Massive galaxies & EROs at z = 1 − 3 in cosmological hydrodynamic simulations : near - IR properties

Recent observations have revealed a population of red massive galaxies at high redshift which are challenging to explain in 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, Js, Ks, K filters, and compare their near-infrared (IR) properties with observations at redshift z = 1−3. We find that the simulated galaxies brighter than the magnitude limit of Kvega = 20 mag have stellar masses M⋆ & 10 hM⊙ and a number density of a few ×10 −4h3Mpc 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 ∼ 1 × 10 Mpc at z = 1 − 2 if we assume a uniform extinction of E(B − V ) = 0.4 for the entire population of simulated galaxies. The critical University of California, San Diego, Center for Astrophysics & Space Sciences, 9500 Gilman Dr., La Jolla, CA 92093-0424, U.S.A. Princeton University Observatory, Princeton, NJ 08544, U.S.A. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, U.S.A. Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85740 Garching bei München, Germany