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1. Using gravitational waves & multi-messenger astronomy to reverse-engineer the properties of galactic nuclei

   https://doi.org/10.1088/1361-6382/ae0aad  

   Ford, K_E Saavik; McKernan, Barry (February 2026, Classical and Quantum Gravity)

   Abstract Active galactic nuclei (AGN) are powered by accretion disks onto supermassive black holes (SMBHs) in the centers of galaxies. AGN are believed to play important roles in the evolution of both SMBHs and their host galaxies over cosmic time. AGN and the nuclear star clusters (NSCs) that interact with them remain unresolved with present and planned telescopes. As a result, the properties of AGN and NSCs are highly uncertain. Here we review how binary black hole (BBH) mergers can occur in AGN disks and how both the gravitational wave and electromagnetic wave properties of such mergers allow us to reverse-engineer the properties of AGN disks and NSCs over cosmic time. We point out that the feature in the BBH mass spectrum around $\sim 35M_{\odot }$ is an excellent probe of hierarchical merger models. Likewise constraints on the spins of upper-mass gap BH ( $\gtrsim 50M_{\odot }$ ) test the AGN channel. The effective spin ( ${\chi }_{\mathrm{eff}}$ ) distribution, including asymmetry, islands of structure and magnitudes are excellent tests of AGN model predictions. We also argue, that the rate of AGN-driven BBH mergers as a function of redshift should scale slightly shallower than the AGN number density, at least out to redshifts of $\sim 2$ , and should turnover at the same redshift as the AGN number density. Finally, we emphasize a determination of an AGN fraction of observed BBH mergers ( $f_{\mathrm{BBH},\mathrm{AGN}}$ ),regardless of the actual value, allows us to infer the average properties of AGN disks and NSCs out to high redshift. 

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2. Limits on Hierarchical Black Hole Mergers from the Most Negative χ _eff Systems

   https://doi.org/10.3847/2041-8213/ac86c4  

   Fishbach, Maya; Kimball, Chase; Kalogera, Vicky (August 2022, The Astrophysical Journal Letters)

   Abstract It has been proposed that some black holes (BHs) in binary black hole (BBH) systems are born from “hierarchical mergers” (HMs), i.e., earlier mergers of smaller BHs. These HM products have spin magnitudes χ ∼ 0.7, and, if they are dynamically assembled into BBH systems, their spin orientations will sometimes be antialigned with the binary orbital angular momentum. In fact, as Baibhav et al. showed, ∼16% of BBH systems that include HM products will have an effective inspiral spin parameter, χ eff < −0.3. Nevertheless, the LIGO–Virgo–KAGRA (LVK) gravitational-wave (GW) detectors have yet to observe a BBH system with χ eff ≲ −0.2, leading to upper limits on the fraction of HM products in the population. We fit the astrophysical mass and spin distribution of BBH systems and measure the fraction of BBH systems with χ eff < −0.3, which implies an upper limit on the HM fraction. We find that fewer than 26% of systems in the underlying BBH population include HM products (90% credibility). Even among BBH systems with primary masses m 1 = 60 M ⊙ , the HM fraction is less than 69%, which may constrain the location of the pair-instability mass gap. With 300 GW events (to be expected in the LVK’s next observing run), if we fail to observe a BBH with χ eff < −0.3, we can conclude that the HM fraction is smaller than 2.5 − 2.2 + 9.1 % . 

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3. McFACTS I: Testing the LVK AGN Channel with Monte Carlo for AGN Channel Testing and Simulation ( McFACTS )

   https://doi.org/10.3847/1538-4357/adf114  

   McKernan, Barry; Ford, KE Saavik; Cook, Harrison E; Delfavero, Vera; McPike, Emily; Nathaniel, Kaila; Postiglione, Jake; Ray, Shawn; O’Shaughnessy, Richard (September 2025, The Astrophysical Journal)

