ar X iv : 0 80 4 . 19 59 v 1 [ as tr o - ph ] 1 1 A pr 2 00 8 Submitted to ApJ

Thüringer Landessternwarte Tautenburg, Sternwarte 5, D–07778 Tautenburg, Germany Department of Physics and Astronomy, University of Nevada, Las Vegas, NV 89154 Department of Astronomy, University of Texas, Austin, TX 78712, USA Townes Fellow, Space Sciences Laboratory, University of California, Berkeley, CA, 94720-7450, USA Astronomy Department, University of California, 445 Campbell Hall, Berkeley, CA 94720-3411, USA Dark Cosmology Centre, Juliane Maries Vej 30, DK-2100 København Ø, Denmark Division of Physics, Mathematics, and Astronomy, California Institute of Technology, Pasadena, CA 91125 INAF, Osservatorio Astronomico di Roma, via Frascati 33, 00040, Monteporzio Catone (RM), Italy INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy Università degli studi di Milano-Bicocca, Dipartimento di Fisica, piazza delle Scienze 3, 20126 Milano, Italy Department of Astronomy, Yale University, P.O. Box 208101, New Haven, CT 06520, USA Dipartimento di Fisica e Matematica, Università dell’Insubria, via Valleggio 11, 22100 Como, Italy INAF, Osservatorio Astrofisico di Arcetri, largo E. Fermi 5, I-50125 Firenze, Italy Instituto de Astrof́ısica de Andalućıa (IAA-CSIC), Apartado de Correos, 3.004, E-18.080 Granada, Spain INAF, Istituto di Radioastronomia, Largo E. Fermi 5, 50125 Firenze, Italy Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, United Kingdom