Comparison between the computed iron and nickel parent scale lengths (with 3σ error bars) and the photoionisation values from Huebner & Mukherjee (2015). Different outflow velocities are shown: the highlighted region corresponds to the range of parent scale lengths obtained using the HCN outflow velocity from Biver et al. (2026); the segmented lines indicate the HCN scale lengths derived from ALMA observations (Roth et al. 2026); and the solid lines represent the scale lengths computed using the standard $0.85r_h^{-0.5}$ km s⁻¹ outflow velocity law.