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

We employ a high-resolution Lambda CDM N-body simulation to present merger rate predictions for dark matter (DM) halos and investigate how common merger-related observables for galaxies-such as close pair counts, starburst counts, and the morphologically disturbed fraction-likely scale with luminosity, stellar mass, merger mass ratio, and redshift from z = 0 to z = 4. We investigate both rate at which subhalos first enter the virial radius of a larger halo (the infall rate), and the rate at which subhalos become destroyed, losing 90% of the mass they had at infall (the destruction rate). For both merger rate definitions, we provide a simple universal fitting formula that describes our derived merger rates for DM halos a function of dark halo mass, merger mass ratio, and redshift, and go on to predict galaxy merger rates using number density matching to associate halos with galaxies. For example, we find that the instantaneous (destruction) merger rate of m/M > 0.3 mass-ratio events into typical L greater than or similar to f L-* galaxies follows the simple relation dN/dt similar or equal to 0.03(1 + f) Gyr(-1) (1 + z)(2.1). Despite the rapid increase in merger rate with redshift, only a small fraction of > 0.4 L-* high-redshift galaxies (similar to 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 several observational indications. In contrast, a fairly high fraction (similar to 20%) of those z = 2 galaxies should have experienced a morphologically transformative merger within a virial dynamical time (similar to 500 Myr at z = 2). We compare our results to observational merger rate estimates from both morphological indicators and pair-fraction-based determinations between z = 0 and 2 and show that they are consistent with our predictions. However, we emphasize that great care must be made in these comparisons because the predicted observables depend very sensitively on galaxy luminosity, redshift, overall mass ratio, and uncertain relaxation timescales for merger remnants. We show that the majority of bright galaxies at z = 3 should have undergone a major merger (> 0.3) in the previous 700 Myr and conclude that mergers almost certainly play an important role in delivering baryons and influencing the kinematic properties of Lyman break galaxies (LBGs).