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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 subgrid dynamical friction for massive black holes (BHs). Furthermore, we use a super-Lagrangian refinement scheme to resolve AGN feedback coupling to the ISM at $$\sim 10-100\,$$ pc scales. By comparison between merging and isolated galaxies, with and without AGN feedback, we identify trends in the complex interplay between dynamics, BH fuelling and feedback, and star formation and feedback. We consider three galaxy types: Milky Way analogues, Sbc-type galaxies, and Small Magellanic Cloud (SMC) analogues. 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 discs 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 subgrid dynamical friction prescription effectively retains BHs in galactic nuclei during more massive mergers, the clumpy multiphase ISM induces significant wandering of low-mass BHs $$\mathrm{(< 10^5\, M_\odot)}$$ in the shallow potentials of the SMC-like galaxies. These low-mass BHs wander at distances $$\gtrsim 2$$ kpc from the galactic centre, yielding negligible BH accretion and feedback. This has implications for Laser Interferometer Space Antenna event rates and present a further challenge to understanding the rapid growth of $$z\sim 7-10$$ quasars discovered by James Webb Space Telescope. 

Abstract We analyze the dynamics of low-mass black hole (BH) seeds in the high-redshift (z ≳ 5) Universe using a suite of [4.5 Mpc]³and [9 Mpc]³BRAHMAcosmological hydrodynamic simulations. The simulations form seeds with massM_seed = 2.2 × 10³M_⊙in halos that exceed critical thresholds of dense and metal-poor gas mass (5–150M_seed) and the halo mass (1000–10,000M_seed). While the initialBRAHMAboxes pinned the BHs to the halo centers, here we implement a subgrid dynamical friction (DF) model. We also compare simulations where the BH is allowed to wander without the added DF. We investigate the spatial and velocity offsets of BHs in their host subhalos, as well as BH merger rates. We find that subgrid DF is crucial to ensure that a significant fraction of BHs effectively sink to halo centers byz ∼ 5, thereby enabling them to get gravitationally bound and merge with other BHs at separations close to the spatial resolution (∼0.2–0.4 kpc) of the simulation. For the BHs that merge, the associated merger timescales lag between ∼100 and 1000 Myr after their host halos merge. Compared to predictions using BH repositioning, the overallz ≳ 5 BH merger rates under subgrid DF decrease by a factor of ∼4–10. Under subgrid DF, the different seed models predict merger rates between ∼100 and 1000 events per year atz ≳ 5. These mergers dominate early BH growth, assembling BHs up to ∼10⁴–10⁵M_⊙byz ∼ 5, wherein ≲2% of their mass is assembled via gas accretion. Our results highlight the promise for constraining seeding mechanisms using gravitational waves from future facilities such as the Laser Interferometer Space Antenna. 

ABSTRACT JWST has revealed a large population of accreting black holes (BHs) in the early Universe. Recent work has shown that even after accounting for possible systematic biases, the high-z$$M_*{\!-\!}M_{\rm \rm bh}$$ relation can be above the local scaling relation by $$\gt 3\sigma$$. To understand the implications of these overmassive high-z BHs, we study the BH growth at $$z\sim 4{\!-\!}7$$ using the $$[18~\mathrm{Mpc}]^3$$BRAHMA cosmological simulations with systematic variations of heavy seed models that emulate direct collapse black hole (DCBH) formation. In our least restrictive seed model, we place $$\sim 10^5~{\rm M}_{\odot }$$ seeds in haloes with sufficient dense and metal-poor gas. To model conditions for direct collapse, we impose additional criteria based on a minimum Lyman Werner flux (LW flux $$=10~J_{21}$$), maximum gas spin, and an environmental richness criterion. The high-z BH growth in our simulations is merger dominated, with a relatively small contribution from gas accretion. The simulation that includes all the above seeding criteria fails to reproduce an overmassive high-z$$M_*{\!-\!}M_{\rm bh}$$ relation consistent with observations (by factor of $$\sim 10$$ at $$z\sim 4$$). However, more optimistic models that exclude the spin and environment based criteria are able to reproduce the observed relations if we assume $$\lesssim 750~\mathrm{Myr}$$ delay times between host galaxy mergers and subsequent BH mergers. Overall, our results suggest that current JWST observations may be explained with heavy seeding channels if their formation is more efficient than currently assumed DCBH conditions. Alternatively, we may need higher initial seed masses, additional contributions from lighter seeds to BH mergers, and / or more efficient modes for BH accretion. 

ABSTRACT We study gas inflows on to supermassive black holes using hydrodynamics simulations of isolated galaxies and idealized galaxy mergers with an explicit, multiphase interstellar medium (ISM). Our simulations use the recently developed ISM and stellar evolution model called Stars and MUltiphase Gas in GaLaxiEs (SMUGGLE). We implement a novel super-Lagrangian refinement scheme that increases the gas mass resolution in the immediate neighbourhood of the black holes (BHs) to accurately resolve gas accretion. We do not include black hole feedback in our simulations. We find that the complex and turbulent nature of the SMUGGLE ISM leads to highly variable BH accretion. BH growth in SMUGGLE converges at gas mass resolutions ≲3 × 103 M⊙. We show that the low resolution simulations combined with the super-Lagrangian refinement scheme are able to produce central gas dynamics and BH accretion rates very similar to that of the uniform high resolution simulations. We further explore BH fueling by simulating galaxy mergers. The interaction between the galaxies causes an inflow of gas towards the galactic centres and results in elevated and bursty star formation. The peak gas densities near the BHs increase by orders of magnitude resulting in enhanced accretion. Our results support the idea that galaxy mergers can trigger AGN activity, although the instantaneous accretion rate depends strongly on the local ISM. We also show that the level of merger-induced enhancement of BH fueling predicted by the SMUGGLE model is much smaller compared to the predictions by simulations using an effective equation of state model of the ISM.