tokens: We thank an anonymous referee for valuable comments that helped improve the paper. We thank Santiago García-Burillo for insightful discussions and Sebastian Hönig and Leonard Burtscher for making their fully reduced spectra available to us. We acknowledge financial support from the Spanish Ministry of Economy and Competitiveness through the Plan Nacional de Astronomía y Astrofísica grants AYA2012-31447 (AA-H and AH-C), and AYA2015-64346-C2-1-P (AA-H), which were partly funded by the FEDER programme, and AYA2015-70815-ERC and AYA2012-31277 (AH-C) and AYA2012-32295 (MP-S and LC). AA-H is also partly funded by CSIC/PIE grant 201650E036. RP acknowledges the Oxford University Summer Research Programme. IA and MM-P are partly supported by Mexican CONACyT through research grant CB-2011-01-167291. MM-P acknowledges support by the CONACyT PhD fellowship programme. CRA acknowledges the Ramón y Cajal Programme of the Spanish Ministry of Economy and Competitiveness. NAL is supported by the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., on behalf of the international Gemini partnership of Argentina, Brazil, Canada, Chile, and the United States of America. TD-S acknowledges support from ALMA-CONICYT project 31130005 and FONDECYT 1151239. Based on observations made with the GTC, installed in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, in the island of La Palma. This research has made use of the NED which is operated by JPL, Caltech, under contract with the National Aeronautics and Space Administration. This work is based in part on observations made with the Spitzer Space Telescope , which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA.
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