tokens: ACKNOWLEDGEMENTS We thank the referee for the useful and constructive comments that improved the presentation of the results.This work is based on observations carried out at the Large Binocular Telescope (LBT) under program ID 2015_2016_28. The LBT is an international collaboration among institutions in the US, Italy and Germany. We acknowledge the support from the LBT-Italian Coordination Facility for the execution of the observations, the data distribution and for support in data reduction. We thank D. Thomas and J. Lian for having provided us with the median metallicity values of eBOSS galaxies from Comparat et al. ( 2017 ). PS would like to thank T. Durden for the useful suggestions.
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