Spatially resolved relations between the magnetic field strength and the SFR surface density (B–Σ_SFR) (a), the atomic gas mass surface density (B–Σ_H I) (b), molecular gas mass surface density (B–Σ_H2) (c), and the combined atomic and molecular gas mass surface density (B–Σ_{H I + H2}) (d). Data points are coloured according to the radio spectral index and, in panel (b), according to the point density. Best-fitting relations are shown as sold lines and the theoretical expectations as dashed lines.