  We analyse a high-resolution, fully cosmological, hydrodynamical disc galaxy
simulation, to study the source of the double-exponential light profiles seen
in many stellar discs, and the effects of stellar radial migration upon the
spatio-temporal evolution of both the disc age and metallicity distributions.
We find a "break" in the pure exponential stellar surface brightness profile,
and trace its origin to a sharp decrease in the star formation per unit surface
area, itself produced by a decrease in the gas volume density due to a warping
of the gas disc. Star formation in the disc continues well beyond the break. We
find that the break is more pronounced in bluer wavebands. By contrast, we find
little or no break in the mass density profile. This is, in part, due to the
net radial migration of stars towards the external parts of the disc. Beyond
the break radius, we find that ~60% of the resident stars migrated from the
inner disc, while ~25% formed in situ. Our simulated galaxy also has a minimum
in the age profile at the break radius but, in disagreement with some previous
studies, migration is not the main mechanism producing this shape. In our
simulation, the disc metallicity gradient flattens with time, consistent with
an "inside-out" formation scenario. We do not find any difference in the
intensity or the position of the break with inclination, suggesting that
perhaps the differences found in empirical studies are driven by dust
extinction.
