Mergers and Mass Accretion Rates in Galaxy Assembly: The Millennium Simulation Compared to Observations of z~2 Galaxies

TL;DR

This study uses the Millennium Simulation to compare galaxy merger rates and mass accretion with observations at z~2, suggesting secular evolution plays a key role in galaxy development beyond mergers.

Contribution

The paper provides a detailed comparison of simulated merger rates and mass accretion with observational data, highlighting the importance of secular evolution in galaxy assembly.

Findings

Mass accretion rates in simulations match observed star formation rates at z~2.

Major mergers account for submillimeter galaxy counts.

Mergers alone cannot explain the evolution of galaxies to z=0.

Abstract

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.