Electron densities (n_e) estimated based on the C III (top) and Si III (bottom) doublet line flux ratios for a fixed electron temperature of T_e = 10⁴ K. The error bars (and limits) correspond to the 1σ uncertainties on the doublet flux ratios. For comparison, the theoretical curves (e.g., Osterbrock & Ferland 2006; Kewley et al. 2019a,b) for electron temperatures of 5000 K, 10 000 K and 20 000 K are shown as the solid red curves. The filled histograms count the number of objects where n_e could be determined from the observed flux ratio. The open histograms add the subset of objects where the n_e estimates are upper limits. The single open symbol to the right in the top panel indicates the C III flux ratio of object 605172634 (Fig. E.3) which results in an unconstrained electron density estimate for T_e = 10⁴ K.