Massive Galaxies in Cosmological Simulations: Ultraviolet‐selected Sample at Redshift z = 2 (original) (raw)

2005, The Astrophysical Journal

We study the properties of galaxies at redshift z = 2 in a Λ cold dark matter (ΛCDM) 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 and 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 2×10 −2 h 3 Mpc −3 for E(B −V ) = 0.15 in our most representative run, roughly one order of magnitude larger than that of Lyman break galaxies at z = 3. The most massive galaxies at z = 2 have stellar masses 10 11 M ⊙ , and their observed-frame G − R colors lie in the range 0.0 < G − R < 1.0. They typically have been continuously forming stars with a rate exceeding 30 M ⊙ yr −1 over a few Gyrs from z = 10 to z = 2, although the TVD simulation indicates a more sporadic star formation history than the SPH simulations. Of order half of their stellar mass was already assembled by z ∼ 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 ⊙ , and lack a prominent old stellar population. On the other hand, the reddest massive galaxies at z = 2 with G − R ≥ 1.0 and M star > 10 10 h −1 M ⊙ completed the build-up of their stellar mass by z ∼ 3. Interestingly, our study indicates that -2the majority of the most massive galaxies at z = 2 should be detectable at restframe ultra-violet 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 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 Vband luminosity function of z = 2 galaxies can be characterized by a Schechter function with parameters (Φ * , M * V , α) = (1.8 × 10 −3 , −23.4, −1.85), while the TVD simulation suggests a flatter faint-end slope of α ∼ −1.2. A comparison with z = 3 shows that the rest-frame V -band luminosity function has brightened by about 0.5 magnitude 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 z 1.