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Abstract Massive black hole binaries (MBHBs) form through galaxy mergers and are among the loudest sources of gravitational waves (GWs) in the Universe. If the binary inspiral time is long, a subsequent galaxy merger can introduce a third black hole, forming a triple system. In the Illustris cosmological simulation, 6% of MBHBs form such triples at parsec scales, where strong three-body interactions are likely. We apply results from numerical simulations of triple massive black holes (MBHs) to strong triples identified in Illustris to assess their impact on MBH mergers and recoils. We find that triple interactions raise the merger fraction, reduce the median merger timescales in strong triple systems, and increase the overall merger fraction slightly (by 4%). Furthermore, massive major mergers are over three times more likely to be facilitated by strong triple interactions than mergers in general. We also compare GW recoil kicks to gravitational slingshot kicks from triple interactions. Both mechanisms can produce kicks exceeding host escape speeds, ejecting MBHs and producing wandering or offset black holes. Although slingshots yield the kicks with the highest velocity, GW recoils dominate the ejected population when assuming random MBH spin orientations. Under this assumption, ejections from GW recoil and slingshot kicks reduce the total number of mergers by 6%. Our results highlight the impact of strong triple dynamics and GW recoils on MBH evolution and support their inclusion in cosmological simulations. 

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 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 Recent pulsar timing array (PTA) observations detected nanohertz gravitational waves, likely originating from massive black hole binaries (MBHBs). The detected amplitude is unexpectedly higher than inferred from the electromagnetic measurements. We present new gravitational-wave background (GWB) results from the ASTRID simulation. Its large volume and on-the-fly dynamics for massive black holes (MBHs) provide new insights into the MBHB population, offering a more accurate assessment of its contribution to the observed GWB. ASTRID predicts a GWB from MBHBs ofh_c = 2.8 × 10⁻¹⁵, or ∼45% of the observed amplitude at ∼4 nHz with a slope consistent withf^−2/3, andh_c = 2.5 × 10⁻¹⁶withh_c ∝ f^−1.6at ∼30 nHz. These predictions remain below current PTA constraints but align with empirical models based on the observed MBH mass functions. By comparison, TNG300 with postprocessed MBH dynamics yields a range between 70% and 90% (20% and 30%) of the observed levels at low (high) frequencies. At low frequencies, ASTRID predicts that the bulk of the GWB originates from MBHBs with massesM_tot = 1–3 × 10⁹M_⊙peaking atz ≈ 0.3, consistent with TNG300. Notably, both simulations predict significant contributions from minor mergers (q < 0.2) by up to ∼40%. By tracing the full merger trees of local MBHs in ASTRID, we show that they generate gravitational waves at ∼10%–80% of the maximum signal assuming no accretion and recent equal-mass mergers. Finally, we demonstrate the importance of on-the-fly MBH dynamics, the lack of which leads to 3–5 times excessive mass growth by merger, and a boost to the GWB prediction from this overestimated mass function, especially at high frequencies. 

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 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. 

ABSTRACT Massive black hole (MBH) binaries can form following a galaxy merger, but this may not always lead to a MBH binary merger within a Hubble time. The merger time-scale depends on how efficiently the MBHs lose orbital energy to the gas and stellar background, and to gravitational waves (GWs). In systems where these mechanisms are inefficient, the binary inspiral time can be long enough for a subsequent galaxy merger to bring a third MBH into the system. In this work, we identify and characterize the population of triple MBH systems in the Illustris cosmological hydrodynamic simulation. We find a substantial occurrence rate of triple MBH systems: in our fiducial model, 22 per cent of all binary systems form triples, and $$\gt 70{{\ \rm per\ cent}}$$ of these involve binaries that would not otherwise merge by z = 0. Furthermore, a significant subset of triples (6 per cent of all binaries, or more than a quarter of all triples) form a triple system at parsec scales, where the three BHs are most likely to undergo a strong three-body interaction. Crucially, we find that the rate of triple occurrence has only a weak dependence on key parameters of the binary inspiral model (binary eccentricity and stellar loss-cone refilling rate). We also do not observe strong trends in the host galaxy properties for binary versus triple MBH populations. Our results demonstrate the potential for triple systems to increase MBH merger rates, thereby enhancing the low-frequency GW signals detectable with pulsar timing arrays and with LISA. 

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.