Numerical Simulations of Galaxy Formation: Angular Momentum Distribution and Phase Space Structure of Galactic Halos

Abstract

Within the past decade, the CDM model has emerged as a standard paradigm of structure formation. While it has been very successful in explaining the structure of the Universe on large scales, on smaller (galactic) scales problems have surfaced. In this thesis, we investigate several of these problems in more detail. The thesis is organized as follows. In Chapter 1, we give a brief introduction about structure formation in the universe and discuss some of the problems being faced by the current CDM paradigm of galaxy formation.In Chapter 2, we analyze the angular momentum properties of virialized halos obtained from hydrodynamical simulations. We describe an analytical function that can be used to describe a wide variety of angular momentum distributions (AMDs), with just one parameter α. About $90-95% of halos turn out to haveα < 1.3, while exponential disks in cosmological halos would require 1.3 < α < 1.6. This implies that a typical halo in simulations has an excess of low angular momentum material as compared to that of observed exponential disks, a result which is consistent with the findings of earlier works.In Chapter 3, we perform controlled numerical experiments of merging galactic halos in order to shed light on the results obtained in cosmological simulations. We explore the properties of shape parameter α of AMDs and the spin ratio λGas/λDM in merger remnants and also their dependence on orbital parameters. We find that the shape parameter α is typically close to 1 for a wide range of orbital parameters, less than what is needed to form an exponential disk.The last chapter of the thesis (Chapter 4) is devoted to the analysis of phase space structure of dark matter halos. We first present a method to numerically estimate the densities of discretely sampled data based on a binary space partitioning tree. We implement an entropy-based node splitting criterion that results in a significant improvement in the estimation of densities compared to earlier work. We use this technique to analyze the phase space structure of halos.