Top panels: light-weighted metallicity as a function of stellar mass surface density Σ_⋆. Dots are colour-coded in tones of grey according to their distance r/r_e. The left panel corresponds to galaxies in the field, the central panel shows galaxies in groups, and the right panel shows the iso-contour levels for galaxies in groups (red lines) and galaxies in the field (blue lines). The bottom panels show the median value of log(Z_L/Z_⊙) as a function of Σ_⋆ for the five stellar-mass bins considered. Vertical error bars were computed using the bootstrap re-sampling technique.