when the age of the Universe was less than a Giga year i.e. the Epoch of Reionization (EoR). Indeed, galaxy dynamics in such a distant universe is an uncharted field and we do not have solid answers to several fundamental questions about the structure of EoR galaxies: 1- What are the dynamical and morphological properties of EoR galaxies? 2- Do we expect galaxies to form their disk structure as early as the EoR? 3- Are there any signatures of galaxy-galaxy mergers in such early epochs? 4- Are EoR galaxies rotationally supported or dispersion-dominated systems? 5- What is the energy source powering the gas velocity dispersion in EoR galaxies? Atacama Large Millimeter/submillimeter Array (ALMA), being the most powerful millimeter/ sub-millimeter interferometer on Earth, has been playing a revolutionary role in the field of high-z galaxy dynamics by providing spatially-resolved emission-line observations. However, as we aim at studying the first galaxies that appeared in the Universe, the limiting angular resolutions and signal-to-noise ratio of the observations significantly limit such studies. These limitations can be overcome either by performing deeper observations or targeting lensed galaxies. While waiting for such high-quality data to be available for EoR galaxies, we can address the problem theoretically. This Thesis focuses on the study of structural and dynamical properties of EoR galaxies by utilizing analytical modelings (when possible) as well as state-of-the-art hydrodynamical simulations of galaxies. Since our aim is to provide a solid comparison and/or prediction for the upcoming observations, we develop a framework in which a common cross-talk among observations and pure theoretical works becomes possible. The Thesis is structured as follows: 1- In Chapter 1, we give the reader the necessary theoretical and observational background information to follow the whole Thesis. A brief review of the conventional galaxy formation in the context of the standard model of cosmology is followed by explaining the current status of observations of distant galaxies with particular attention to the EoR. 2- In Chapter 2, we introduce the ISM physics including far-infrared (FIR) line emissions, then we introduce semi-analytical models of galaxies that we have developed to get the first insights on the properties of [C II] emission line coming from high-z galaxies either as the kinematics or star formation tracer. We end this Chapter by explaining the modeling features of a suite of hydrodynamical simulations used in the rest of the Thesis. 3- In Chapter 3, we explore different kinematical features of EoR galaxies and their connection with the assembly process as imprinted in the FIR line emission profiles. This is achieved by tracing the evolution of a simulated galaxy from z = 7 to z = 6 through the FIR [C II] emission. 4- In Chapter 4, we study the structure of spatially resolved, line-of-sight velocity dispersion in EoR galaxies traced by [C II] line emission in the redshift range of 6 < z < 8. We also quantify the contribution of the different energy sources powering such velocity dispersions in EoR galaxies. 5- In Chapter 5, we address one of the recent puzzling issues in high-z galaxy dynamics studies, which is related to the observations of surprisingly cold galactic disks at high redshift universe. This problem is addressed by studying the dynamical properties of a large sample of simulated EoR galaxies in the redshift range of 6 ≤ z < 9 probed by [C II] and nebular Hα emission. 6- Finally in Chapter 6, we present the conclusions and the future prospects.