Massive Galaxies and Extremely Red Objects atz= 1–3 in Cosmological Hydrodynamic Simulations: Near‐Infrared Properties

Recent observations have revealed a population of red massive galaxies athigh redshift which are challenging to explain in hierarchical galaxy formationmodels. We analyze this ``massive galaxy problem'' with two different types ofhydrodynamic simulations -- Eulerian TVD and SPH -- of a concordance Lambda CDMuniverse. We consider two separate but connected aspects of the problem posedby these extremely red objects (EROs): (1) the mass-scale, and (2) red colors.We perform spectrophotometric analyses of simulated galaxies in B, z, R, I, Js,Ks, K filters, and compare their near-IR properties with observations atredshift z=1-3. We find that the simulated galaxies brighter than the magnitudelimit of K_vega=20 mag have stellar masses Mstar >= 10^{11} Msun/h and a numberdensity of a few \times 10^{-4} h^3 Mpc^{-3} at z~2, in good agreement with theobserved number density in the K20 survey. Therefore, our hydrodynamicsimulations do not exhibit the ``mass-scale problem''. The answer to the``redness problem'' is less clear because of our poor knowledge of the amountof dust extinction in EROs and the uncertain fraction of star-forming EROs.However, our simulations can account for the observed comoving number densityof ~ 1e-4 Mpc^{-3} at z=1-2 if we assume a uniform extinction of E(B-V)=0.4 forthe entire population of simulated galaxies. The critical question of theexistence of a significant population of old, passively evolving massive EROsshould continue to be tested with future hydrodynamic simulations. Upcomingobservations of the thermal emission of dust in 24 micron by the Spitzer SpaceTelescope will help to better estimate the dust content of EROs at z=1-3, andthus to further constrain the star formation history of the Universe, andtheoretical models of galaxy formation. (abridged)Comment: 32 pages, 7 figures, ApJ, in press. One paragraph added to the Discussion sectio