Massive Galaxies in Cosmological Simulations: Ultraviolet-selected Sample at Redshift Z = 2

Abstract

We study the properties of galaxies at redshift z = 2 in a Lambda cold dark matter (LambdaCDM) universe, using two different types of hydrodynamic simulation methods-Eulerian total variation diminishing (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. and Steidel et al. 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 2 x 10(-2) h(3) Mpc(-3) for E(B-V) = 0.15 in our most representative run, roughly 1 order of magnitude larger than that of Lyman break galaxies at z = 3. The most massive galaxies at z = 2 have stellar masses of greater than or similar to 10(11) M-circle dot, and their observed-frame G-R colors lie in the range 0.0 < G-R < 1.0. They typically have been continuously forming stars at a rate exceeding 30 M-circle dot yr(-1) over a few gigayears from z = 10 to z = 2, although the TVD simulation indicates a more sporadic star formation history than the SPH simulations. On the order of half of their stellar mass was already assembled by z similar to 4. The bluest galaxies with colors -0.2 < G-R < 0.0 at z = 2 are somewhat less massive, with M-star < 10(11) h(-1) M-circle dot, and lack a prominent old stellar population. On the other hand, the reddest massive galaxies at z = 2 with G-R greater than or equal to 1.0 and M-star > 10(10) h(-1) M-circle dot completed the build-up of their stellar mass by z similar to 3. Interestingly, our study indicates that the majority of the most massive galaxies at z = 2 should be detectable at rest-frame ultraviolet wavelengths, contrary to some recent claims made on the basis of near-infrared studies of galaxies at the same epoch, provided the median extinction is less than E(B-V) < 0.3 as indicated by surveys of Lyman break galaxies at z = 3. However, our results also suggest that the fraction of stellar mass contained in galaxies that pass the color-selection criteria used by Steidel et al. (2004) could be as low as 50% of the total stellar mass in the universe at z = 2. Our simulations imply that the missing stellar mass is contained in fainter ( R > 25.5) and intrinsically redder galaxies. The bright end of the rest-frame V-band luminosity function of z 2 galaxies can be characterized by a Schechter function with parameters (Phi*, M*(V), alpha) = (1.8 x 10(-3), -23.4, -1.85), while the TVD simulation suggests a flatter faint-end slope of alpha similar to -1.2. A comparison with z = 3 shows that the rest-frame V-band luminosity function has brightened by about 0.5 mag from z = 3 to z = 2, without a significant change in the shape. Our results do not imply that hierarchical galaxy formation fails to account for the massive galaxies at zgreater than or similar to1.