Dust spectral index β from the pixel-by-pixel single-T MBB fit versus the molecular gas surface density, ${\mathrm{\Sigma }}_{\mathrm{gas}}^{\mathrm{mol}}$ (left panel), and the average ISRF, ⟨U⟩ (right panel). Each point represents an 8″ ∼ 560 pc pixel, colour-coded by THEMIS χ_red² (left) or ${\mathrm{\Sigma }}_{\mathrm{gas}}^{\mathrm{mol}}$ (right). Analysis is limited to pixels within the CO(1–0) mask (Fig. B.4); angular resolution is 25″. Pearson correlation coefficients (r) are shown at the top. Filled stars mark the centre (magenta), spiral arms (blue), and disc (cyan) regions (Sect. 4.2); the red diamond indicates integrated values over the whole galaxy (Sect. 4.1).