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 TVD and SPH -- of a concordance Lambda CDM universe. We consider two separate but connected aspects of the problem posed by 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 at redshift z=1-3. We find that the simulated galaxies brighter than the magnitude limit of K_vega=20 mag have stellar masses Mstar >= 10^{11} Msun/h and a number density of a few \times 10^{-4} h^3 Mpc^{-3} 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 ~ 1e-4 Mpc^{-3} 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 question of the existence of a significant population of old, passively evolving massive EROs should continue to be tested with future hydrodynamic simulations. Upcoming observations of the thermal emission of dust in 24 micron 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. (abridged)