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, 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)

💡 Research Summary

The paper investigates how major mergers contribute to the baryonic assembly and morphological evolution of galaxies within the ΛCDM framework. Using high‑resolution N‑body simulations, the authors track the merger histories of dark‑matter halos in the mass range 10^11–10^13 M☉. They define a “major merger” as an event with a mass ratio m/M ≥ 0.3 and then assign stellar and cold‑gas masses to each halo by applying empirically calibrated relations between halo mass, stellar mass, and gas mass that are validated from z ≈ 0 to z ≈ 2. This hybrid approach allows them to estimate the gas fraction (f_gas) of each merger without directly simulating the hydrodynamics of the gas.

The key findings are as follows. First, for Milky Way‑size halos (M_DM ≈ 10^12 M☉) the majority of major mergers are gas‑rich (f_gas > 0.5). The prevalence of gas‑rich mergers rises with redshift; at z > 1 most major mergers have f_gas > 0.7. Conversely, low‑mass halos (M_DM ≈ 10^11 M☉) experience a higher proportion of gas‑poor mergers, but the absolute number of mergers is low, so the threat to disk survival is modest. Second, the fraction of gas‑poor (dry) major mergers increases with halo mass. For massive halos (M_DM ≈ 10^13 M☉) roughly 40 % of all major mergers are dry, a fraction that matches the observed abundance of spheroid‑dominated galaxies in the same mass range. This mass‑dependent trend supports the hypothesis that gas‑rich mergers can preserve or rebuild disks, while dry mergers are primarily responsible for creating elliptical systems.

Third, the authors quantify how much of a galaxy’s cold baryonic budget is delivered directly by major mergers. In galaxies with stellar mass M_* ≈ 10^10 M☉ (halo mass ≈ 10^11.5 M☉), about 30 % of the total cold baryons (stars + cold gas) are accreted through major mergers, and more than half of that accreted material arrives as gas. For more massive systems (M_* ≈ 10^11 M☉, halo mass ≈ 10^13 M☉) the merger‑contributed fraction rises to roughly 50 %, but the bulk of the accreted baryons are in stellar form, reflecting the dominance of dry mergers at high mass.

These results place an upper limit on the contribution of other channels—such as smooth cold‑flow accretion or cooling from hot halos—to a galaxy’s cold baryon inventory. For a 10^10 M☉ galaxy, at most ~70 % of its cold baryons can originate from non‑merger processes; for a 10^11 M☉ galaxy the ceiling drops to ~50 %.

Overall, the study provides robust, simulation‑backed evidence that gas‑rich major mergers are a natural and inevitable component of galaxy evolution, playing a crucial role in maintaining disk structures, while gas‑poor mergers account for the observed population of spheroidal galaxies. By explicitly linking merger histories to baryonic content, the work bridges a gap between theoretical predictions and observational constraints on galaxy morphology and mass assembly across cosmic time.