GALAXY MERGERS AND DARK MATTER HALO MERGERS IN ΛCDM: MASS, REDSHIFT, AND MASS-RATIO DEPENDENCE

Abstract

A high-resolution LCDM N-body simulation is employed to present merger rate predictions for dark matter halos and to investigate how common merger-related observables—such as close pair counts, starburst counts, and the morphologically disturbed fraction—are likely scale with luminosity, stellar mass, and redshift from z = 0 to z = 4. We provide a simple ‘universal’ fitting formula that describes our derived merger rates for dark matter halos a function of halo mass, merger mass ratio, and redshift. Using number density-matching as a means of assigning galaxy properties to halos, we show, for example, that the instantaneous merger rate of r > 0.3 mass ratio events into typical L & f L∗ galaxies follows the simple relation dN/dt ≃ 0.03(1 + f)Gyr −1 (1 + z) 2.1 . Despite the rapid increase in merger rate with redshift, only a small fraction of > 0.4L∗ high-redshift galaxies (∼ 3% at z = 2) should have experienced a major merger (m/M > 0.3) in the very recent past (t < 100 Myr). This suggests that short-lived, merger-induced bursts of star formation should not contribute significantly to the global star formation rate at early times, in agreement with observational indications. In contrast, a fairly high fraction (∼ 20%) of those z = 2 galaxies should have experienced a morphologically transformative merger within a virial dynamical time. We compare our results to observational merger rate estimates from both morphological indicators and pair-fraction based determinations between z = 0 − 2 and show that they agree with our predictions to within observational uncertainties. We find that a majority of bright z = 3 Lyman Break Galaxy halos should have undergone a major merger in the last 500 Myr and conclude that mergers almost certainly play an important role in delivering baryons and influencing the kinematic properties of the highest redshift galaxies. Subject headings: cosmology: theory — dark matter — galaxies: formation — galaxies: halos — methods: N-body simulations