  We use cosmological hydrodynamical simulations of the formation of Milky
Way-mass galaxies to study the relative importance of the main stellar
components, i.e., discs, bulges, and bars, at redshift zero. The main aim of
this work is to understand if estimates of the structural parameters of these
components determined from kinematics (as is usually done in simulations) agree
well with those obtained using a photometric bulge/disc/bar decomposition (as
done in observations). To perform such a comparison, we have produced synthetic
observations of the simulation outputs with the Monte-Carlo radiative transfer
code SUNRISE and used the BUDDA code to make 2D photometric decompositions of
the resulting images (in the i and g bands). We find that the kinematic
disc-to-total ratio (D/T) estimates are systematically and significantly lower
than the photometric ones. While the maximum D/T ratios obtained with the
former method are of the order of 0.2, they are typically >0.4, and can be as
high as 0.7, according to the latter. The photometric decomposition shows that
many of the simulated galaxies have bars, with Bar/T ratios in the range
0.2-0.4, and that bulges have in all cases low Sersic indices, resembling
observed pseudo-bulges instead of classical ones. Simulated discs, bulges and
bars generally have similar (g-i) colours, which are in the blue tail of the
distribution of observed colours. This is not due to the presence of young
stars, but rather to low metallicities and poor gas content in the simulated
galaxies, which makes dust extinction low. Photometric decompositions thus
match the component ratios usually quoted for spiral galaxies better than
kinematic decompositions, but the shift is insufficient to make the simulations
consistent with observed late-type systems.
