GAS-RICH MERGERS IN LCDM: DISK SURVIVABILITY AND THE BARYONIC ASSEMBLY OF GALAXIES

We use N-body simulations and observationally normalized relations between dark matter halo mass, stellar mass, and cold gas mass to derive robust expectations about the baryonic content of major mergers out to redshift z similar to 2. First, we find that the majority of major mergers (m/M > 0.3) experienced by the Milky Way size dark matter halos should have been gas-rich, and that gas-rich mergers are increasingly common at high redshifts. Though the frequency of major mergers into galaxy halos in our simulations greatly exceeds the observed early-type galaxy fraction, the frequency of gas-poor major mergers is consistent with the observed fraction of bulge-dominated galaxies across the halo mass range M(DM) similar to 10(11)-10(13) M(circle dot). These results lend support to the conjecture that mergers with high-baryonic gas fractions play an important role in building and/or preserving disk galaxies in the universe. Second, we find that there is a transition mass below which a galaxy's past major mergers were primarily gas-rich and above which they were gas-poor. The associated stellar mass scale corresponds closely to that marking the observed bimodal division between blue, star-forming, disk-dominated systems and red, bulge-dominated systems with old populations. Finally, we find that the overall fraction of a galaxy's cold baryons deposited directly via major mergers is significant. Approximately similar to 20%-30% of the cold baryonic material in M(star) similar to 10(10.5) M(circle dot) (M(DM) similar to 10(12) M(circle dot)) galaxies is accreted as cold gas or stars via major mergers since z = 2, with most of this accretion in the form of cold gas. For more massive galaxies with M(star) similar to 10(11) M(circle dot) (M(DM) similar to 10(13) M(circle dot)), the fraction of baryons amassed in mergers since z = 2 is even higher, similar to 40%, but most of these accreted baryons are delivered directly in the form of stars. This baryonic mass deposition is almost unavoidable, and provides a limit on the fraction of a galaxy's cold baryons that can originate in cold flows or from hot halo cooling.