  We perform a spectrophotometric analysis of galaxies at redshifts z = 4 - 6
in cosmological SPH simulations of a Lambda CDM universe. Our models include
radiative cooling and heating by a uniform UV background, star formation,
supernova feedback, and a phenomenological model for galactic winds. Analysing
a series of simulations of varying boxsize and particle number allows us to
isolate the impact of numerical resolution on our results. Specifically, we
determine the luminosity functions in B, V, R, i', and z' filters, and compare
the results with observed galaxy surveys done with the Subaru telescope and the
Hubble Space Telescope. We find that the simulated galaxies have UV colours
consistent with observations and fall in the expected region of the
colour-colour diagrams used by the Subaru group. Assuming a uniform extinction
of E(B-V) = 0.15, we also find reasonable agreement between simulations and
observations in the space density of UV bright galaxies at z = 3 - 6, down to
the magnitude limit of each survey. For the same moderate extinction level of
E(B-V) ~ 0.15, the simulated luminosity functions match observational data, but
have a steep faint-end slope with alpha ~ -2.0. We discuss the implications of
the steep faint-end slope found in the simulations.
