Abstract
We study the properties of galaxies at z=2 in a Lambda CDM universe, using two different types of hydrodynamic simulation methods (Eulerian TVD and SPH) and a spectrophotometric analysis in the Un, G, R filter set. The simulated galaxies at z=2 satisfy the color-selection criteria proposed by Adelberger 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 2e-2 h^3/Mpc^3, 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 >~1e11 Msun, 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 Msun/yr 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 reddest massive galaxies at z=2 with G-R >= 1.0 and Mstar>1e10 Msun/h finished the build-up of their stellar mass by z~3. Interestingly, our study suggests that the majority of the most massive galaxies at z=2 should be detectable at rest-frame UV wavelengths, contrary to some recent claims made on the basis of near-IR studies of galaxies at the same epoch, provided the median extinction is less than E(B-V)<0.3. However, our results also suggest that the fraction of stellar mass contained in galaxies that pass the color-selection criteria could be as low as 50% of the total stellar mass in the Universe at z=2. Our simulations suggest that the missing stellar mass is contained in fainter (R>25.5) and intrinsically redder galaxies. Our results do not suggest that hierarchical galaxy formation fails to account for the massive galaxies at z>=1. (abridged)
Highlights
A number of recent observational studies have revealed a new population of red, massive galaxies at redshift z ∼ 2 (e.g. Chen et al 2003; Daddi et al 2004; Franx et al 2003; Glazebrook et al 2004), utilizing near-infrared (IR) wavelengths which are relatively less affected by dust extinction
We have used two different types of hydrodynamic cosmological simulations (Eulerian total variation diminishing (TVD) and smoothed particle hydrodynamics (SPH)) to study the properties of massive galaxies at z = 2 in a Λ cold dark matter (ΛCDM) universe, with particular emphasis on an observationally inspired selection based on the Un, G, R filter set
We find that the fraction of stellar mass contained in galaxies that pass the color-selection criteria could be as low as 50% of the total
Summary
A number of recent observational studies have revealed a new population of red, massive galaxies at redshift z ∼ 2 (e.g. Chen et al 2003; Daddi et al 2004; Franx et al 2003; Glazebrook et al 2004), utilizing near-infrared (IR) wavelengths which are relatively less affected by dust extinction. Recently Adelberger et al (2004) and Steidel et al (2004) have introduced new techniques for exploring the ‘redshift desert’, making it possible to identify a large number of galaxies efficiently with the help of a color selection criteria in the color-color plane of Un −G vs G−R In this technique, galaxies at z = 2−2.5 are located photometrically from the mild drop in the Un filter owing to the Ly-α forest opacity, and galaxies at z = 1.5 − 2 are recognized from the lack of a break in their observed-frame optical spectra. Lilly et al 1996; Connolly et al 1997; Sawicki et al 1997; Treyer et al 1998; Pascarelle et al 1998; Cowie et al 1999) These recent observational studies of galaxies at z = 2, both in the UV and near-IR wavelengths, imply a range of novel tests for the hierarchical structure formation theory.
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