Haggar, Roan
(2022)
Infall and accretion of substructure in hydrodynamical simulations of galaxy clusters.
PhD thesis, University of Nottingham.
Abstract
The evolution of galaxies is heavily influenced by the environment in which they are found. Dense regions of the Universe can quench a galaxy's star formation by removing its gas, and transform a galaxy's morphology from being disk-dominated to bulge-dominated. Galaxy clusters represent the most extreme example of this: compared to the cosmic field, these highly dense environments contain a much greater fraction of red, elliptical galaxies. However, a subtlety of this is that a galaxy's evolution is not just driven by the environment in which it is currently found, but is also influenced by the environments through which it has previously passed. For example, a galaxy in the outskirts of a cluster may also have been quenched during pre-processing, meaning that it was accreted onto the cluster through a cosmic filament or as a member of a group, both of which can quench star formation. Alternatively, it could be a backsplash galaxy: one that has passed through the dense cluster environment in the past, but subsequently left and now resides in the cluster outskirts.
Pre-processing, backsplash galaxies, and the direct impact of a cluster environment are difficult to disentangle from each other, because observations cannot provide us with the full histories of galaxies. This makes observational studies of galaxy environment difficult, particularly nearby to clusters. For instance, it is not clear how common are backsplash galaxies around clusters, how their frequency varies between clusters, or whether we can identify which galaxies are indeed backsplash. It is also not fully understood what happens to galaxy groups nearby to clusters, and how a cluster can influence the impact of a group on its constituent galaxies. These questions are challenging to answer with observational data, but can be approached using cosmological simulations that complement the available observations.
In this thesis, we use data from The Three Hundred project, a suite of hydrodynamical simulations of large galaxy clusters, to study the environmental histories of galaxies in and around clusters. We begin by establishing that these simulations are fit-for-purpose, by comparing them to equivalent dark matter-only simulations. We find that, compared to the hydrodynamical runs, our dark matter-only simulations underestimate the number density of galaxies in the central regions of both groups and clusters, for which we discuss several potential causes. This indicates that hydrodynamical simulations are necessary in studying cluster substructure, as the evolution of galaxy groups will be different in dark matter-only simulations.
Having established this, we then use these hydrodynamical simulations to examine how galaxy groups evolve as they approach, enter, and pass through a cluster. These galaxy groups become gravitationally unbound very quickly, losing most of their member galaxies less than 1 Gyr after entering a cluster. In fact, the overwhelming majority of groups do not survive a full passage through a cluster, meaning that any groups nearby to a cluster are almost certainly on their first infall towards the cluster centre.
We then investigate backsplash galaxies and find that, on average, over half of all galaxies between R200 and 2R200 from their host at z=0 are backsplash galaxies. However, this fraction depends on the dynamical state of a cluster; dynamically relaxed clusters, which are isolated and accreting new material slowly, have a far greater fraction of backsplash galaxies in their outskirts. This backsplash population is mostly developed in the last few Gyr, and is dependent on the recent dynamical history of a cluster.
This work uses simulations to shed light on the different processes that galaxies can experience during their accretion onto a galaxy cluster. More importantly though, the findings from these simulations can be applied to real, observational studies. The dynamical state of a cluster is a measurable property, and so can be used to infer how 'contaminated' the population of infalling galaxies in an observed cluster's outskirts will be by backsplash galaxies. Furthermore, galaxy groups observed nearby to a cluster are on their first infall, and so will contain very few backsplash galaxies. Any galaxies that are members of a group inside of a cluster will have experienced the central region of a group, but have likely only joined the cluster very recently. Simulations will be a valuable tool to complement upcoming surveys like the WEAVE Wide-Field Cluster Survey and Euclid -- due to begin in late 2022 and 2023 respectively -- and will allow us to more deeply interpret this observational data, and infer the environmental histories of galaxies.
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