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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 A major open question in astrophysics is the mechanisms by which massive black holes (BHs) form in the early Universe, which pose constraints on seeding models. We study BH formation and evolution in a flexible model combining the cosmological IllustrisTNG (TNG) simulations with semi-analytic modeling in post-processing. We identify our TNG model hosts based on various criteria including a minimum gas mass of 107–109 M⊙, total host mass of 108.5–1010.5 M⊙, and a maximum gas metallicity of 0.01–0.1 Z⊙. Each potential host is assigned a BH seed with a probability of 0.01–1. The populations follow the TNG galaxy merger tree. This approach improves upon the predictive power of the simple TNG BH seeding prescription, narrowing down plausible seeding parameter spaces, and it is readily adaptable to other cosmological simulations. Several model realizations predict z ≲ 4 BH mass densities that are consistent with empirical data as well as the TNG BHs. However, high-redshift BH number densities can differ by factors of ∼ 10 to ≳ 100 between seeding parameters. In most model realizations, ≲ 105 M⊙ BHs substantially outnumber heavier BHs at high redshifts. Mergers between such BHs are prime targets for gravitational-wave detection with LISA. The z = 0 BH mass densities in most realizations of the model agree well with observations, but our strictest seeding criteria fail at high redshift. Our findings strongly motivate the need for better empirical constraints on high-z BHs, and they underscore the significance of recent AGN discoveries with JWST. 

Abstract From the luminous quasars atz∼ 6 to the recentz∼ 9–11 active galactic nuclei (AGN) revealed by JWST, observations of the earliest black hole (BH) populations can provide unique constraints on BH evolution. We use theBRAHMAsimulations with constrained initial conditions to investigate BH assembly in extreme overdense regions. The simulations implement heavy ∼10⁴–10⁵M_⊙seeds forming in dense, metal-poor gas exposed to sufficient Lyman–Werner flux. With gas accretion modeled via the Bondi–Hoyle formalism and BH dynamics with a subgrid dynamical friction scheme, we isolate the impact of seeding, dynamics, accretion, and feedback on BH evolution. With fiducial stellar and AGN feedback inherited fromIllustrisTNG, accretion is suppressed atz≳ 9, leaving mergers as the dominant growth channel. Gas accretion dominates atz≲ 9, where permissive models (super-Eddington or low radiative efficiency) build ∼10⁹M_⊙BHs powering quasars byz∼ 6, while stricterIllustrisTNG-based prescriptions yield much smaller BHs (∼10⁶–10⁸M_⊙). Our seed models strongly affect mergers atz≳ 9: only the most lenient models (with ∼10⁵M_⊙seeds) produce enough BH mergers to reach ≳10⁶M_⊙byz∼ 10, consistent with current estimates for GN-z11. Our dynamical friction model gives low merger efficiencies. Therefore, even in such extreme regions, we are unable to produce ≳10⁷M_⊙BHs byz∼ 9–10, as currently inferred for GHZ9, UHZ1, and CAPERS-LRD-z9. If the BH-to-stellar mass ratios of these sources are indeed so extreme, they would require either very short BH merger timescales or reduced AGN thermal feedback. Weaker stellar feedback boosts both star formation and BH accretion and cannot raise these ratios. 

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 The origin of the ‘seeds’ of supermassive black holes (BHs) continues to be a puzzle, as it is currently unclear if the imprints of early seed formation could survive to today. We examine the signatures of seeding in the local Universe using five $$[18~\mathrm{Mpc}]^3$$BRAHMA simulation boxes run to $z=0$. They initialize $$1.5\times 10^5~\rm {M}_{\odot }$$ BHs using different seeding models. The first four boxes initialize BHs as heavy seeds using criteria that depend on dense and metal-poor gas, Lyman–Werner radiation, gas spin, and environmental richness. The fifth box initializes BHs as descendants of lower mass seeds ($$\sim 10^3~\rm {M}_{\odot }$$) using a new stochastic seed model built in our previous work. In our simulations, we find that the abundances and properties of $$\sim 10^5-10^6~\rm {M}_{\odot }$$ local BHs hosted in $$M_*\lesssim 10^{9}~\rm {M}_{\odot }$$ dwarf galaxies, are sensitive to the assumed seeding criteria. This is for two reasons: (1) there is a substantial population of local $$\sim 10^5~\rm {M}_{\odot }$$ BHs that are ungrown relics of early seeds from $$z\sim 5-10$$; (2) BH growth up to $$\sim 10^6~\rm {M}_{\odot }$$ is dominated by mergers in our simulations all the way down to $$z\sim 0$$. As the contribution from gas accretion increases, the signatures of seeding start to weaken in more massive $$\gtrsim 10^6~\rm {M}_{\odot }$$ BHs, and they are erased for $$\gtrsim 10^7~\rm {M}_{\odot }$$ BHs. The different seed models explored here predict abundances of local $$\sim 10^6~\rm {M}_{\odot }$$ BHs ranging from $$\sim 0.01-0.05~\mathrm{Mpc}^{-3}$$ with occupation fractions of $$\sim 20-100~{{\ \rm per\ cent}}$$ for $$M_*\sim 10^{9}~\rm {M}_{\odot }$$ galaxies. These results highlight the potential for placing constraints on seeding models using local $$\sim 10^5-10^6~\rm {M}_{\odot }$$ BHs hosted in dwarf galaxies. Since merger dynamics and accretion physics impact the persistence of seeding signatures, and both high and low mass seed models can produce similar local BH populations, disentangling their roles will require combining high and low redshift constraints. 

