AGN feedback in merging galaxies with a SMUGGLE multiphase ISM

TL;DR

This work investigates how AGN feedback couples to a realistically multiphase ISM during galaxy mergers by using AREPO with SMUGGLE and a super-Lagrangian refinement to resolve scales of $\sim10$–$100$ pc. By studying MW-, Sbc-, and SMC-type progenitors in equal-mass mergers, and comparing runs with and without AGN feedback, the study reveals that BH dynamics and merger-induced gas morphologies critically shape BH growth and feedback efficacy. MW mergers exhibit the strongest AGN–ISM coupling, with substantial BH growth and powerful outflows, while Sbc mergers show delayed, post-merger quenching, and SMC mergers experience BH wandering that suppresses accretion and feedback. These results advance our understanding of SMBH–galaxy co-evolution in merging systems and have implications for LISA event rates and high-redshift quasar growth observed by JWST.

Abstract

We study fast nuclear winds driven by Active Galactic Nucleus (AGN) feedback in merging galaxies using high-resolution hydrodynamics simulations. We use Stars and MUltiphase Gas in GaLaxiEs (SMUGGLE) to explicitly model the multiphase interstellar medium (ISM) and employ sub-grid dynamical friction for massive black holes (BHs). Furthermore, we use a super-Lagrangian refinement scheme to resolve AGN feedback coupling to the ISM at $\sim10-100\,$pc scales. By comparison between merging and isolated galaxies, with and without AGN feedback, we identify trends in the complex interplay between dynamics, BH fueling and feedback, and star formation and feedback. We consider three galaxy types: Milky Way analogs, Sbc-type galaxies, and Small Magellanic Cloud (SMC) analogs. The synergy between AGN feedback and merger dynamics is strongest in the Milky Way-like mergers, where the AGN winds are energetically dominant and entrain more gas when the initially thin disks become thick and amorphous during the merger. In contrast, the merger of thicker, vigorously star-forming Sbc galaxies is not strongly impacted by AGN feedback until star formation declines in the post-merger phase. Finally, while the sub-grid dynamical friction prescription effectively retains BHs in galactic nuclei during more massive mergers, the clumpy multiphase ISM induces significant wandering of low-mass BHs $\mathrm{(<10^5M_\odot)}$ in the shallow potentials of the SMC-like galaxies. These low-mass BHs wander at distances $\gtrsim 2$ kpc from the galactic center, yielding negligible BH accretion and feedback. This has implications for LISA event rates and present a further challenge to understanding the rapid growth of $z\sim7-10$ quasars discovered by JWST.

Figures (13)

• Figure 1: Distance between the BHs in the two galaxies in the MW, Sbc and SMC merger simulations with and without AGN feedback.
• Figure 2: Gas density (top left), stellar density (top right), gas temperature (bottom left) and gas velocity (bottom right) map of gas in the merging MW galaxies just before the second pericentric passage. A slice of thickness 71 kpc was used to average the quantities. Temperature and speed are mass weighted.
• Figure 3: Distance between the BHs and the local center of mass (calculated iteratively as described in the text) in the MW, Sbc and SMC isolated and merger simulations with AGN feedback. In the pre-merger stages, the position of only one of the BHs is shown. The thin black vertical lines show the times of pericentric passages and the coalescence. Only one of the BHs in the merger simulation is plotted here. Because the merger simulation involves identical progenitor galaxies, there is no significant difference between the dynamics of the two BHs.
• Figure 4: BH mass as a function of time in the MW, Sbc and SMC isolated and merger simulations with AGN feedback. The solid and dashed lines corresponds to the two BHs in the merger simulations with AGN feedback. The dotted lines show the BH mass for the isolated galaxy with AGN feedback; one is the original curve, and the other is the same curve scaled by a factor of two. BHs in MW and Sbc mergers have enhanced growth relative to the isolated galaxies whereas in the case of SMC, there is no enhancement. This difference is primarily due to the wandering of the of the BHs in SMC due to inefficient dynamical friction.
• Figure 5: BH Eddington ratio as a function of time in the MW, Sbc and SMC isolated and merger simulations with AGN feedback. Eddington ratio of only one BH from the merger run is plotted. Because the merger simulations involves identical progenitor galaxies, there is no significant difference between the Eddington ratios of the two BHs. In MW and Sbc, we can see increased average Eddington ratios during the pericentric passages and coalescence. After the coalescence, strong AGN feedback suppressed accretion rates. In SMC there is no clear differences between the merging and isolated systems due to the wandering of the BHs. The cyan vertical lines represent the pericentric passages and the final coalescence.