H0LiCOW II. Spectroscopic survey and galaxy-group identification of the strong gravitational lens system HE0435-1223

D. Sluse, A. Sonnenfeld, N. Rumbaugh, Cristian E. Rusu, Christopher D. Fassnacht, T. Treu, Sherry H. Suyu, K. C. Wong, Matthew W. Auger, V. Bonvin, T. Collett, Frederic Courbin, S. Hilbert, L. V. E. Koopmans, Philip J. Marshall, Georges Meylan, C. Spiniello, M. Tewes