The density profile of clusters of galaxies : Abell 1351 & Abell 1995

Two galaxy clusters, Abell 1351 and Abell 1995, are examined using weak gravitational lensing techniques. Mass maps are created, and the clusters' density and shear profiles are compared to theoretical predictions. Gravitational lensing is important for studying the mass distribution of the Universe. It pro- vides methods of estimating the masses of everything acting as a gravitational lens, without making assumptions about the dynamical state and the nature of the gravitating matter. In particular, this includes the dark matter, which is currently believed to make up for about 90% of the entire matter in the Universe, evading any other means of direct detection. Dark matter and its gravitation therefore govern the architecture and evolution of the largest bounds objects known in the Universe, from galaxies up to supercluster of galaxies. This thesis uses data from the CFH12K wide-field imager at the Canada-France-Hawaii Telescope. In order to obtain mass estimates for the galaxy clusters by weak lensing methods, distortions in the images of faint and distant background galaxies are measured. These distortions are introduced into the images by the intervening tidal gravitational fields of the clusters along the line of sight. Comparing the strength of the distortions to theoretical models, mass maps and density profiles of the lensing clusters are created. The work includes standard image reduction techniques using the IMCAT software package. Details about the astrometric calibration, shear measurements, point spread function (PSF) corrections and the method of mass distribution reconstruction are given, together with a description of the principles of weak gravitational lensing. The main results are that both clusters investigated contain a relatively large mass within the scale of their virial radius. Yet their density profiles are significantly different, in particular in their central parts. Similar results have been found in the recent years also for different clusters. The finding of this result was only possible due to the very large field of view of the CFH12K imager, which allowed to probe the shear field at very different angular scales