BPT diagnostic diagrams [OIII]/Hβ vs. [NII]/Hα (left), and [SII]/Hα (right) derived from the composite spectra extracted from the five regions of the Cartwheel galaxy. The small grey points are the individual HII regions from Zaragoza-Cardiel et al. (2022). The grey contours are obtained from the nuclear spectra of a sample of 50 000 SDSS galaxies at 0.03 < z < 0.08 whose emission lines are detected at S/N > 5. The dashed lines indicate the separation between the stellar photoionisation processes (towards to left) and AGN/shock ionisation (top and right) as defined in Kauffmann et al. (2003) (K03, left panel) and in Kewley et al. (2006) (K06, right panel). The black solid lines in the left panel show the tracks from photoionisation models of Kewley et al. (2001) for three different metallicities (0.2, 0.4, and 1 Z_⊙).