Mass accretion and merger rates in galaxy assembly: insights from the Millennium Simulation
We have developed a new method to build halo merger trees and extract the halo merger rate and fraction as well as the halo mass accretion rate from the large LCDM "Millennium Simulation". By removing superfluous mergers that are artifacts of the standard Friends-Of-Friends (FOF) halo identification algorithm, we find a lower merger rate compared to previous work. This correction results in a better agreement with predictions from the extended Press-Schechter model. We quantify the merger rate per progenitor halo, which is the quantity that should be related to observed galaxy merger fractions when they are measured via pair counting. At low mass/redshift the merger rate increases moderately with mass and steeply with redshift. At high enough mass/redshift these trends break down, and the merger rate decreases with mass and increases only moderately with redshift. Furthermore, 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 major merger fraction is sufficient to account for the number counts of submillimeter galaxies (SMGs) in support of observational evidence that these are major mergers.