We perform a spectrophotometric analysis of galaxies at redshifts z = 4 - 6 in cosmological SPH simulations of a \Lambda cold dark matter ( \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 ^ { \prime } , and z ^ { \prime } filters , and compare the results with observational surveys of Lyman break galaxies ( LBGs ) performed 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 .
The stellar masses of the most massive galaxies in our largest simulation increase their stellar mass from M _ { \star } \sim 10 ^ { 11 } M _ { \odot } at z = 6 to M _ { \star } \sim 10 ^ { 11.7 } M _ { \odot } at z = 3 .
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 ) \sim 0.15 , the simulated luminosity functions match observational data , but have a steep faint-end slope with \alpha \sim - 2.0 .
We discuss the implications of the steep faint-end slope found in the simulations .
Our results confirm the generic conclusion from earlier numerical studies that UV bright LBGs at z \geq 3 are the most massive galaxies with E ( B - V ) \sim 0.15 at each epoch .