Cosmological simulations show that dark matter halos contain a wealth of substructure. These subhalos are assumed have a mass distribution that extends down to the smallest mass in the Cold Dark Matter (CDM) hierarchy, which lies below the current resolution limit of simulations. Substructure has important ramifications for indirect dark matter detection experiments as the signal depends sensitively on the small-scale density distribution of dark matter in the Galactic halo. A clumpy halo produces a stronger signal than halos where the density is a smooth function of radius. However, the small-scale Universe presents a daunting challenge for models of structure formation. In the CDM paradigm, structures form in a hierarchical fashion, with small-scale perturbations collapsing first to form halos that then grow via mergers. However, near the bottom of the hierarchy, dark matter structures form nearly simultaneously across a wide range of scales. To explore these small scales, I use a series of simulations of scale-free cosmological models, where the initial density power spectrum is a power-law. I can effectively examine various scales in the Universe by using the index in these artificial cosmologies as a proxy for scale. This approach is not new, but my simulations are larger than previous such simulations by a factor of 3 or more. My results call into question the often made assumption that the subhalo population is scale-free. The subhalo population does depend on the mass of the host. By combining my study with others, I construct a phenomenological model for the subhalo mass function. This model shows that the full subhalo hierarchy does not greatly boost the dark matter annihilation flux of a host halo. Thus, the enhancement of the Galactic halo signature due to substructure can not alone account the observed flux of cosmic rays produced by annihilating dark matter. Finally, I examine the nonlinear power spectrum, which is used to determine cosmological parameters based on large-scale, observational surveys. I find that in this nonlinear regime, my results are not consistent with currently used fitting formulae and present my own empirical formula. / Thesis (Ph.D, Physics, Engineering Physics and Astronomy) -- Queen's University, 2009-09-25 01:01:39.714

Using data from the Next Generation Virgo Cluster Survey and high-resolution simulations of Virgo cluster-like halos, we determine the stellar-to-halo mass relation (SHMR) for subhalos, using the technique of abundance matching. The subhalo SHMR differs markedly from its field galaxy counterpart, regardless of how the subhalo mass is defined (mass at z = 0, mass at infall, or maximum mass while in the field). The slope of the relation at low mass (M⋆<10^10 Msun) is in all cases steeper than the same for the field. We find conflicting indicators of whether this difference in slope indicates an increasing or decreasing dark-to-stellar ratio; further modelling is required to reach a definitive conclusion. We also find evidence for the existence of a measurable age gradient in velocity, such that older subhalos have lower velocities than their younger peers. This opens the possibility that good quality redshifts of the lower mass galaxies of the Virgo cluster might provide additional constraints on the SHMR at high redshift and its evolution. Finally, we investigate the degree to which mergers, particularly major mergers, cause mixing of old and new material in halos, which has implications for the robustness of any implied radial age gradient. We find only a slight increase in mixing for major mergers over minor mergers, and little evidence for any large amount of mixing being induced by mergers of any ratio.

Mode of access: World Wide Web. / Thesis (Ph. D.)--University of Hawaii at Manoa, 2005. / Includes bibliographical references (leaves 136-146). / Electronic reproduction. / Also available by subscription via World Wide Web / xvi, 146 leaves, bound col. ill. 29 cm

Thesis (Ph.D.)--Michigan State University. Dept. of Physics and Astronomy, 2008. / Title from PDF t.p. (viewed on July 22, 2009) Includes bibliographical references (p. 243-249). Also issued in print.

Thesis (Ph. D.)--University of Hawaii at Manoa, 2005. / Includes bibliographical references (leaves 136-146).

Thesis (Ph. D.)--University of Washington, 2005. / Vita. Includes bibliographical references (leaves 115-121).

Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

Caracterizamos estatisticamente a estrutura dos halos de matéria escura extraídos da Simulação do Millennium, uma das maiores simulações do modelo cosmológico CDM realizadas até hoje, com 10 bilhões de partículas num volume de 500h−1Mpc3. Nossos resultados confirmam que na época atual, z = 0, o parâmetro de concentração dos halos se correlaciona com a massa. Esta correlação é ajustada por uma lei de potência no intervalo de massas que corresponde a halos de galáxias (1012h−1M) até aglomerados de galáxias (1015h−1M), log10 c ∞ −α log10M, com α = 0.1. Comparamos a previsão de três modelos existentes na literatura para esta correlação. Medimos a distribuição de probilidade do parâmetro de concentração, que é ajustada com boa aproximação por uma função lognormal com dispersão σlog10 c = 0.1 em todo o intervalo de massas. Verificamos que o parâmetro de concentração é independente do spin do halo quando sistemas claramente fora de equilíbrio são removidos da análise. Comparamos modelos que relacionam o parÂmetro de concentração com histórico de formação extraído da simulação. Mostramos que quando definimos a época de formação levando em conta a distribuição de progenitores do halo e não apenas o progenitor mais massivo, obtemos uma correlação melhor entre densidade média do universo nessa época e a densidade característica do halo. Finalmente, testamos a validade de dois modelos que predizem o parâmetro de concentração a z = 0 com base no histórico de acreção de massa do progenitor mais massivo do halo. / We use the Millennium Simulation (MS) to study the statistics of CDM halo concentrations at z = 0. Our results confirm that the average halo concentration declines monotonically with mass; the concentration-mass relation is well fit by a power-law over 3 decades in mass, up to the most massive objects that form in a CDM universe (~1015h−1M). We compare this relation with three models in the literature. The large volume surveyed, together with the unprecedented numerical resolution of the MS, allow us to estimate with confidence the distribution of concentrations and, consequently, the abundance of systems with unusual properties. We find that halo concentrations are independent of spin once haloes manifestly out of equilibrium have been removed from the sample. The concentrations of out-ofequilibrium haloes tend to be lower and have more scatter, while their spins tend to be higher. A number of previously noted trends within the halo population are induced primarily by these properties of unrelaxed systems. Finally, we compare the result of predicting halo concentrations using the mass assembly history of the main progenitor with predictions based on simple arguments regarding the assembly time of all progenitors. The latter are typically as good or better than the former, suggesting that halo concentration depends not only on the evolutionary path of a halo’s main progenitor, but on how and when all of its constituents collapsed to form non-linear objects.

