| We use observations and simulations to study the structure of galaxy clusters and to further clarify the relationship between cluster substructure and the cosmological density parameter {dollar}Omegasb0.{dollar} We start by developing a new statistic which uses clues contained in the cluster X-ray emission as dynamical constraints. The statistic {dollar}wsb{lcub}vec x{rcub}{dollar} measures the centroid variation of the cluster X-ray emission which reflects variations in the center of mass of the intracluster gas, a non-equilibrium feature. Primarily because {dollar}wsb{lcub}vec x{rcub}{dollar} relies on the projected X-ray emission, the centroid variation is an indicator of recent merging, rather than a tight constraint on the merger history.; In collaboration with A. Evrard (U. Michigan), we study an ensemble of clusters simulated within three generic cosmological models to demonstrate that cosmologically induced differences in cluster merger histories are reflected in the cluster X-ray morphologies. Specifically, the ensemble distributions of centroid variation {dollar}wsb{lcub}vec x{rcub},{dollar} axial ratio {dollar}eta,{dollar} and radial falloff {dollar}beta{dollar} clearly discriminate between low {dollar}Omegasb0{dollar} and critical density {dollar}Omega=1{dollar} cosmological models.; With a sample of Einstein Imaging Proportional Counter (IPC) X-ray images of 65 cluster, we use this new X-ray morphology-cosmology connection to place constraints on {dollar}Omegasb0.{dollar} First, we use the ensemble of cluster images to determine the range of observed cluster morphologies; over half the cluster sample exhibits significant centroid variations, indicating that a higher fraction of clusters than previously known have experienced recent mergers. Second, we show that clusters evolved in two low {dollar}Omegasb0{dollar} models are more centrally condensed and spherically symmetric than observed clusters. This observation stands as a significant challenge to the standard low {dollar}Omegasb0{dollar} cosmological models.; Finally, we study the dynamics and galaxy population in the cluster Abell 576. The observational constraints include X-ray observations, 281 (230 new) redshifts of galaxies projected within {dollar}1.5hsp{lcub}-1{rcub}{dollar} Mpc, and CCD R band galaxy photometry over a {dollar}2hsp{lcub}-1{rcub} {lcub}rm Mpc{rcub}times2hsp{lcub}-1{rcub}{dollar} Mpc region centered on the cluster. We find evidence that the galaxy population consists of (a minimum of) two kinematically distinct populations. The galaxies exhibiting evidence of recent star formation are consistent with an unvirialized, near-core population, and the galaxies with no evidence of recent star formation are more clustered and have a lower velocity dispersion. Although there is no evidence for gross substructure, the clustered population contains a cold core which suggests that the cluster core may contain the remnants of a low mass subcluster. In addition, the analysis underscores the uncertainties of cluster masses and the dangers inherent in making statistical background corrections when calculating luminosity functions and galaxy distributions. |
