PhD Thesis, June 2001: Dark Halo Merging and Galaxy Formation

Please contact me if you would like a copy of the entire thesis.

Abstract:

This dissertation investigates several related aspects in the theory of hierarchical galaxy formation, using high resolution cosmological simulations and analytic and semi-analytic models for forming galaxies within dark-matter halos to make theoretical predictions that are closely tied to observations. I have concentrated primarily on two topics, both relating to the effect of merging on dark-matter halo properties and on mechanisms for galaxy formation.
Recent advances in observational techniques have aided in the discovery of a large population of high-redshift galaxies, identified using the Lyman-break technique. In Part I, I discuss how this population of Lyman-break galaxies (LBGs) can constrain models of structure formation and galaxy formation. N-body simulations are combined with semi-analytic models for populating high-redshift dark matter halos with LBGs. I examine scenarios in which high-redshift star formation is driven by collisional starbursts and scenarios in which quiescent star formation dominates, make predictions for the clustering properties of each model, and confront these predictions with the clustering properties of observed LBGs. I also present an analytic formalism that can be used to constrain the halo occupation function for a population galaxies, and discuss present constraints using observational quantities of LBGs. Models for high redshift halo occupation can be constrained using these methods but at present the data cannot rule out most realistic models. Full Table of Contents

Chapter 1: Motivation and Outline

Part I: Understanding High-Redshift Galaxies

Chapter 2: Implications of Spikes in the Redshift Distribution of z~3 Galaxies

published as: ApJ, 506, 19 and also at astro-ph/9712141

Chapter 3: Galaxy Formation at z~3: Constraints from Spatial Clustering

published as: ApJ, 554, 85 and also at astro-ph/0011261

Chapter 4: Galaxy Halo Occupation at High Redshift

published as: MNRAS, 329,246 and also at astro-ph/0106293

Part II: Evolution of Dark Matter Halo Properties

Chapter 5: A Structural Merger Tree

Chapter Abstract:

We use high-resolution N-body simulations of an LCDM cosmology to develop a halo finder that fits density profiles and determines halo angular momentum properties for a statistical sample of halos, including both distinct halos and subhalos within larger halos. We describe the development of a ``structural'' merger tree, which determines detailed halo mass assembly histories and keeps track of the evolution of structural parameters for this sample of halos.

Chapter 6: Dark Halo Profiles and Mass Assmbly History

Chapter Abstract:

We study the relation between halo mass assembly histories and the build-up of their density profiles, using a statistical sample of dark-matter halos in a high-resolution N-body simulation of a LCDM cosmology. We find that halo mass accretion histories can be characterized by a simple one-parameter form; this parameter can be related to a characteristic formation epoch a_c=1/(1+z_c) where the log-slope of the mass accretion trajectory dlog(M/M_0)/dlog(a/a_0) is equal to S. We then find that the halo concentration parameter c_vir=R_vir/R_s can be related to its formation history via c_vir=c_0a/Sa_c. A physical interpretation of this finding is that for high mass infall rates the central density is related to the background density; when the mass infall rate slows, the central density stays approximately constant and the halo concentration just grows as R_vir. We find that this relation holds with very little scatter when errors in the concentration and the formation time are taken into account, and show that scatter in c_vir for a given mass can be explained almost exclusively by scatter in the characteristic formation epoch a_c for halos of that mass. We demonstrate how this model can be used to predict the mass and redshift dependence of c_v, and the scatter about these relations, using mass accretion histories derived from the extended Press-Schechter (EPS) formalism, after adjusting for the later formation times EPS predicts.
Chapters 5 and 6 are now published in a somewhat different form, as ApJ, 568, 52 and also at astro-ph/0108151

Chapter 7: Future Directions

Chapter Abstract:

In this chapter I present preliminary results and plans for future work based on the structural merger tree discussed in Chapter 5. I first discuss efforts to understand the origin and evolution of angular momentum. I have compared the evolution of halo spin parameters with halo merger and mass accretion histories, and show that angular momentum is strongly affected by the merging history of halos. High mass accretion rates and major mergers tend to either strongly increase or slightly decrease the angular momentum of halos. Once angular momentum is built up in a halo, small encounters will tend to decrease it over time. I also discuss some preliminary work and prospects for studying the build-up of angular momentum profiles in halos, which may have crucial implications for the formation of disk galaxies. I discuss future directions for work related to understanding the role of environment in halo and galaxy structural properties and to analyzing merger rates in simulations and comparing to analytic estimates, and I describe plans to combine the structural merger tree developed here with semi-analytic models for galaxy formation.