  We study the properties of galaxies at z=2 in a Lambda cold dark matter
universe, using two different types of hydrodynamic simulation methods --
Eulerian TVD and smoothed particle hydrodynamics (SPH) -- and a
spectrophotometric analysis in the U_n, G, R filter set. The simulated galaxies
at z=2 satisfy the color-selection criteria proposed by Adelberger et al.
(2004) and Steidel et al. (2004) when we assume Calzetti extinction with
E(B-V)=0.15. We find that the number density of simulated galaxies brighter
than R<25.5 at z=2 is about 1e-2 h^3 Mpc^-3 for E(B-V)=0.15, which is roughly
twice that of the number density found by Erb et al. (2004) for the UV bright
sample. This suggests that roughly half of the massive galaxies with M*>10^{10}
Msun/h at z=2 are UV bright population, and the other half is bright in the
infra-red wavelengths. The most massive galaxies at z=2 have stellar masses >=
10^{11-12} Msun. They typically have been continuously forming stars with a
rate exceeding 30 Msun/yr over a few Gyrs from z=10 to z=2, together with
significant contribution by starbursts reaching up to 1000 Msun/yr which lie on
top of the continuous component. TVD simulations indicate a more sporadic star
formation history than the SPH simulations. Our results do not imply that
hierarchical galaxy formation fails to account for the observed massive
galaxies at z>=1. The global star formation rate density in our simulations
peaks at z>=5, a much higher redshift than predicted by the semianalytic
models. This star formation history suggests early build-up of the stellar mass
density, and predicts that 70 (50, 30)% of the total stellar mass at z=0 had
already been formed by z=1 (2, 3). Upcoming observations by Spitzer and Swift
might help to better constrain the star formation history at high redshift.
