1. Universitäts-Sternwarte, Ludwig-Maximilians-Universität München, Scheinerstr 1, D-81679 München, Germany
2. Excellence Cluster Origin and Structure of the Universe, Boltzmannstr 2, D-85748 Garching bei München, Germany
3. Astrophysics Group, Imperial College London, Blackett Laboratory, Prince Consort Road, London SW7 2AZ, UK