The evolution of a dark halo substructure
Creators: name: Gill, Stuart P. D.
Publisher: name: Swinburne
Year: 2024

description: In this dissertation we analyse the dark matter substructure dynamics within a series of high-resolution cosmological galaxy clusters simulations generated with the N-body code MLAPM. Two new halo finding algorithms were designed to aid in this analysis. The first of these was the 'MLAPM-halo-finder' MHF), built upon the adaptive grid structure of MLAPM. The second was the 'MLAPM-halo-tracker' (MHT), an extension of MHF which allowed the tracking of orbital characteristics of gravitationally bound objects through any given cosmological N-body-simulation. Using these codes we followed the time evolution of hundreds of satellite galaxies within the simulated clusters. These clusters were chosen to sample a variety of formation histories, ages, and triaxialities; despite their obvious differences, we and striking similarities within the associated substructure populations. Namely, the radial distribution of these substructure satellites follows a 'universal' radial distribution irrespective of the host halo's environment and formation history. Further, this universal substructure profile is anti-biased with respect to the underlying dark matter profile. All satellite orbits follow nearly the same eccentricity distribution with a correlation between eccentricity and pericentre. The destruction rate of the substructure population is nearly independent of the mass, age, and triaxiality of the host halo. There are, however, subtle differences in the velocity anisotropy of the satellite distribution. We find that the local velocity bias at all radii is greater than unity for all halos and this increases as we move closer to the halo centre, where it varies from 1.1 to 1.4. For the global velocity bias we find a small but slightly positive bias, although when we restrict the global velocity bias calculation to satellites that have had at least one orbit, the bias is essentially removed. Following this general analysis we focused on three specific questions regarding the evolution of substructures within dark matter halos. Observations of the Virgo and Coma clusters have shown that their galaxies align with the principal axis of the cluster. Further, a recent statistical analysis of some 300 Abell clusters confirm this alignment, linking it to the dynamical state of the cluster. Within our simulations the apocentres of the satellite orbits are preferentially found within a cone of opening angle 40 degrees around the major axis of the host halo, in accordance with the observed anisotropy found in galaxy clusters. We do, however, note that a link to the dynamical age of the cluster is not well established. Further analysis connects this distribution to the infall pattern of satellites along the filaments, rather than some 'dynamical selection' during their life within the host's virial radius. We then focused our attention on the outskirts of clusters investigating the socalled 'backsplash population', i.e. satellite galaxies that once were inside the virial radius of the host but now reside beyond it. We and that this population is significant in number and needs to be appreciated when interpreting empirical galaxy morphology-environmental relationships and decoupling the degeneracy between nature and nurture. Specifically, we and that approximately half of the galaxies with current clustercentric distance in the interval 1- 2 virial radii of the host are backsplash galaxies which once penetrated deep into the cluster potential, with 90% of these entering to within 50% of the virial radius. These galaxies have undergone significant tidal disruption, losing on average 40% of their mass. This results in a mass function for the backsplash population different to those galaxies infalling for the first time. We further show that these two populations are kinematically distinct and should be observable spectroscopically. Finally we present a detailed study of the real and integrals-of-motion space distributions of a disrupting satellite obtained from one of our self-consistent highresolution cosmological simulations. The satellite has been re-simulated using various analytical halo potentials and we and that its debris appears as a coherent structure in integrals-of-motion space in all models ('live' and analytical potential) although the distribution is significantly smeared for the live host halo. The primary mechanism for the dispersion is the mass growth of the host. However, when quantitatively comparing the effects of 'live' and time-varying host potentials we conclude that not all of the dispersion can be accounted for by the steady growth of the host's mass. We ascribe the remaining differences to additional effects in the 'live' halo such as non-sphericity of the host and interactions with other satellites, which have not been modelled analytically.