Iron abundance profiles of Cepheids (top) and open clusters (bottom). Data sources are provided in Table 1. Symbols in the upper panel correspond to Ref. 3 and in the lower panel to Refs. 9 (blue filled squares), 10 (brown asterisks) and 11 (green open squares), respectively. Curves corespond to model results at time 4, 8 and 12 Gyr (thick curve), respectively). The thick red curve in the top panel corresponds to the average metallicity of a young stellar population of age 0.2 ± 0.2 Gyr (Cepheids) and the shaded aerea represents the corresponding ±1σ dispersion.