Dark matter halos of galaxies studied with weak gravitational lensing

Abstract

The main goal of this thesis is to investigate the dark matter halos of galaxies. Current theories of galaxy formation predict that galaxies are embedded in large dark matter halos with much larger extents than the visible parts of the galaxies. Testing this prediction from observations is challenging because - by definition - the dark matter halos do not emit any radiation. Therefore, they can only be studied using gravity. <br /> We choose weak gravitational lensing for our studies. The method is also known as galaxy-galaxy lensing. Lensing has the advantage that no assumptions concerning the dynamical state of the galaxies have to be made but that the dark matter distribution can be probed directly. We will use data from the COMBO-17 survey which is ideally suited for this kind of investigation. The most important advantage of COMBO-17 is that it combines deep observations with a large field-of-view and also gives redshifts and spectral classification for objects brighter than R=24. This data set allows us to extend existing studies of the local population of lens galaxies to higher redshifts. The most interesting questions in these studies concern not only the density profiles of dark matter halos of galaxies but also the connection between the luminous parts of galaxies and their dark matter halos. We will investigate how the dark matter halos depend on luminosity and spectral type of the galaxies. <br /> In weak lensing studies, shapes of source galaxies have to be measured accurately. Usually, these galaxies are small and cover only a few pixels on the CCD images. Further, they are distorted by e.g. defocussing, guiding errors and, most important for ground-based observations, the atmosphere. Special software is available to correct for these distortions. However, so far it has not been tested on observational data how well the corrections work and if galaxy shapes can be recovered reliably. We will use multiple observations from one of the COMBO-17 fields to compare the shape measurements from independent sumframes and thus test the reliablitiy of shape measurements.