Enhanced Multiphase Circumgalactic Medium and Gas Cycling in Galaxy Mergers

TL;DR

This study uses cosmological simulations to show that galaxy mergers significantly enhance the multiphase structure, cool gas content, and gas cycling rates in the circumgalactic medium, fueling star formation.

Contribution

It demonstrates that mergers accelerate CGM processing and increase cool gas, revealing new insights into gas dynamics during galaxy interactions.

Findings

Mergers increase cool/cold gas content by amplifying radiative cooling.

Both inflow and outflow mass fluxes are elevated by at least 1 dex in mergers.

Gas cycling from cosmic inflow to galaxy is accelerated by a factor of ~30 in mergers.

Abstract

We investigate the impact of galaxy mergers on the circumgalactic medium (CGM) using the FIREbox cosmological hydrodynamic simulation. By comparing matched samples of merging and isolated galaxies with stellar masses $M_\star \sim 10^{10}$--$10^{11} M_{\odot}$ at $z=0$ and mass ratio of merging galaxies larger than $1:10$, we find that mergers significantly alter CGM properties. Merging systems exhibit enhanced radiative cooling, leading to shorter cooling times than free-fall times across large CGM volumes. This results in amplified multiphase structure and increased cool/cold gas content ($T \sim 10^4K$) compared to isolated galaxies. Both inflow and outflow mass fluxes are elevated by at least $\sim$1 dex in mergers across all temperature phases, with cool gas primarily generated in-situ via radiative cooling rather than from pre-existing streams. Gas cycling analysis reveals that mergers fundamentally accelerate CGM processing, amplifying the effective transfer rate from cold/cool cosmic inflow to galaxy inflow by factors of $\sim 30$, through rapid cycling of inflowing gas through intermediate CGM phases, efficiently fueling the ISM and star formation. The enhanced cool gas content in mergers produces elevated column densities for low- and intermediate-temperature ion species in the inner CGM, while high-temperature ones remain largely unaffected.