Radial ionization stratification in the LISs of NGC 7009. Isocontours normalized to the maximum value of the emission lines at the position of knot k1, overlaid on the MUSE [N II] 16584 emission line map from Akras et al. (2022b). Upper left panel: Hα (red), Brγ (dashed red) and H₂ (yellow) emission lines. Lower left panel: Brγ and H₂ (dashed red and yellow; Akras et al. 2020a), [Fe II] (cyan; Akras et al. 2024), [O I], [S II] and [O III] (dashed green, magenta and blue, respectively; Akras et al. 2022b) and [C I] (green, this work). The field of view of the upper and lower panels is 7.5×6 arcsec and 6.5×4.8 arcsec, respectively. The arrow indicates the direction of the central star. North is up and east is to the left.