   Abstract Active galactic nuclei (AGNs) are a promising source of the binary black hole mergers observed in gravitational waves with LIGO-Virgo-Kagra (LVK). Constraining the AGN channel allows us to limit AGN parameter space (disk density, size, average lifetime) and nuclear star cluster (NSC) parameter space. Constraints on AGNs and NSCs have implications for Λ cold dark matter models of AGN feedback and models of AGN-driven supermassive black hole merger and growth. Here we present several qualitative studies of the AGN channel using new public, open-source, fast, reproducible codeMcFACTS(https://github.com/mcfacts/mcfacts): Monte Carlo for AGN channel Testing and Simulation. We demonstrate several important features for testing the AGN channel, including: (i) growth to large-mass intermediate-mass black hole is helped by the presence of migration traps or swamps, (ii) flat BH initial mass functions highlight hierarchical merger features in the mass spectrum, (iii) the (q, χ_eff) anticorrelation is a strong test of the bias to prograde mergers in the AGN channel, (iv) spheroid encounters can drive a fraction of mergers with high in-plane spin components (χ_p), (v) a high rate of extreme mass ratio inspirals are driven by an initial population of embedded retrograde BH, and (vi) both LVK and LISA are powerful probes of models of AGN disks and their embedded populations. 

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4. Spin–Orbit Alignment in Merging Binary Black Holes Following Collisions with Massive Stars

   https://doi.org/10.3847/2041-8213/adc263  

   Kıroğlu, Fulya; Lombardi, James C; Kremer, Kyle; Vanderzyden, Hans D; Rasio, Frederic A (April 2025, The Astrophysical Journal Letters)

   Abstract Merging binary black holes (BBHs) formed dynamically in dense star clusters are expected to have uncorrelated spin–orbit orientations since they are assembled through many random interactions. However, measured effective spins in BBHs detected by LIGO/Virgo/KAGRA hint at additional physical processes that may introduce anisotropy. Here we address this question by exploring the impact of stellar collisions and accretion of collision debris on the spin–orbit alignment in merging BBHs formed in dense star clusters. Through hydrodynamic simulations, we study the regime where the disruption of a massive star by a BBH causes the stellar debris to form individual accretion disks bound to each black hole (BH). We show that these disks, which are randomly oriented relative to the binary orbital plane after the initial disruption of the star, can be reoriented by strong tidal torques in the binary near pericenter passages. Following accretion by the BHs on longer timescales, BBHs with small but preferentially positive effective spin parameters (χ_eff≲ 0.2) are formed. Our results indicate that BBH collisions in young massive star clusters could contribute to the observed trend toward small positiveχ_eff, and we suggest that the standard assumption often made that dynamically assembled BBHs should have isotropically distributed BH spins is not always justified. 

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5. McFACTS III: Compact Binary Mergers from Active Galactic Nucleus Disks over an Entire Synthetic Universe

   https://doi.org/10.3847/1538-4357/ade4c1  

   Delfavero, Vera; Ford, K_E Saavik; McKernan, Barry; Cook, Harrison E; Nathaniel, Kaila; Postiglione, Jake; Ray, Shawn; McPike, Emily; O’Shaughnessy, Richard (August 2025, The Astrophysical Journal)

   Abstract The active galactic nucleus (AGN) channel for the formation of binary black hole (BBH) mergers has been previously studied as a potential formation channel for the merging compact binaries observed by the LIGO–Virgo–KAGRA (LVK) scientific collaboration. The first two papers in this series explored theMcFACTScode for the evolution of black hole orbits in AGN accretion disks for individual galaxy models and described the characteristics of predicted BBH populations in realizations of those models (such as the correlation between mass ratio and aligned spin). In this work, we explore the impact of the properties of AGN host galaxies and assume an AGN lifetime and cosmological model for the density of AGN in a universe like our own. By sampling from an inferred population of AGN, we marginalize over galaxy mass to predict a population of BBH mergers observable by modern ground-based gravitational-wave observatories. We find that for reasonable assumptions, AGN disk environments may account for massive BBH mergers such as GW190521 and GW190929_012149. We find that the majority of observable BBH mergers from our simulation are expected to originate in galaxies with a supermassive black hole between 10⁷M_⊙and 10^9.4M_⊙. We also find that if hierarchical mergers from AGN disks account for a substantial part of the LVK population, our current models require an AGN lifetime of 0.5–2.5 Myr. 

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