Sensitivity analysis for the metallicity gradients. The reference analysis (black) adopts BC03 models, the MILES stellar library, a Chabrier IMF, and z_f = 6. The result we obtained when we adopted a different z_f = 11 is reported in violet, a different metallicity prior (uniform) in red, a different age prior (logaritmically uniform) in cyan, an extended star formation timescale (0.5 Gyr) in grey, a different stellar library (STELIB) in light blue, and a different model (Maraston) in brown. The blue, yellow, and green symbols are gradients derived by adopting a different IMF (−1.5 slope, Salpeter, and −3.5 slope, respectively). These slopes are systematically flatter because the range of metallicities considered in this comparison is smaller.