  A quantitative derivation of the intrinsic properties of galaxies related to
their fundamental building blocks, gas, dust and stars is essential for our
understanding of galaxy evolution. A fully self-consistent derivation of these
properties can be achieved with radiative transfer (RT) methods that are
constrained by panchromatic imaging observations. Here we present an
axi-symmetric RT model of the UV-optical-FIR/submm spectral and spatial energy
distribution of the face-on spiral galaxy M51. The model reproduces reasonably
well the azimuthally averaged radial profiles derived from the imaging data
available for this galaxy, from GALEX, SDSS, 2MASS, Spitzer and Herschel. We
model the galaxy with three distinct morphological components: a bulge, an
inner disc and a main disc. We derive the length parameters of the stellar
emissivity and of the dust distribution. We also derive the intrinsic global
and spatially resolved parameters of M51. We find a faint \lq\lq outer
disc\rq\rq\ bridging M51 with its companion galaxy M51b. Finally, we present
and discuss an alternative model, with dust properties that change within the
galaxy.
