  We study the photometric properties of Lyman-break galaxies (LBGs) formed by
redshift z=3 in a set of large cosmological smoothed-particle hydrodynamics
simulations of the Lambda cold dark matter (CDM) model. Our numerical
simulations include radiative cooling and heating with 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. We
compute spectra of simulated galaxies using a population synthesis model, and
derive colours and luminosity functions of galaxies at z=3 after applying local
dust extinction and absorption by the intergalactic medium (IGM). We find that
the simulated galaxies have U-G and G-R colours consistent with observations,
provided that intervening absorption by the IGM is applied. The observed
properties of LBGs, including their number density, colours, and luminosity
functions, can be explained if LBGs are identified with the most massive
galaxies at z=3, having typical stellar mass of M_{star} ~ 1e10 Msun/h, a
conclusion broadly consistent with earlier studies based on hydrodynamic
simulations of the Lamda CDM model. We also find that most simulated LBGs were
continuously forming stars at a high rate for more than one Gyr up until z=3,
but with numerous starbursts lying on top of the continuous component.
Interestingly, our simulations suggest that more than 50% of the total stellar
mass and star formation rate in the Universe are accounted for by galaxies that
are not detected in the current generation of LBG surveys.
