Illustration of the spatial distribution of the atomic carbon that reveals the clumpy structures superimposed on the extended emission of NGC 7009. Upper panel: [C I] λ8727 line MUSE flux map at a 5×5 spatial binning. Six different clumpy structures on the nebula are indicated as red crosses and labeled from k1 to k6 (Gonçalves et al. 2004a). The color bar corresponds to the intensity of the emission line. Bottom panels: observed spectra (solid blue lines) in the range from 8720 to 8740 Å extracted from the six distinct spaxels in units of 10⁻²⁰ erg s⁻¹ cm⁻¹ Å⁻¹. The three emission lines in this particular spectral range are [C I] at 8727.1 Å, [Fe III] at 8728.8 Å, and He i at 8733.4 Å, and they were fitted assuming a Gaussian profile (dashed orange lines). The vertical dashed green lines indicate the wavelengths in the rest frame.