We study the angular momentum, shape and density structures of dark matter haloes using very large dark matter simulations, and use smaller, higher-resolution simulations to investigate how the distributions of these properties are changed by the physical processes associated with baryons and galaxy formation. We begin with a brief review of the necessary background theory, including the growth of cosmic structures, the origin of their angular momenta, and the techniques used to simulate galaxies, haloes and the large scale structure. In Chapter 2, we use the Millennium Simulation (MS) to investigate the distributions of the spin and shape parameters of millions of dark matter haloes. We compare results for haloes identified using three different algorithms, including one based on the branches of the halo merger trees. In addition to characterising the relationships between halo spin, shape and mass, we also study their impact on halo clustering and bias. We go on in Chapter 3 to investigate the internal angular momentum structure of dark matter haloes. We look at the radial profiles of the dark matter angular momentum in terms of both magnitude and direction, again using large-volume dark matter simulations including the MS. We then directly compare dark matter haloes simulated both with and without baryonic physics, studying how this changes the dark matter angular momentum. After relating the spin orientation of galaxies to their haloes, we consider the shape of the projected, stacked mass distribution of haloes oriented according to their central galaxy, mimicking attempts to measure halo ellipticity by weak gravitational lensing. We consider the density structure of dark matter haloes in Chapter 4. For the dark matter simulations, we focus our interest on the source of the scatter in the distribution of concentration parameters, correlating it with both the halo spin and formation time. We compare different algorithms for predicting the concentration distribution using different aspects of the merger histories. We again go on to directly compare high-resolution haloes in simulations run with and without baryons and galaxy formation, looking at how these additional physical processes transform the density profiles. Finally, we compare the circular velocity curves of the haloes simulated with galaxies to the rotation curves of observed galaxies, using the Universal Rotation Curve model.

An important, open research topic today is to understand the relevance that dark matter halo substructure may have for dark matter searches. In the standard cosmological model, halo substructure or subhalos are predicted to be largely abundant inside larger halos, for example, galaxies such as ours, and are thought to form first and later merge to form larger structures. Dwarf satellite galaxies—the most massive exponents of halo substructure in our own galaxy—are already known to be excellent targets for dark matter searches, and indeed, they are constantly scrutinized by current gamma-ray experiments in the search for dark matter signals. Lighter subhalos not massive enough to have a visible counterpart of stars and gas may be good targets as well, given their typical abundances and distances. In addition, the clumpy distribution of subhalos residing in larger halos may boost the dark matter signals considerably. In an era in which gamma-ray experiments possess, for the first time, the exciting potential to put to test the preferred dark matter particle theories, a profound knowledge of dark matter astrophysical targets and scenarios is mandatory should we aim for accurate predictions of dark matter-induced fluxes for investing significant telescope observing time on selected targets and for deriving robust conclusions from our dark matter search efforts. In this regard, a precise characterization of the statistical and structural properties of subhalos becomes critical. In this Special Issue, we aim to summarize where we stand today on our knowledge of the different aspects of the dark matter halo substructure; to identify what are the remaining big questions, and how we could address these; and, by doing so, to find new avenues for research.

In this work we use the N-body resimulation technique to address aspects of structure formation. In the first chapter we study the influence of the local environment of DM haloes on their properties. In the second chapter we address the so-called "substructure problem" which is one of the major challenges of the CDM model of cosmology. We perform ultra-high resolution simulations of the assembly of a Milky Way type dark matter halo within its full cosmological context and propose a new analytical fitting formula (SWTS) which provides a better fit to the simulated Milky Way halo than the NFW or Moore profiles do. In the third chapter we use our ultra-high resolution simulations to study the possible -ray signal from dark matter annihilation. If such a signal was detected, the nature of the dark matter, the answer to one of the most important questions of modern cosmology, would be known. (urn: nbn: de: bvb:19-16446)"

