Residuals of the fit given by Eq. (4) plotted against specific SFR (left panel) and 12 + log(O/H) (right). The metal-poor portion of the piece-wise regression in the right panel is given by , and the “fit” refers to Eq. (4), both shown by long-dashed (gray) lines. The gray shadowed region in the right panel indicates the range of uncertainty on the slope in the above equation, and the short-dashed (gray) line corresponds to a power-law slope of 2.0 (compared to the best-fit value here of 1.55). As in Fig. 2, galaxies are color-coded by 12 + log(O/H).