The Dynamical States and Mass Accretion Histories of Galaxy Clusters in IllustrisTNG

Abstract

The concordance cosmological model describes the history and large-scale structure of the universe using a few key parameters. Two of these parameters, σ8 and Ωm, determine the clustering of matter due to the growth of density fluctuations in the early universe. Current constraints on these parameters measured from nearby large structures and from the early universe are in statistical tension. Our ability to resolve this tension is limited by the degeneracy between the parameters when measured from observations of nearby structure. Since galaxy clusters are the most massive gravitationally bound objects in the universe, their formation is sensitive to σ8 and Ωm. An improved understanding of the formation histories of galaxy clusters can break the measurement degeneracy, thus providing new insights into this tension in our cosmological model. Since the formation time scales of galaxy clusters are unobservable, we must use the structure of clusters to probe their formation histories.

In this thesis, we relate the observable structural properties of galaxy clusters to the mass accretion histories of their surrounding dark matter halos in the IllustrisTNG cosmo- logical simulations. Structure formation in the universe is hierarchical, so recently formed galaxy clusters will have experienced recent mergers with other systems. We examine a set of structural properties that are related to the dynamical states of clusters as indications of recent mergers to relate the structures of clusters to their formation histories. Using the cluster formation history information that is available in IllustrisTNG, we classify clusters as dynamically relaxed or unrelaxed based on their structural properties and compare the mean mass accretion histories of the resulting groups.

We establish in this work that the stellar mass asymmetry and magnitude gaps of galaxy clusters are readily observable structural parameters that most effectively predict the mass accretion histories of halos. By comparing the gravitational lensing profiles of dynamically relaxed and unrelaxed clusters classified using different structural parameters, we demonstrate that the stellar mass asymmetry most reliably distinguishes between halos in different dynamical states with different density profiles. We also show that line of sight galaxy projection does not significantly affect IllustrisTNG cluster samples and that differences between 3D-identified clusters and optically selected clusters can be accounted for with accurate cluster mass estimates. However, we find that the density profiles traced by the weak gravitational lensing around relaxed and unrelaxed clusters in IllustrisTNG simulations and DLIS x UNIONS observations are discrepant. The structural differences between the simulated and observed galaxy clusters will be further explored in future work to better relate this work’s findings to real astrophysical systems.

Overall, we find through cosmological simulations that the structural properties of galaxy clusters can be used to effectively trace their mass accretion histories. The findings of this thesis establish which observable properties of clusters can be targeted in both observations and simulations to grant us insight into the formation histories of the largest structures in the universe.