An observer's view of simulated galaxies: disc-to-total ratios, bars and (pseudo-)bulges

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 Letter 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 (D/T) ratio 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 due 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.