The Structure of Dark Matter Haloes in Cosmological Simulations

Abstract

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.