High‐Redshift Galaxies in Cold Dark Matter Models

We use hydrodynamic cosmological simulations to predict the star formationproperties of high-redshift galaxies (z=2-6) in five variants of theinflationary cold dark matter scenario, paying particular attention to z=3, theredshift of the largest "Lyman-break galaxy" (LBG) samples. Because we link thestar formation timescale to the local gas density, the rate at which a galaxyforms stars is governed mainly by the rate at which it accretes cooled gas fromthe surrounding medium. At z=3, star formation in most of the simulatedgalaxies is steady on 200 Myr timescales, and the instantaneous star formationrate (SFR) is correlated with total stellar mass. However, there is enoughscatter in this correlation that a sample selected above a given SFR thresholdmay contain galaxies with a fairly wide range of masses. The redshift historyand global density of star formation in the simulations depend mainly on theamplitude of mass fluctuations in the underlying cosmological model. The threemodels whose mass fluctuation amplitudes agree with recent analyses of theLyman-alpha forest also reproduce the observed luminosity function of LBGsreasonably well, though the dynamic range of the comparison is small and thetheoretical and observational uncertainties are large. The models with higherand lower amplitudes appear to predict too much and too little star formation,respectively, though they are not clearly ruled out. The intermediate amplitudemodels predict SFR ~ 30-40 Msun/yr for galaxies with a surface density 1 perarcmin^2 per unit redshift at z=3. They predict much higher surface densitiesat lower SFR, and significant numbers of galaxies with SFR > 10 Msun/yr at z >=5.