Top panel: panchromatic SED of M 51. The black line is the best-fitting radiative transfer model, run at high-resolution. The green diamonds are the observed broadband luminosities of M 51 (see Table A.1). The red, cyan, and violet lines represent the SEDs for simulations with only the stellar components of M 51a: old, young non-ionising, and young ionising stellar population, respectively. The orange line represents the SED of M 51b. The dust component, which includes the dusty discs of both galaxies, is still present in these simulations. Bottom panel: difference in dex values between the observations and the mock luminosities.