1. Centre for Extragalactic Astronomy, Department of Physics, Durham University , South Road, Durham DH1 3LE , UK
2. Astronomical Institute, Czech Academy of Sciences , Boční II 1401, CZ-141 00 Prague , Czech Republic
3. School of Physics and Astronomy, University of Southampton , University Road, Southampton SO17 1BJ , UK
4. Dipartimento di Fisica e Astronomia, Università di Firenze , via G. Sansone 1, I-50019 Sesto Fiorentino, Firenze , Italy
5. INAF – Osservatorio Astrofisico di Arcetri , L.go Enrico Fermi 5, I-50125 Firenze , Italy