Near-IR-to-radio SED modelling of the spiral arms (blue), disc (cyan), and centre (magenta) of M 99 with HerBIE. Filled circles show observed photometry; grey diamonds indicate synthetic photometry. Left panel: Full SED fits (solid lines) including stellar emission (starBB, yellow areas); dust emission, using powerU with THEMIS (blue, cyan, magenta areas); and the radio continuum (radio, red areas). Right panel: Fits restricted to λ ≥ 100 μm, modelling dust with single-T MBB (MBB1, coloured areas), and excluding stellar emission. Empty circles were not included in the fit. Monochromatic luminosities (ν L_ν) assume a distance of 14.4 Mpc (see Table 1). Residuals are shown below each panel.