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. 

While the first “seeds” of supermassive black holes (BH) can range from ~10^2-10^6 M_\odot, the lowest mass seeds (< 10^3 M_\odot) are inaccessible to most cosmological simulations due to resolution limitations. We present our new BRAHMA simulations that use a novel flexible seeding approach to predict the z>7 BH populations for low-mass seeds. We ran two types of boxes that model ~10^3 M_\odot seeds using two distinct but mutually consistent seeding prescriptions at different simulation resolutions. First, we have the highest resolution [9 Mpc]^3 (BRAHMA-9-D3) boxes that directly resolve ~10^3 M_\odot seeds and place them within haloes with dense, metal-poor gas. Second, we have lower resolution, larger volume [18 Mpc]^3 (BRAHMA-18-E4), and ~[36 Mpc]^3 (BRAHMA-36-E5) boxes that seed their smallest resolvable ~10^4 & 10^5 M_\odot BH descendants using new stochastic seeding prescriptions calibrated using BRAHMA-9-D3. The three boxes together probe key BH observables between ~10^3, and,10^7 M_\odot. The active galactic nuclei (AGN) luminosity function variations are small (factors of ~2-3) at the anticipated detection limits of potential future X-ray facilities (~10^{43} ergs/s at z~7). Our simulations predict BHs ~10-100 times heavier than the local M_* versus M_bh relations, consistent with several JWST-detected AGN. For different seed models, our simulations merge binaries at ~1-15 kpc, with rates of ~200-2000 yr−1 for >10^3 M_\odot BHs, ~6-60 yr−1 for >10^4 M_\odot BHs, and up to ~10 yr−1 amongst >10^5 M_\odot BHs. These results suggest that Laser Interferometer Space Antenna mission has promising prospects for constraining seed models. 

The physical origin of the seeds of supermassive black holes (SMBHs), with postulated initial masses ranging from ∼105 M⊙ to as low as ∼102 M⊙, is currently unknown. Most existing cosmological hydrodynamic simulations adopt very simple, ad hoc prescriptions for BH seeding and seed at unphysically high masses ∼105–106 M⊙. In this work, we introduce a novel sub-grid BH seeding model for cosmological simulations that is directly calibrated to high-resolution zoom simulations that explicitly resolve ∼103 M⊙ seeds forming within haloes with pristine, dense gas. We trace the BH growth along galaxy merger trees until their descendants reach masses of ∼104 or 105 M⊙. The results are used to build a new stochastic seeding model that directly seeds these descendants in lower resolution versions of our zoom region. Remarkably, we find that by seeding the descendants simply based on total galaxy mass, redshift and an environmental richness parameter, we can reproduce the results of the detailed gas-based seeding model. The baryonic properties of the host galaxies are well reproduced by the mass-based seeding criterion. The redshift-dependence of the mass-based criterion captures the combined influence of halo growth, dense gas formation, and metal enrichment on the formation of ∼103 M⊙ seeds. The environment-based seeding criterion seeds the descendants in rich environments with higher numbers of neighbouring galaxies. This accounts for the impact of unresolved merger dominated growth of BHs, which produces faster growth of descendants in richer environments with more extensive BH merger history. Our new seed model will be useful for representing a variety of low-mass seeding channels within next-generation larger volume uniform cosmological simulations. 

ABSTRACT We explore implications of a range of black hole (BH) seeding prescriptions on the formation of the brightest $$z$$ ≳ 6 quasars in cosmological hydrodynamic simulations. The underlying galaxy formation model is the same as in the IllustrisTNG simulations. Using constrained initial conditions, we study the growth of BHs in rare overdense regions (forming $$\gtrsim 10^{12}\, {\rm M}_{\odot }\,h^{-1}$$ haloes by $$z$$ = 7) using a  (9 Mpc h−1)3 simulated volume. BH growth is maximal within haloes that are compact and have a low tidal field. For these haloes, we consider an array of gas-based seeding prescriptions wherein $$M_{\mathrm{seed}}=10^4\!-\!10^6\, {\rm M}_{\odot }\,h^{-1}$$ seeds are inserted in haloes above critical thresholds for halo mass and dense, metal-poor gas mass (defined as $$\tilde{M}_{\mathrm{h}}$$ and $$\tilde{M}_{\mathrm{sf,mp}}$$, respectively, in units of Mseed). We find that a seed model with $$\tilde{M}_{\mathrm{sf,mp}}=5$$ and $$\tilde{M}_{\mathrm{h}}=3000$$ successfully produces a $$z$$ ∼ 6 quasar with $$\sim 10^9\, {\rm M}_{\odot }$$ mass and ∼1047 erg s−1 luminosity. BH mergers play a crucial role at $$z$$ ≳ 9, causing an early boost in BH mass at a time when accretion-driven BH growth is negligible. With more stringent seeding conditions (e.g. $$\tilde{M}_{\mathrm{sf,mp}}=1000$$), the relative paucity of BH seeds results in a much lower merger rate. In this case, $$z$$ ≳ 6 quasars can only be formed if we enhance the maximum allowed BH accretion rates (by factors ≳10) compared to the accretion model used in IllustrisTNG. This can be achieved either by allowing for super-Eddington accretion, or by reducing the radiative efficiency. Our results demonstrate that progenitors of $$z$$ ∼ 6 quasars have distinct BH merger histories for different seeding models, which will be distinguishable with Laser Interferometer Space Antenna observations. 

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.