Density plots for the [Fe II]λ5.3403  μm/[Ar II]λ6.9853  μm versus H₂ S(5)/[Ar II]λ6.9853  μm (top panels) and [Fe II]λ5.3403  μm/[Ar II]λ6.9853  μm versus [Ar III]λ8.9914  μm/[Ar II]λ6.9853  μm (bottom panels). The dark red lines (bottom-right panel) show predictions from photoionisation models of star-forming regions, assuming stellar clusters aged 6 and 7 Myr. Solid lines indicate different gas densities, dotted lines correspond to varying ionisation parameters (as labelled in the figure), and the predictions for both cluster ages are connected by dashed lines. Similarly, results from AGN photoionisation models are shown as dark blue lines (bottom-central panel). The gray lines and filled circles (bottom-left panel) are predictions of fast shock models from Pereira-Santaella et al. (2024) using the MAPPINGS V code (Sutherland & Dopita 2017) for log R_P = 6 (dashed) and log R_P = 8 (continuous). The numbers indicate the shock velocities in kms⁻¹.