The Evolution of Halo Properties Through Binary Major Mergers

Abstract

Galaxy clusters are massive, gravitationally bound objects composed of a large population of galaxies. Each of these galaxies occupies a dark matter halo and collectively the cluster has its own extended halo. Cluster halos can be described by many structural properties including their mass, concentration, shape, spin, and asymmetry. These properties, among others, can be used as proxies to constrain cosmology. The issue with galaxy clusters is that they are still assembling at the present-day. These clusters primarily grow through mergers, where smaller systems coalesce to form larger ones. If the mass ratio between the two merging components is sufficiently large (i.e. 3:1 or below), this is known as a major merger. The effect of major mergers is to significantly redistribute the matter distribution in the host system. This leads to pronounced fluctuations in the cluster's structural properties during the merger, making measurements of these properties hard to interpret. Therefore, accurately predicting how cluster properties vary during mergers is important in order to use them as a cosmological tool.

In this thesis, we use simulations to study how the structure of remnant systems evolves during mergers. These simulations consider the merger of two isolated components, each represented by truncated Navarro-Frenk-White (NFW) profiles. We find that mergers produce oscillations in structural parameters for both the overall remnant and the host system. For example, the host halo's concentration experiences one of two types of responses to the satellite's motion depending primarily on the pericentric passage distance of the orbit. Given the simulation results, we present a semi-analytic model for the evolution of structure in remnant systems due to isolated, binary mergers. The model consists of two components, a treatment for the orbital evolution of the satellite and a prescription for changes in the host halo's potential. This second component is often neglected when modeling satellite orbits in minor mergers. Interestingly, we find that adding a host halo response model has little impact on the orbital evolution of the satellite and its mass loss. In contrast, this model must be incorporated in order to accurately predict how the remnant's structure changes after the satellite first passes pericentre. While our model generally works well at replicating the median concentration for the first two orbits, it is unable to recreate any of the remnant's anisotropy properties (i.e. shape, spin, and asymmetry). Overall, our results provide a framework for analyzing the response of cluster halo properties to mergers in more realistic scenarios.