We propose to study dark matter halos of galaxies with weak gravitational lensing using the VIRMOS- Descart survey. The results of these studies will provide a valuable test for current theories of galaxy- formation. These theories are able to make detailed predictions on the dark matter halos in which galaxies are embedded while observational studies still remain challenging. Galaxy-galaxy lensing is a very promising technique that has been developed within the last decade and that is currently used mainly by researchers and collaborations under US or Canadian leadership. It can probe halos out to large radii and can thus measure their density profiles which can then in turn be compared to the predictions from the simulations.
Further, the relation between mass, size and concentration of dark matter halos and the luminosity, spectral type, redshift and environment of galaxies will be studied. The VIRMOS-Descart survey is currently the best suited data set for these investigations. It has already been used successfully for other weak lensing studies including the first detection of "cosmic shear", the weakest lensing signal. The lensing group at the host institution has broad experience with all requirements of weak lensing studies and will provide the data for the proposed project. The applicant is most familiar with the measurement of galaxy-galaxy lensing from her PhD work in which she already applies it to another, but substantially smaller data set. In view of currently planned weak lensing surveys, the prospects from the research training for the applicant and the European research area are most promising. On the one hand, experienced researchers will be needed to use the huge data sets from planned surveys and make optimal use of it, and on the other hand, the proposed project will offer the opportunity to the applicant to acquire all major qualifications that will be important for pursuing an independent research career.