tokens: We thank the anonymous referee for her/his useful comments. Fruitful discussions with F. Fraternali, J.P. Naiman, R. Sancisi, M. Tosi, and G. Zamorani are also acknowledged. FD'A and AD'E acknowledge support from PRIN INAF 2014 (principal investigator S. Cassisi). EV acknowledges support from grant NASA-NNX13AF45G .
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