Roan Haggar
A large fraction of galaxies are found in groups or clusters, meaning that an understanding of galaxy clusters is crucial in studying galaxy evolution, large-scale structure formation and Cosmology. This work utilises The Three Hundred Project, a suite of 324 simulations of galaxy clusters that include gas, stars and dark matter. These simulations are used to investigate the evolution of galaxies that are either within or nearby to clusters, and in particular the 'backsplash population' of galaxies. These are galaxies that have been gravitationally accreted by a cluster, but have passed through its centre and now reside outside of the cluster. We show that a significant fraction of all galaxies found in the vicinity of a cluster are backsplash galaxies, and that this fraction depends on the properties of a cluster (for example, whether a cluster is relaxed or unrelaxed). This has important implications for observations of cluster outskirts, as galaxies in the same region can have notably different histories, and may have had their properties modified by the environments in which they have previously existed.
Because these infalls take millions of years, we cannot observe the past trajectories of these galaxies, meaning that it is hard to distinguish backsplash and infalling galaxies observationally. This is further complicated by the fact that telescopes can only view a 2D projected image of galaxy clusters, making it challenging to determine the position of galaxies relative to their cluster. However, cosmological simulations allow us to look at the full histories of clusters and their member galaxies in 3D, in order to find how common backsplash galaxies are. This work utilises simulations of 324 clusters, which we use to calculate the fraction of galaxies in the cluster outskirts that are backsplash galaxies.
We find that backsplash galaxies comprise approximately half of all galaxies between R200 and twice R200 from a cluster. We also find that this fraction depends on properties of a galaxy cluster itself. One such property is how 'relaxed' a cluster is. Clusters are defined as being more relaxed if they are isolated from other clusters, accrete galaxies and other material relatively slowly, and have not recently collided with another cluster.
In this work we show that relaxed clusters have a higher fraction of backsplash galaxies in their outskirts compared to unrelaxed clusters: approximately 70% and 40% respectively. This is due to a number of factors. For instance, as unrelaxed clusters are often accreting large numbers of galaxies, there are generally many infalling galaxies in their outskirts, which dilutes the population of backsplash galaxies.
We can measure the relaxation of a cluster observationally, and so this work allows us to use this observable property to estimate how many backsplash galaxies are around a cluster. This is very useful for surveys of cluster galaxies. It is well known that galaxies within clusters have different properties to those far away from clusters; cluster galaxies are typically redder and more spheroidal. However, it is not fully understood if these differences arise due to the processes taking place within a cluster (such as 'ram pressure stripping' and tidal effects) or those occurring before a galaxy enters a cluster. Backsplash galaxies have already been processed by a cluster environment, in contrast with infalling galaxies, which have not. Therefore, disentangling the populations of backsplash and infalling galaxies around a cluster allows us to further understand which galaxy properties are influenced by the cluster, and which are affected by processes occurring prior to the infall of a galaxy into the cluster.