Galactic Stellar Haloes in the CDM Model

Abstract

This thesis studies galactic stellar haloes built up through the tidal disruption of accreted dwarf galaxies. Numerical simulations are used to explore this process in the context of the Cold Dark Matter model of cosmological structure formation. We predict the properties of stellar structures that the next generation of surveys may discover in the Milky Way halo and the haloes of other nearby galaxies.

We present six simulations based on the Aquarius project, a suite of high resolution N-body simulations of individual dark matter haloes in a fully cosmological setting. We tag subsets of particles in these simulations with stellar populations predicted by the GALFORM semi-analytic model of galaxy formation. Our method self-consistently tracks the dynamical evolution and disruption of satellites from high redshift. The luminosity function and structural properties of surviving satellites, which agree well with observations, suggest that this technique is appropriate.

We find that accreted stellar haloes are assembled between redshifts 1 and 7 from less than 5 significant progenitors. These progenitors are old, metal-rich satellites with stellar masses similar to the brightest Milky Way dwarf spheroidals (100-100 million Solar masses). In contrast to previous stellar halo simulations, we find that several of these major contributors survive as self-bound systems to the present day. Both the number of these significant progenitors and their infall times are inherently stochastic. This results in great diversity among our stellar haloes, which amplifies small differences between the formation histories of their dark halo hosts. The masses (0.1-1 billion Solar masses) and density/surface-brightness profiles of the stellar haloes (from
10 to 100 kpc) are consistent with expectations from the Milky Way and M31.

Each halo has a complex structure, consisting of well-mixed components, tidal streams, shells and other subcomponents. This structure is not adequately described by smooth models. The central regions (within 10 kpc) of our haloes are highly prolate (c/a approx. 0.3), although we find one example of a massive accreted thick disc. Metallicity gradients in our haloes are typically significant only where the halo is built from a small number of satellites. We contrast the ages and metallicities of halo stars with surviving satellites, finding broad agreement with recent observations.

We examine these simulations from the perspective of an observer located at the position of the Sun. We discuss the apparent smoothness of the halo relative to simple 3D star counts derived from photometric tomography. We then describe a simple correlation function statistic that quantifies the amount of spatial and kinematic substructure in the distant stellar halo. We test this statistic with the simulations we have developed, and find that it can distinguish between a range of realistic alternatives for the global structure of the stellar halo. We show that current observational data from pencil beam surveys of approximately 100 tracer stars (such as the Spaghetti Survey) are not sufficient to constrain the degree of structure in the Milky Way halo with this statistic. Larger area surveys with more than 1000 tracer stars (such BHB stars in the Sloan Digital Sky Survey) provide much tighter constraints on comparisons between CDM models and the Milky Way.

Finally, we explore the kinematic structure of accreted stellar haloes in the CDM model. We demonstrate that multicomponent haloes like those of the Milky Way and M31 arise naturally through the accretion of stars from tidally disrupted satellite galaxies. Accreted haloes can reproduce the gross properties of the velocity ellipsoid measured in the Solar neighbourhood, although they can be far from dynamical equilibrium and have complex anisotropy profiles. In particular, halo stars do not trace the dark matter velocity distribution up to the escape velocity in the Solar
neighbourhood. This suggests that mass estimates of the Milky Way based on related kinematic measurements may deviate significantly from the true mass, if the stellar halo is built largely though accretion.