UV-mm spectral energy distribution of NGC 3628. The red data points are collected from the literature, as discussed in Sect. 2. The solid green line is the input SED model, which consists of an evolved (5 Gyr) stellar population normalised to the IRAC 3.6 μm flux density, and a young (50 Myr) stellar population normalised to the GALEX FUV flux density. The solid blue line is the SED from our panchromatic radiative transfer model, with a thin and thick disc. The solid yellow line is the SED corresponding to a model with only a thin disc; it systematically underestimates the observed flux densities in the far-infrared and submm range.