Synchrotron spectral index (α_sync) versus SFR surface density (Σ_SFR), colour-coded by total gas surface density (${\mathrm{\Sigma }}_{\mathrm{gas}}^{\mathrm{tot}}$). The index α_sync is constrained to the range [0.5, 1.2], as indicated by the grey shaded areas. Each circle is an 8″ (∼560 pc) pixel, and the angular resolution is 25″ (∼1.75 kpc). Stars show integrated values for the disc (cyan), arms (blue), centre (magenta), and the red diamond, the global value. Green crosses mark eight SF regions from the sSFR map (Appendix F), measured within 25″ apertures. Pearson coefficients are in the top right. Grey circles are low S/N pixels excluded from the analysis (Appendix E).