Mergers and mass accretion rates in galaxy assembly: The millennium simulation compared to observations of z ≈ 2 galaxies

Recent observations of UV/optically selected, massive star-forming galaxies at z ≈ 2 indicate that the baryonic mass assembly and star formation history is dominated by continuous rapid accretion of gas and internal secular evolution, rather than by major mergers. We use the Millennium Simulation to build new halo merger trees and extract halo merger fractions and mass accretion rates. We find that, even for halos not undergoing major mergers, the mass accretion rates are plausibly sufficient to account for the high star formation rates observed in z ≈ 2 disks. On the other hand, the fraction of major mergers in the Millennium Simulation is sufficient to account for the number counts of submillimeter galaxies (SMGs), in support of observational evidence that these are major mergers. When following the fate of these two populations in the Millennium Simulation to z = 0, we find that subsequent mergers are not frequent enough to convert all z ≈ 2 turbulent disks into elliptical galaxies at z = 0. Similarly, mergers cannot transform the compact SMGs/red sequence galaxies at z ≈ 2 into observed massive cluster ellipticals at z = 0. We argue therefore, that secular and internal evolution must play an important role in the evolution of a significant fraction of z ≈ 2 UV/optically and submillimeter-selected galaxy populations.