tokens: Based on observations made with the WHT operated on the island of La Palma by the Isaac Newton Group of Telescopes in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. This research has been supported by the Spanish Ministry of Economy and Competitiveness (MINECO) under the grants AYA2007-67625-CO2-01, AYA2009-12903 and AYA2012-39408-C02-02. JEB acknowledge financial support to the DAGAL network from the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme FP7/2007-2013/ under REA grant agreement number PITN-GA-2011-289313. This research made use of astropy , a community-developed core python package for astronomy (Astropy Collaboration et al. 2013 ), aplpy , an open-source plotting package for python hosted at http://aplpy.github.com , and astrodendro , a python package to compute dendrograms of astronomical data ( http://www.dendrograms.org/ ). We acknowledge the usage of the HyperLeda data base ( http://leda.univ-lyon1.fr ). We thank Roberto Terlevich, the referee of the paper, whose comments have led to important improvements on the original version of the paper.
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