Logarithmic map of the ratio of [Fe II] lines at 17.9 and 5.3 µm. Cyan contours show the [FeII] (5.3 µm) line flux at 3, 10, and 50σ (0.1, 0.4, 2.1 mJy pixel⁻¹). Colour code reports the logarithmic value of the ratio (bottom) and the corresponding n_e (in cm⁻³) for an average T_e of 1000 K. Only pixels within 3σ line contours are displayed. The magenta dot shows the position of the ALMA mm continuum source.