Thesis Defense: Douglas Applegate

Abstract: Clusters of galaxies have become a cornerstone of the experimental evidence supporting the standard Lambda-CDM cosmological model, and will be a mainstay in testing the nature of dark energy with upcoming surveys. To constrain cosmological models, these surveys indirectly measure cluster masses as part of cluster counting experiments, requiring surveys to calibrate their mass proxies. In this talk, I will present weak-lensing masses for 51 of the most X-ray luminous galaxy clusters known. This cluster sample spans redshifts 0.15 < z < 0.7, and is well suited to calibrate mass proxies for current cluster cosmology experiments.

To this end, I will discuss a new weak-lensing method that exploits all information available in the photometric redshift posterior probability distributions of individual galaxies.  Using simulations based on the COSMOS-30 catalog, I will demonstrate control of systematic biases in the mean mass of the sample with this method, from photometric redshift biases and associated uncertainties, to better than 3%. The performance of this new photometric redshift-based method allows us to calibrate `color-cut` masses for all 51 clusters in the present sample to a total systematic uncertainty of ~7% on the mean mass, a level sufficient to significantly improve current cosmology constraints from galaxy clusters. Finally, I will discuss how these results bode well for future cosmological studies of clusters.