Position-velocity plots of the logarithm of the electron density n_e. Contours indicate regions inside which: cyan – [S ii]λ6716 ≥ 3σ; orange – [S ii]λ6731 ≥ 3σ; green – [S ii]31/16 ratio at the high density limit. Where only [S ii]λ6731 ([S ii]λ6731) ≥ 3σ, the derived n_e is a lower (upper) limit. Inside the green contours n_e is derived with the modified HDL-BE procedure (cf. Sect. 2.2).