The Growth of Galaxies in Cosmological Simulations of Structure Formation

We use hydrodynamic simulations to examine how the baryonic components ofgalaxies are assembled, focusing on the relative importance of mergers andsmooth accretion in the formation of ~L_* systems. In our primary simulation,which models a (50\hmpc)^3 comoving volume of a Lambda-dominated cold darkmatter universe, the space density of objects at our (64-particle) baryon massresolution threshold, M_c=5.4e10 M_sun, corresponds to that of observedgalaxies with L~L_*/4. Galaxies above this threshold gain most of their mass byaccretion rather than by mergers. At the redshift of peak mass growth, z~2,accretion dominates over merging by about 4:1. The mean accretion rate pergalaxy declines from ~40 M_sun/yr at z=2 to ~10 M_sun/yr at z=0, while themerging rate peaks later (z~1) and declines more slowly, so by z=0 the ratio isabout 2:1. We cannot distinguish truly smooth accretion from merging withobjects below our mass resolution threshold, but extrapolating our measuredmass spectrum of merging objects, dP/dM ~ M^a with a ~ -1, implies thatsub-resolution mergers would add relatively little mass. The global starformation history in these simulations tracks the mass accretion rate ratherthan the merger rate. At low redshift, destruction of galaxies by mergers isapproximately balanced by the growth of new systems, so the comoving spacedensity of resolved galaxies stays nearly constant despite significant massevolution at the galaxy-by-galaxy level. The predicted merger rate at z<~1agrees with recent estimates from close pairs in the CFRS and CNOC2 redshiftsurveys.Comment: Submitted to ApJ, 35 pp including 15 fig