Left: strength of the optical Fe II emission (R_FeII) shown with respect to the distribution in cloud column density (N_H) from CLOUDY. The model uses a log U = −3.5 and log n = 11.75. The colours represent nine different cases of metallicity (Z). The observed estimates from Persson (1988) and Marinello et al. (2016) for I Zw 1 are shown in cyan and orange bars (the errors in these estimates are depicted by the widths of the bars), respectively. The inset plot zooms in on a portion of the base plot to highlight the modelled trends that recover the R_FeII within the observed values. We note that the R_FeII is shown in log scale in the base plot while for the inset plot we show the ratio in linear scale. Right: corresponding R_CaT distribution for the same modelled parameters as in the previous panel.