Abstract
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, Js, Ks, and K 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 KVega = 20 mag have stellar masses M⋆ ≳ 1011 h-1 M☉ and a number density of a few times 10-4 h3 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 ~1 × 10-4 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. 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.
Highlights
Multiband photometry including the near infrared (IR) band makes it possible to estimate the stellar mass of high redshift galaxies when the observed photometric results are fitted with artificial galaxy spectra generated by a population synthesis model
Our emphasis has been on the stellar mass and near-IR colors of galaxies, and on a comparison of our results with observations, including those of the GDDS, K20, FIRES, and Great Observatories Origins Deep Survey (GOODS) surveys
Our results suggest that hydrodynamic simulations based on the Λ cold dark matter (ΛCDM) model do not exhibit the “mass-scale problem”; i.e. there is no obvious difficulty for the simulations to produce a sufficiently large number density of massive galaxies at high redshift, unlike some of the semi-analytic models
Summary
Multiband photometry including the near infrared (IR) band makes it possible to estimate the stellar mass of high redshift galaxies when the observed photometric results are fitted with artificial galaxy spectra generated by a population synthesis model. Working in the near-IR allows one to construct a mass-selected sample, because near-IR band emission is less affected by dust extinction than at shorter wavelengths and closely traces the total stellar mass. Using this technique, a number of recent observational studies have revealed a seemingly new population of very red, massive galaxies at redshift z = 1 − 3 We first give a brief review of the results of some of the major surveys that detected such a population of galaxies (see Section 2) All of these recent observational studies, at z = 1 − 3, both in the UV and near-IR wavelengths, imply a range of novel tests for the hierarchical structure formation theory.
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