Theoretical population ratios for different ionic species at T_e = 15 000 K (solid curves). The vertical part of each cross represents the observed column density ratio and its error for each outflowing system, while the horizontal part represents the associated n_e error. The arrows indicate lower limits obtained from possibly saturated troughs corresponding to these species. The case of J0542 is explained in Section 6.1. The top axis shows the distance scale based on the outflow detected in J1426+3151. The black stars and arrows correspond to objects from Xu et al. (2019).