Mass–excitation diagram for our sample galaxies. The blue circles represent individual measurements. The red and blue diamonds represent stacked measurements in cluster and field galaxies, respectively. The blue and orange curves indicate the lower and upper mass-excitation demarcation by Coil et al. (2015). The AGNs (star-forming galaxies) are above (below) the demarcation. The gray shaded region indicates the high-mass end, which we did not analyze due to uncertainties in the metallicity calibrations in this regime.