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Weak gravitational lensing
Detection of mass concentrations in wide field imaging data

dc.contributor.advisorSchneider, Peter
dc.contributor.authorSchirmer, Mischa
dc.description.abstractThe deflection and distortion of a light bundle by tidal gravitational fields is described in the framework of General Relativity. There, light bundles from a source follow exactly the curvature of spacetime, and are distorted when passing by a large mass concentration such as a galaxy cluster. This weak gravitational lensing effect can be used to reconstruct the (dark) matter distribution in the lens, and to obtain a mass estimate independent of its luminosity and virialisation. A description of the basic (cosmological) physics behind gravitational lensing is given in the first Chapter of the present work, together with an introduction into the subject of gravitational lensing itself.
With the rapid improvement of detector and telescope technology, and the advent of wide field imagers at the end of the nineties, it became possible to turn the lensing argument around and use the weak lensing effect to search for mass concentrations in the universe. This method has the advantage that the galaxy clusters are detected directly by their most fundamental property, the mass, and not by their luminosity. The mass of a cluster, in turn, is a sensitive measure of cosmology and thus a mass-selected sample of galaxy clusters is highly desireable in this respect. A significant fraction of clusters detected in this way is rather enigmatic, since they appear to be entirely dark, i.e. they are not associated with any light. The physical nature of these objects, if they are indeed real and not due to some yet unknown systematics, is still unclear. This work establishes for the first time a larger sample of such objects.
For the latter purpose, our group conducted a weak lensing survey with the Wide Field Imager at the 2.2m MPG/ESO telescope. 20 square degrees of the southern sky were mapped to great depth in excellent observational conditions. For the reduction of the very large amount of data (several TB), a fully automatic pipeline had to be developed that meets all specifications required for an analysis of the weak lensing effect. Four men years were used for the development and thorough testing of this tool. A description of the pipeline and the techniques required especially for multi-chip cameras is given in Chapter 2.
In Chapter 3 the focus of interest is on the analysis of the reduced images, and the extraction of the desireable weak lensing signal. The basic quantity which is to be obtained from the images are the shapes of the potentially lensed galaxies, which is a highly non-trivial task as is outlined in the text. Having measured image shapes at hand, a statistics is introduced that allows the identification of a distortion pattern characteristic for galaxy clusters, and an estimate of the signal-to-noise of its detection. This statistics involves a filter function, for which several suggestions are given in the literature. Further possibilities are introduced in this work, and the various filters are tested extensively against each other. A very effective new filter was found in this process for the detection of mass concentrations.
Chapter 4, finally, verifies the used evaluation methods and presents the results obtained from the conducted survey. It is shown that the galaxies in the survey fields are not randomly oriented, but show significant coherent shear patterns, which are consistent with simulations. Besides, a sample of 100 mass concentrations with a signal-to-noise of at least 4 is presented. About 60% of the mass concentrations found are dark, whereas 30% are bright, having obvious optical counterparts. Half of the latter are already spectroscopically confirmed. The remaining 10% of the sample could not be classified. The populations of the bright and dark mass peaks show very similar distributions in their size as well as in their significance, apart from the very smallest angular scale probed where disproportionate many dark peaks are found. It is shown that true mass peaks can be discerned from spurious peaks in the sense that they appear on a significant level for a broader range of filter scales. Thereafter, various verification methods for the mass detections found are presented by means of an example.
dc.rightsIn Copyright
dc.subjectWeak gravitational lensing
dc.subjectDark matter
dc.subjectClusters of galaxies
dc.subjectData reduction
dc.subject.ddc520 Astronomie, Kartografie
dc.titleWeak gravitational lensing
dc.title.alternativeDetection of mass concentrations in wide field imaging data
dc.typeDissertation oder Habilitation
dc.publisher.nameUniversitäts- und Landesbibliothek Bonn
ulbbnediss.affiliation.nameRheinische Friedrich-Wilhelms-Universität Bonn
ulbbnediss.fakultaetMathematisch-Naturwissenschaftliche Fakultät
dc.contributor.coRefereeBartelmann, Matthias

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