Enhanced Multiphase Circumgalactic Medium and Gas Cycling in Galaxy 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.

Figures (10)

• Figure 1: Projected distributions of key physical properties across three galactic systems. From top to bottom: stellar surface density, gas density, gas density-weighted temperature, entropy, and radial velocity. The panels show a typical galaxy merger hosting three galaxies with stellar masses of $2.2\times10^{10}\,\mathrm{M_\odot}$, $8.4\times10^{9}\,\mathrm{M_\odot}$ and $8.0\times10^{9}\,\mathrm{M_\odot}$ (left column), a starburst merger with stellar mass of $2.1\times10^{10}\,\mathrm{M_\odot}$ (middle column), and a corresponding isolated control galaxy with stellar mass of $4.1\times10^{10}\,\mathrm{M_\odot}$ (right column) at $z=0$. The merger systems exhibit more disturbed CGM structures than the isolated galaxy, with more cool gas and higher inflow velocities. The starburst merger shows particularly strong outflow velocities due to intense stellar feedback triggered by the interaction.
• Figure 2: Virial mass distributions of host halos, showing the counts of halos hosting merging galaxies (red histogram) and halos hosting isolated galaxies (blue histogram) within given virial mass bins. The red (blue) dashed vertical lines indicate the median virial masses of merging (isolated) galaxy host halos. Virial masses of selected halos range from $10^{11}\,\mathrm{M_\odot}$ to $10^{12}\,\mathrm{M_\odot}$.
• Figure 3: Stellar mass and gas mass distribution of the primary galaxies and secondary galaxies in merging pairs (blue dots and red squares, respectively). Our sample covers a stellar mass range of $3\times10^9\,\mathrm{M_\odot}$ to $4\times10^{10}\,\mathrm{M_\odot}$, and a gas mass range of $5\times10^8\,\mathrm{M_\odot}$ to $2\times10^{10}\,\mathrm{M_\odot}$
• Figure 4: Fig. \ref{['fig:SFR_absolute']}: Specific star formation rate of merging galaxies and control galaxies versus virial masses of host halos at $z=0$. Merging galaxies are indicated with stars while control galaxies are indicated with blue octagons. A notable merging pair with high sSFR is marked in green and labeled as "starburst merger," while other merging samples are marked in red. Fig. \ref{['fig:SFR_enhance']}: Enhancement of specific star formation rate (sSFR), defined as $\mathrm{sSFR_{merger}}/\mathrm{mean}(\mathrm{sSFR_{isolated}})$, for merging galaxies relative to the mean sSFR of isolated galaxies, plotted as a function of the separation between merging galaxies; systems that have coalesced are assigned a separation of 0. The red dashed line indicates no enhancement (ratio = 1).
• Figure 5: Radial profiles of hydrogen number density (top, volume-weighted) and temperature (bottom, density-weighted) for merging systems (red solid line), the starburst merger (green dash-dotted line), and isolated galaxies (blue dashed line), with galactocentric radius normalized to each halo's virial radius $R_\mathrm{vir}$. Profiles are sampled over all galaxies in each category at $z=0$. Regions identified as galaxies are excluded from the analysis, while we use a gray band near $r/R_\mathrm{vir}=0$ to represent the median galaxy radius in our sample. Outer lighter shaded region colored with red, green and blue denote the 16th to 84th percentile ranges within each corresponding sample category. The inner darker shaded region around each line shows the 16th to 84th percentiles of the median value calculated via bootstrapping. Merging galaxies exhibit marginally higher gas densities in the outer CGM and slightly lower temperatures in the inner CGM compared to isolated systems. However, these differences remain relatively modest, confirming that our selected samples possess comparable halo baryon content.