The growth of galaxies in cosmological simulations of structure formation

We use hydrodynamic simulations to examine how the baryonic components of galaxies are assembled, focusing on the relative importance of mergers and smooth accretion in the formation of similar toL(*) systems. In our primary simulation, which models a (50 h(-1) Mpc)(3) comoving volume of a Lambda-dominated cold dark matter universe, the space density of objects at our (64 particle) baryon mass resolution threshold M-c = 5.4 x 10(10) M-. corresponds to that of observed galaxies with L similar to L-*/4. Galaxies above this threshold gain most of their mass by accretion rather than by mergers. At the redshift of peak mass growth, zapproximate to2, accretion dominates over merging by about 4 : 1. The mean accretion rate per galaxy declines from similar to40 M-. yr(-1) at z = 2 to similar to10 M-. yr(-1) at z = 0, while the merging rate peaks later (z approximate to 1) and declines more slowly, so by z = 0 the ratio is about 2 : 1. We cannot distinguish truly smooth accretion from merging with objects below our mass resolution threshold, but extrapolating our measured mass spectrum of merging objects, dP/dM proportional to M-alpha with alpha similar to 1, implies that subresolution mergers would add relatively little mass. The global star formation history in these simulations tracks the mass accretion rate rather than the merger rate. At low redshift, destruction of galaxies by mergers is approximately balanced by the growth of new systems, so the comoving space density of resolved galaxies stays nearly constant despite significant mass evolution at the galaxy-by-galaxy level. The predicted merger rate at z less than or similar to 1 agrees with recent estimates from close pairs in the Canada-France Redshift Survey and Canadian Network for Observational Cosmology Redshift Survey.