tokens: We thank the anonymous referee for insightful comments which greatly improved the quality of this manuscript. This work is based on observations at the W.M. Keck Observatory, which is operated as a scientific partnership among Caltech, the University of California and NASA. S.H. wishes to thank B. Siana for very constructive comments and I. Shivaei for useful discussions. D.S. acknowledges financial support from the Netherlands Organization for Scientific research (NWO) through a Veni fellowship, from FCT through a FCT Investigator Starting Grant and Start-up Grant (IF/01154/2012/CP0189/CT0010) and from FCT grant PEst-OE/FIS/UI2751/2014.
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