Relation between the stellar mass, in units of the solar masses, and the oxygen abundance at the central regions (r < 0.2 r_e), using the O3N2 calibration, for the galaxies considered in the current study (solid circles). The black-solid line shows the relation found by Tremonti et al. (2004) for the star-forming galaxies in the SDSS sample, corrected for the mismatch between their suggested calibrator and the currently used here (i.e., −0.15 dex). The grey-dashed lines show the ~1σ and ~2σ range around this relation.