Ratio of versus absorption redshift (z_abs) for the very strong Mg ii systems (top) and weak Mg ii systems (bottom). Symbols are equivalent to those in Fig. ⁵. Two opposite trends are present. There is an absence of very strong absorbers with low ratios at low redshift (top) and an absence of weak absorbers with large ratios at high redshift (bottom). In the case of the very strong absorbers, we chose Fe ii λ2587, because it was less often saturated than the Fe ii λ2383 and λ2600 transitions, and was covered more often than the weaker Fe ii λ2374 transition. For weak Mg ii absorbers, Fe ii λ2383 was chosen because, as the strongest Fe ii transition, it is mostly likely to be detected.