SED of M33. Radio spectrum (top): the emission model (solid curve), including primary and secondary synchrotron radiation (dotted curves of decreasing flux) as well as a thermal free–free component (dot-dashed curve), is overlaid with data from Table 2 (black dots). The empty squares represent the estimated radio flux after removing the contribution from ≤200 pc size sources (Tabatabaei et al. 2022). The total radio emission, resulting if the ‘nominal’ primary electron spectrum is used, is indicated by the dashed curve. Broadband SED (bottom): data points are shown as dots, model predictions as curves. Emission components are plotted by the following line types: radio synchrotron, dotted; thermal free–free emission, dot-dashed; total radio, solid; Comptonised CMB, long-dashed; Comptonised starlight (EBL+FGL), short-dashed; non-thermal bremsstrahlung, dotted; and pionic, solid. For each type of non-thermal leptonic yield, the total, primary, and secondary emissions are denoted as curves of progressively lower flux. In the X-ray and γ-ray range, on top of the separate components, a solid line indicates the total emission. The empty squares and dashed curve in the radio are as described in the top panel.