1. INAF/OAS Bologna, Via Piero Gobetti 101, Area della Ricerca CNR/INAF, I-40129 Bologna, Italy
2. INFN, Sezione di Bologna, Via Irnerio 46, I-40126 Bologna, Italy
3. Instituto de Astrofísica de Canarias, C/Via Lactea, s/n, E-38205 La Laguna, Tenerife, Spain
4. Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain
5. Leibniz Institute for Astrophysics Potsdam (AIP), An der Sternwarte 16, D-14482 Potsdam, Germany
6. Max-Planck Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
7. Center for Theoretical Astrophysics and Cosmology, Institute for Computational Science, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland
8. Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
9. Department of Physics, University of Surrey, Guildford GU2 7XH, UK