Cosmology with clusters of galaxies : high-redshift clusters and the evolution of cluster substructure

Abstract

Clusters of galaxies have become an important cosmological tool, yet we do not un-erstand many aspects of their formation and development. In this thesis, I pursue two projects aimed at using clusters to constrain cosmology and better understanding cluster evolution. First, I examine the Chandra observation of MS1054-0321. MS1054-0321 is the highest redshift cluster in the Einstein Medium Sensitivity Survey (EMSS), and it was one of the first high-redshift clusters observed with Chandra. I confirm that this cluster is hot and massive, although its temperature is slightly lower than inferred previously. I also detected an iron line in this cluster, one of the first detections of iron in a cluster at these redshifts, with an abundance consistent with early enrichment of the ICM. MS1054-0321 exhibits significant substructure, which I study in detail for the first time. In X-rays, it appears to be a nearly equal mass double cluster in the process of merging. Both the cluster galaxies and mass associated with the western subclump are offset from the X-ray peak, possibly indicating that the gas in the subclump is being stripped off as it falls into the cluster. Despite the lower temperature, I find that the detection of this cluster in the EMSS constrains Qm to be less than one.

(cont.) In the second project, I investigate the evolution of cluster substructure with redshift, quantifying for the first time cluster structure out to z [approx.] 1. My sample includes 40 X-ray selected, luminous clusters from the Chandra archive, and I quantify cluster morphology using the power ratio method (Buote & Tsai 1995). I find that, as expected qualitatively from hierarchical models of structure formation, high-redshift clusters have more substructure and are dynamically more active than low-redshift clusters. Specifically, the clusters with z > 0.5 have significantly higher average third and fourth order power ratios than the lower redshift clusters. This observation of structure evolution indicates that dynamical state may be an important systematic effect in cluster studies seeking to constrain cosmology, and when calibrated against numerical simulations, structure evolution will itself provide interesting bounds on cosmological models.