Gas-Rich Mergers in LCDM: Disk Survivability and the Baryonic Assembly of Galaxies

TL;DR

This study uses simulations and observational data to show that major galaxy mergers are typically gas-rich, especially at high redshift, and significantly contribute to galaxy baryonic mass assembly and disk survival.

Contribution

It provides the first robust predictions about the baryonic content of major mergers across redshifts, highlighting their role in galaxy evolution and disk preservation.

Findings

Most major mergers for Milky Way-sized halos are gas-rich.

Gas-rich mergers are more common at higher redshifts.

A substantial fraction of a galaxy's cold baryons are acquired through major mergers.

Abstract

We use N-body simulations and observationally-normalized relations between dark matter halo mass, stellar mass, and cold gas mass to derive robust, arguably inevitable expectations about the baryonic content of major mergers out to redshift z~2. First, we find that the majority of major mergers (m/M > 0.3) experienced by Milky Way size dark matter halos should have been gas-rich, and that gas-rich mergers are increasingly common at high redshift. Though the frequency of major mergers into galaxy halos in our simulations greatly exceeds the observed late-type galaxy fraction, the frequency of gas-poor major mergers is consistent with the observed fraction of spheroid-dominated galaxies across the halo mass range M_DM ~ 10^11-10^13 Msun. 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. Also, we find that the overall fraction of a galaxy's cold baryons deposited directly via major mergers is substantial. Approximately ~30% of the cold baryonic material in M_star ~ 10^10 Msun$ (M_DM ~ 10^11.5 Msun) galaxies is accreted as cold gas in major mergers. For more massive galaxies with M_star ~ 10^11 Msun (M_DM ~ 10^13 Msun) the fraction of baryons amassed in mergers is even higher, ~50%, 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. (Abridged)