Weak Lensing Mass Determination of Eight X-ray Selected Galaxy Clusters from the <em>400d</em> Survey

Abstract

Evolution in the mass function of galaxy clusters sensitively traces both the expansion history of the Universe and cosmological structure formation. Hence, measuring the number density of galaxy clusters as a function of redshift provides constraints to cosmological parameters, independent of other methods. Current results from these probes, including clusters of galaxies, are found to agree on a cosmological model dominated by Dark Energy and Cold Dark Matter. Investigating the unknown physical nature of Dark Energy ranks among the foremost questions in current cosmology. In particular, the presence or absence of evolution in Dark Energy density is expressed by the equation-of-state parameter.<br /> This thesis presents the first results from the <i>400d</i> Galaxy Cluster Survey Weak Lensing Programme, in which optical follow-up observations for a sample of relatively distant (<i>0.35 < z < 0.90</i>) X-ray selected galaxy clusters are analysed and presented. Mass determination by weak gravitational lensing uses minute distortions in the images of <i>background</i> galaxies, caused by the relativistic curvature of space-time, to infer the mass of the intervening cluster. The weak lensing follow-up project aims at measuring reliable weak lensing masses for 36 clusters, for which a mass function and resulting cosmological constraints using Chandra X-ray observations have been published. Determining cluster masses by weak lensing makes possible a cross-calibration of the assumptions and systematics related to both the X-ray and weak lensing methods. <br /> As the initial phase of the <i>400d</i> weak lensing programme project, observations of eight clusters were obtained with the Megacam instrument at the 6.5m MMT telescope, which we demonstrated to be well-suited for weak lensing. In this thesis, the successful weak lensing detections of these eight clusters are reported, leading to weak lensing mass estimates which then are compared to X-ray masses. For the pilot object, CL 0030+2618, the data analysis is described in great detail, focussing in particular on the construction of a catalogue of lensed background galaxies by using photometry in three passbands. In a synopsis involving several optical and X-ray methods, the identity of the brightest cluster galaxy is established and found to be consistent with both X-ray and weak lensing cluster centres for CL 0030+2618. Cluster masses are obtained by fitting the tangential weak lensing shear measured as a function of separation from the cluster centre with a profile function derived from the Navarro-Frenk-White Dark Matter density profile.<br /> Performing a similar analysis for the seven further clusters and investigating the spatial distribution of the lensing signal, multiple shear peaks and/or clusters are detected in three cases. In a comparison between the weak lensing and hydrostatic X-ray mass estimates for the eight clusters, good agreement and a power-law relation with remarkably small scatter are found. Preliminary scaling relation between the weak lensing masses and published X-ray observables of the eight clusters indicate the potential of the weak lensing survey, once observations are available for the complete 36 cluster sample. The completion of the <i>400d</i> weak lensing survey is concluded to be feasible and promising.