Galaxy Merger Statistics and Inferred Bulge-to-Disk Ratios in Cosmological SPH Simulations

Abstract

We construct merger trees for galaxies identified in a cosmological hydrodynamical simulation and use them to characterize predicted merger rates as a function of redshift, galaxy mass, and merger mass ratio. At z=0.3, we find a mean rate of 0.054 mergers per galaxy per Gyr above a 1:2 mass ratio threshold for massive galaxies (baryonic mass above 6.4E10 \Msun), but only 0.018 / Gyr for lower mass galaxies. The mass ratio distribution is proportional to R_merg^-1.2 for the massive galaxy sample, so high mass mergers dominate the total merger growth rate. The predicted rates increase rapidly with increasing redshift, and they agree reasonably well with observational estimates. A substantial fraction of galaxies do not experience any resolved mergers during the course of the simulation, and even for the high mass sample only 50% of galaxies experience a greater than 1:4 merger since z=1. Typical galaxies thus have fairly quiescent merger histories. We assign bulge-to-disk ratios to simulated galaxies by assuming that mergers above a mass ratio threshold R_major convert stellar disks into spheroids. With R_major of 1:4, we obtain a fairly good match to the observed dependence of early-type fraction on galaxy mass. However, the predicted fraction of truly bulge-dominated systems (f_bulge} > 0.8) is small, and producing a substantial population of bulge-dominated galaxies may require a mechanism that shuts off gas accretion at late times and/or additional processes (besides major mergers) for producing bulges.