Dependence of the magnesium-to-oxygen abundance ratio relative to the solar value, [Mg/O] = log (Mg/O) – log (Mg/O)_⊙, on oxygen abundance 12+log O/H with error bars for all data points. The solar value log (Mg/O)_⊙ = −1.09 is adopted from Asplund et al. (2009). Dashed line and ⟨[Mg/O]⟩ in both panels are mean values of [Mg/O]. a) The encircled galaxies are those with no clear presence of blue-shifted absorption profiles (Fig. 2). b) The same sample as in a) but with division between bright Mg ii lines with I(λ2797 Å)/I(Hβ) ≥ 0.3 (red filled circles) and weak Mg ii lines with I(λ2797 Å)/I(Hβ) < 0.3 (blue filled circles). Encircled galaxies in b) are those where [O iii]λ4363 Å is measured. Additionally, the mean values of [Mg/O] for bright (⟨[Mg/O]⟩(bright)) and weak (⟨[Mg/O]⟩(weak)) Mg ii lines are denoted.