Abstract
We study the physical and photometric properties of galaxies at z=4 in cosmological hydrodynamic simulations of a lambda-CDM universe. We focus on galaxies satisfying the GOODS B-dropout criteria. Our goals are: (1) to study the nature of high-redshift galaxies; (2) to test the simulations against published measurements of high-redshift galaxies; (3) to find relations between photometric measurements by HST/ACS (0.4 -- 1 micron) and Spitzer/IRAC (3.6 -- 8 micron) and the intrinsic physical properties of GOODS B-dropouts such as stellar mass, stellar age, dust reddening, and star-formation rate; and (4) to assess how representative the GOODS survey is at this epoch. Our simulations predict that high-redshift galaxies show strong correlations in star formation rate versus stellar mass, and weaker correlations versus environment and age, such that GOODS galaxies are predicted to be the most massive, most rapidly star-forming galaxies at that epoch, living preferentially in dense regions. The simulated rest-frame UV luminosity function (LF) and integrated luminosity density are in broad agreement with observations at z~4. The predicted rest-frame optical (observed 3.6 micron) LF is similar to the rest-frame UV LF, shifted roughly one magnitude brighter. We predict that GOODS detects less than 50% of the total stellar mass density formed in galaxies more massive than 10^8.7 M_sun by z=4, mainly because of brightness limits in the HST/ACS bands. The most rapidly star forming galaxies in our simulations have rates exceeding 1000 M_sun yr^-1, similar to observed sub-mm galaxies. The star formation rates of these galaxies show at most a mild excess (2--3x) over the rates that would be expected for their stellar mass. Whether these bright galaxies would be observable as LBGs depends on the uncertain effects of dust reddening.
Highlights
According to the currently-favored hierarchical model of structure formation, the first generations of stars began forming in low-mass galaxies at very high redshift
In this work we present comparisons between the ensemble statistical properties of high-redshift galaxies in our simulations and in the Great Observatories Origins Deep Survey (GOODS) data set; examine the basic physical properties of a simulated GOODS sample and discuss the trends that emerge; investigate how the photometric properties of the GOODS sample relate to the underlying physical properties; determine how the model parameters affect the observed color-magnitude and color-color relations; discuss the extent to which the observed GOODS sample will be representative of the complete galaxy population at z = 4; and compare our results with those of previous simulations and semianalytic models
We examine the properties of galaxies that satisfy the selection criteria for the z ∼ 4 B-dropout sample of the GOODS survey
Summary
According to the currently-favored hierarchical model of structure formation, the first generations of stars began forming in low-mass galaxies at very high redshift. Numerical simulations that include a range of physical processes believed to govern galaxy formation allow the hierarchical scenario to be tested through detailed comparisons with observed galaxies. Our simulations have sufficient dynamic range to numerically resolve the entire “B-dropout” sample within a comoving cosmological volume that is comparable to the volume probed by GOODS at z ∼ 4 These simulations have been shown to broadly match the observed cosmic star formation rate density (Springel & Hernquist 2003b; Hernquist & Springel 2003), and the rest-frame UV luminosity functions of LBGs when a moderate amount of dust extinction is assumed (Nagamine et al 2004a,b, 2005a; Night et al 2005).
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