The goal of this thesis is to (i) explore the physics that drives universal accretion history of dark matter halos; (ii) determine the relation between the halos accretion history and the halos internal structure; and (iii) disentangle the impact of halos accretion history on galaxy evolution. To address these topics, we first use the extended Press-Schechter (EPS) formalism to derive the halo mass accretion history (MAH) from the growth rate of initial density perturbations. We show that the halo MAH can be well described by an exponential function of redshift in the high-redshift regime. However, in the low-redshift regime the mass history growth slows down because the growth of density perturbations is halted in the dark energy dominated era due to the accelerated expansion of the Universe. As a result, in the low-redshift regime the halo MAH can be described by a power-law function of redshift. We complement this study with the analysis of MAHs of dark matter halos using a suite of cosmological simulations. We explore the relation between the density profile of dark matter halos and their MAHs, and confirm that the formation time, defined as the time when the virial mass of the main progenitor equals the mass enclosed within the scale radius, correlates strongly with concentration. We combine both analysis, analytic and numerical, to show that the halo MAH is the link between halo concentration and the initial density perturbation field. The connection found between the halo MAH and its density profile reached in these studies was vital to derive a semi-analytic, physically motivated model for dark matter halo concentration as a function of halo mass and redshift. Because the semi-analytic model is based on EPS theory, it can be applied to wide ranges in mass, redshift and cosmology. The resulting concentration-mass (c-M) relations are found to agree with the simulations, and because the model applies only to relaxed halos, they do not exhibit the upturn at high masses or high redshifts found by some recent works. We predict a change of slope in the z~0 c-M relation at a mass scale of 10^11 solar masses. We find that this is due to the change in the functional form of the halo MAH, which goes from being dominated by an exponential (for high-mass halos) to a power-law (for low-mass halos). During the latter phase, the core radius remains approximately constant, and the concentration grows due to the drop of the background density. We then connect the evolution of dark matter halos to the evolution of galaxies. We investigate the hot hydrostatic halo formation and its dependence on feedback mechanisms. We find that in the presence of energy sources like stellar feedback, the hot halo mass increases and the mass scale of hot halo formation is reduced. Active galactic nuclei (AGN) do not affect the hot halo as strongly. We develop a semi-analytic approach that makes use of both, the hot halo mass and the fraction of shock-heated gas, to calculate a `critical mass scale' for hot halo formation. We find that this mass scale, where the heating rate produced by accretion shocks equals cooling, is the point in mass above which halos develop a stable hot atmosphere. In the redshift range z=0-4, the critical mass is 10^11.7 solar masses, but it then increases for increasing redshift, in very good agreement with our numerical results. Finally, we investigate the physics that drives the gas accretion rate onto galaxies at the center of dark matter halos. We separately analyze the gas accretion rate onto the interstellar medium (ISM) and onto the galaxy. We find that the accretion rate onto the ISM remains roughly constant in halos larger than 10^11.7 solar masses, whereas the accretion rate onto the galaxy increases with increasing halo mass and flattens in the halo mass range 10^11.7-10^12.7 solar masses, and at redshifts z

This book is aimed at helping those that are interested in learning about some of the analytical methods used for the study of the formation of dark matter structures in the Universe. The development of analytical or semi-numerical methods for the very interesting problem of structure formation is a very active research area. Analytical methods -compared to N-body simulations- are very economical and give more light to the physical mechanisms during the process of formation. In spite of focusing on a very specific problem this book is not self-contained. Cosmological models are briefly discussed, and only flat models with a cosmological constant. The history of the Universe up to the epoch which gravity plays the main role in the evolution is also not discussed. Fortunately, classical books of Cosmology are available for the reader who is interested in these subjects. The emphasis of this book is on specific problems such as the mass growth, the density profiles and the random angular momentum of DMH.

A major outstanding problem in physics is understanding the nature of the dark energy that is driving the accelerating expansion of the Universe. This thesis makes a significant contribution by demonstrating, for the first time, using state-of-the-art computer simulations, that the interpretation of future galaxy survey measurements is far more subtle than is widely assumed, and that a major revision to our models of these effects is urgently needed. The work contained in the thesis was used by the WiggleZ dark energy survey to measure the growth rate of cosmic structure in 2011 and had a direct impact on the design of the surveys to be conducted by the European Space Agency's Euclid mission, a 650 million euro project to measure dark energy.