Caracterizamos estatisticamente a estrutura dos halos de matéria escura extraídos da Simulação do Millennium, uma das maiores simulações do modelo cosmológico CDM realizadas até hoje, com 10 bilhões de partículas num volume de 500h−1Mpc3. Nossos resultados confirmam que na época atual, z = 0, o parâmetro de concentração dos halos se correlaciona com a massa. Esta correlação é ajustada por uma lei de potência no intervalo de massas que corresponde a halos de galáxias (1012h−1M) até aglomerados de galáxias (1015h−1M), log10 c ∞ −α log10M, com α = 0.1. Comparamos a previsão de três modelos existentes na literatura para esta correlação. Medimos a distribuição de probilidade do parâmetro de concentração, que é ajustada com boa aproximação por uma função lognormal com dispersão σlog10 c = 0.1 em todo o intervalo de massas. Verificamos que o parâmetro de concentração é independente do spin do halo quando sistemas claramente fora de equilíbrio são removidos da análise. Comparamos modelos que relacionam o parÂmetro de concentração com histórico de formação extraído da simulação. Mostramos que quando definimos a época de formação levando em conta a distribuição de progenitores do halo e não apenas o progenitor mais massivo, obtemos uma correlação melhor entre densidade média do universo nessa época e a densidade característica do halo. Finalmente, testamos a validade de dois modelos que predizem o parâmetro de concentração a z = 0 com base no histórico de acreção de massa do progenitor mais massivo do halo. / We use the Millennium Simulation (MS) to study the statistics of CDM halo concentrations at z = 0. Our results confirm that the average halo concentration declines monotonically with mass; the concentration-mass relation is well fit by a power-law over 3 decades in mass, up to the most massive objects that form in a CDM universe (~1015h−1M). We compare this relation with three models in the literature. The large volume surveyed, together with the unprecedented numerical resolution of the MS, allow us to estimate with confidence the distribution of concentrations and, consequently, the abundance of systems with unusual properties. We find that halo concentrations are independent of spin once haloes manifestly out of equilibrium have been removed from the sample. The concentrations of out-ofequilibrium haloes tend to be lower and have more scatter, while their spins tend to be higher. A number of previously noted trends within the halo population are induced primarily by these properties of unrelaxed systems. Finally, we compare the result of predicting halo concentrations using the mass assembly history of the main progenitor with predictions based on simple arguments regarding the assembly time of all progenitors. The latter are typically as good or better than the former, suggesting that halo concentration depends not only on the evolutionary path of a halo’s main progenitor, but on how and when all of its constituents collapsed to form non-linear objects.

I carried out systematic studies on the assembly history of dark matter halos, using numerical simulations and semi-analytical methods. First, I look into dark halo mass assembly history. I confirmed that the halo mass assembly is divided into a fast accretion phase and a slow accretion phase. These two phases are found to be separated by the epoch when the dark halo potential reaches its maximum. The fast accretion phase is dominated by mergers, especially major mergers; the slow accretion phase is dominated by slow mass accretion. Each halo experiences about 3±2 major mergers since its main progenitor had a mass equal to 1 percent of halo mass. However, the average redshift at which these major mergers occur is strongly mass dependent. Secondly, I investigate the formation times and the assembly bias of dark halos. I use eight different definitions of halo formation times to characterize the different aspects of the halo assembly history. I find that these formation times have different dependence on halo mass. While some formation times characterize well the hierarchical nature of halo formation, the trend is reversed for other definitions of the formation time. In addition, the formation-time dependence of halo bias is quite strong for some definitions of formation time but weak or absent for others. Thirdly, I study sub-halo mass function in the halo assembly history, with the generally known unevolved sub-halo mass functions (USMFs). I find that for subhalos that merge into the main progenitor of a present-day halo, their USMF can be well described by a universal functional form; the same conclusion can also be reached for the USMF of all sub-halos that have merged during the entire halo merging history. In these two cases, the USMFs do not seem to depend on the redshift of the host halo either. However, due to the mass loss caused by dynamical effects, only small part of the accreted halos survived and became sub-structures in the present-day dark halos. In the cluster-sized halos, 30% survived sub-halos are sub-subhalos. The sub-halo mass function at given accretion time (redshift) is also investigated to find the origin of the statistics mentioned above.