2005ApJ...627..608N

Recent observations have revealed a population of red massive galaxies at high redshift that are challenging to explain in terms of hierarchical galaxy formation models. We analyze this ``massive galaxy problem'' with two different types of hydrodynamic simulations–Eulerian total variation diminishing (TVD) and smoothed particle hydrodynamics (SPH)–of a concordance Λ cold dark matter (ΛCDM) universe. We consider two separate but connected aspects of the problem posed by these extremely red objects (EROs): (1) the mass scale of these galaxies and (2) their red colors. We perform spectrophotometric analyses of simulated galaxies in B,z,R,I,J<sub>s</sub>,K<sub>s</sub>,andK filters and compare their near-infrared (near-IR) properties with observations at redshift z=1-3. We find that the simulated galaxies brighter than the magnitude limit of K<sub>Vega</sub>=20 mag have stellar masses M<sub>*</sub>≳10<sup>11</sup>h<sup>–1</sup>M<sub>☉</sub>and a number density of a few times 10<sup>–4</sup>h<sup>3</sup>/Mpc3 at z∼2, in good agreement with the observed number density in the K20 survey. Therefore, our hydrodynamic simulations do not exhibit the mass-scale problem. The answer to the ``redness problem'' is less clear because of our poor knowledge of the amount of dust extinction in EROs and the uncertain fraction of star-forming EROs. However, our simulations can account for the observed comoving number density of ∼1x10^–4/Mpc3 at z=1-2 if we assume a uniform extinction of E(B-V)=0.4 for the entire population of simulated galaxies. Upcoming observations of the thermal emission of dust in 24 µm by the Spitzer Space Telescope will help to better estimate the dust content of EROs at z=1-3 and thus to further constrain the star formation history of the universe and theoretical models of galaxy formation.