tokens: The first author wish to thank the School of Astronomy, IPM for providing support while working on this paper. AM also acknowledges the Isaac Newton Group of Telescopes (ING) and the Instituto de Astrofísica de Canarias (IAC) for hospitality and support while this paper was in progress. We also thank an anonymous referee for a lot of constructive comments. The authors acknowledge support from the Spanish Ministry of Economy and Competitiveness (MINECO) through grants AYA2009-11137, AYA2013-48226-C3-1-P, AYA2013-46886-P and AYA2014-58308-P. JFB also acknowledges financial support from the DAGAL network from the People Programme (Marie Curie Actions) of the European Unions Seventh Framework Programme FP7/2007-2013/ under REA grant agreement number PITN-GA-2011-289313. Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the US Department of Energy Office of Science. The SDSS-III web site is http://www.sdss3.org/ . SDSS-III is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS-III Collaboration including the University of Arizona, the Brazilian Participation Group, Brookhaven National Laboratory, Carnegie Mellon University, University of Florida, the French Participation Group, the German Participation Group, Harvard University, the Instituto de Astrofísica de Canarias, the Michigan State/Notre Dame/JINA Participation Group, Johns Hopkins University, Lawrence Berkeley National Laboratory, Max Planck Institute for Astrophysics, Max Planck Institute for Extraterrestrial Physics, New Mexico State University, New York University, Ohio State University, Pennsylvania State University, University of Portsmouth, Princeton University, the Spanish Participation Group, University of Tokyo, University of Utah, Vanderbilt University, University of Virginia, University of Washington and Yale University.
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