MERGERS AND BULGE FORMATION IN ΛCDM: WHICH MERGERS MATTER?

Abstract

We use a suite of semi-empirical models to predict galaxy merger rates and\ncontributions to bulge growth as functions of merger mass, redshift, and mass\nratio. The models use empirical halo occupation constraints to identify\nmergers, together with high-resolution simulations to quantify how mergers with\ndifferent properties contribute to the bulge population. We find good agreement\nwith a variety of observational constraints, and provide fitting functions for\nmerger rates and contributions to bulge growth. We identify several robust\nconclusions. (1) Major mergers dominate formation and assembly of L* bulges and\nthe spheroid mass density, minor mergers contribute ~30%. (2) This is\nmass-dependent: bulge formation is dominated by more minor mergers in\nlower-mass systems. At higher masses, bulges form in major mergers near L*, but\nsubsequently assemble in minor mergers. (3) The minor/major contribution is\nalso morphology-dependent: higher B/T systems form in more major mergers, lower\nB/T systems form in situ from minor mergers. (4) Low-mass galaxies, being\ngas-rich, require more major mergers to reach the same B/T as high-mass\nsystems. (5) Absolute merger rates increase with galaxy mass. (6) Predicted\nrates agree well with observations, but suggest that some morphology-selected\nsamples include contamination from minor mergers. (7) Predicted rates agree\nwith integrated growth in bulge mass with cosmic time, but with factor ~2\nuncertainty - half the bulge mass density could come from non-mergers. We\nconsider ~1000 model variations and quantify resulting uncertainties.\nConclusions regarding the major/minor contribution to bulge growth are very\nrobust, absolute merger rates have systematic factor ~2 uncertainties.\n