Eric Hallman, Ph.D. 2004

Observations and Numerical Simulations of Large Scale Structure Formation

Eric Hallman

Under the supervision of Dr. T.W. Jones

ABSTRACT

The results of a study of the relationship between X-ray substructure in clusters of galaxies and the dynamics and evolution of large scale structure are presented. I use X-ray observational data, cosmological simulations, and synthetic observations to clarify the interpretation of X-ray features as resulting from shocks and cold fronts. I also explore observational strategies for identifying and classifying shocks and cold fronts. I examine the role of shocks not only as tracers of the dynamics and evolution of cosmic flows, but as the primary mechanism for dissipating gravitational energy in the gas. The principal findings are the following: 1) Analysis of Chandra observations of the cool cluster A168 shows an unusual cold front. The cold feature leads the subcluster core in the direction of motion. The analysis suggests that A168's cold front is in a late stage of its evolution, on the verge of mixing into the external medium; 2) High-resolution N-body/hydro cosmological simulations of a λCDM universe are analyzed. Weak shocks (M < 3) that form as a result of large-scale structure formation have very similar frequency as a function of Mach number in volumes isolated by temperature and density in the simulation. The dissipation rates in clusters at the current epoch are a generally increasing function of mass, but are strongly affected by the relative states of cluster dynamical activity. The more dynamically active clusters/groups have higher ratios of cosmic-ray to thermal dissipation than those that are relaxed. This is consistent with the observed correlation between disturbed X-ray morphology and non-thermal radio emission in clusters; 3) Synthetic observations of the simulated clusters/groups that form in the simulation volume are also performed. A blind search for shocks in the synthetic X-ray images reveals a distribution of shocks as a function of Mach number that compares favorably with the three-dimensional distribution of shocks in the hot gas in cluster volumes. The use of carefully chosen X-ray energy bands for observation increases the probability of finding shocks. Cold fronts identified in the synthetic observations have a similar distribution as a function of the steepness of the temperature jump as do shocks.