[1] Univ Ferrara, Dipartimento Fis & Sci Terra, Via Saragat 1, I-44122 Ferrara, Italy
[2] INAF, Osservatorio Astron Bologna, Via Gobetti 93-3, I-40129 Bologna, Italy
[3] Univ Milan, Dipartimento Fis, Via Celoria 16, I-20133 Milan, Italy
[4] Univ Copenhagen, Niels Bohr Inst, Dark Cosmol Ctr, Juliane Maries Vej 30, DK-2100 Copenhagen, Denmark
[5] Ist Nazl Fis Nucl, Sez Bologna, Viale Berti Pichat 6-2, I-40127 Bologna, Italy
[6] INAF, Osservatorio Astron Capodimonte, Via Moiariello 16, I-80131 Naples, Italy
[7] Univ Observ Munich, Scheinerstr 1, D-81679 Munich, Germany
[8] INAF, Osservatorio Astron Trieste, Via GB Tiepolo 11, I-34143 Trieste, Italy
[9] Acad Sinica, Inst Astron & Astrophys, POB 23-141, Taipei 10617, Taiwan
[10] Univ Trieste, Dipartimento Fis, Via GB Tiepolo 11, I-34143 Trieste, Italy
[11] INAF, Osservatorio Astrofis Arcetri, I-50125 Florence, Italy
[12] Heidelberg Univ, Zentrum Astron, Inst Theoret Astrophys, Philosophenweg 12, D-69120 Heidelberg, Germany
[13] Space Telescope Sci Inst, 3700 San Martin Dr, Baltimore, MD 21208 USA
[14] Univ Groningen, Kapteyn Astron Inst, Postbus 800, NL-9700 AV Groningen, Netherlands
[15] Johns Hopkins Univ, Dept Phys & Astron, 3400 North Charles St, Baltimore, MD 21218 USA
[16] INAF, Ist Astrofis Spaziale & Fis Cosm IASF, Via E Bassini 15, I-20133 Milan, Italy
[17] Korea Inst Adv Study, Sch Phys, Hoegiro 85, Seoul 02455, South Korea
[18] Max Planck Inst Extraterr Phys, Giessenbachstr, D-85748 Garching, Germany
[19] Ben Gurion Univ Negev, Phys Dept, POB 653, IL-84105 Beer Sheva, Israel