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1. Uncovering Hidden Massive Black Hole Companions with Tidal Disruption Events

   https://doi.org/10.3847/1538-4357/ad0234  

   Mockler, Brenna; Melchor, Denyz; Naoz, Smadar; Ramirez-Ruiz, Enrico (November 2023, The Astrophysical Journal)

   Abstract Dynamical perturbations from supermassive black hole (SMBH) binaries can increase the rates of tidal disruption events (TDEs). However, most previous work focuses on TDEs from the heavier black hole in the SMBH binary (SMBHB) system. In this work, we focus on the lighter black holes in SMBHB systems and show that they can experience a similarly dramatic increase in their TDE rate due to perturbations from a more massive companion. While the increase in TDEs around the more massive black hole is mostly due to chaotic orbital perturbations, we find that, around the smaller black hole, the eccentric Kozai–Lidov mechanism is dominant and capable of producing a comparably large number of TDEs. In this scenario, the mass derived from the light curve and spectra of TDEs caused by the lighter SMBH companion is expected to be significantly smaller than the SMBH mass estimated from galaxy scaling relations, which are dominated by the more massive companion. This apparent inconsistency can help find SMBHB candidates that are not currently accreting as active galactic nuclei and that are at separations too small for them to be resolved as two distinct sources. In the most extreme cases, these TDEs provide us with the exciting opportunity to study SMBHBs in galaxies where the primary SMBH is too massive to disrupt Sun-like stars. 

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2. High-redshift Merger Model for Low-frequency Gravitational Wave Background

   https://doi.org/10.3847/1538-4357/ad5c68  

   Wu, Zhao-Feng; Giannios, Dimitrios (August 2024, The Astrophysical Journal)

   Abstract In 2023, the Pulsar Timing Array Collaborations announced the discovery of a gravitational wave background (GWB), predominantly attributed to supermassive black hole binary (SMBHB) mergers. However, the detected GWB is several times stronger than the default value expected from galactic observations at low and moderate redshifts. Recent findings by the James Webb Space Telescope have unveiled a substantial number of massive, high-redshift galaxies, suggesting more massive SMBHB mergers at these early epochs. Motivated by these findings, we propose an “early merger” model that complements the standard merger statistics by incorporating these early, massive galaxies. We compare the early and standard “late merger” models, which assume peak merger rates in the local Universe, and match both merger models to the currently detected GWB. Our analysis shows that the early merger model has a significantly lower detection probability for single binaries and predicts a ∼30% likelihood that the first detectable single source will be highly redshifted and remarkably massive with rapid frequency evolution. In contrast, the late merger model predicts a nearly monochromatic first source at low redshift. The future confirmation of an enhanced population of massive high-redshift galaxies and the detection of fast-evolving binaries would strongly support the early merger model, offering significant insights into the evolution of galaxies and SMBHs. 

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3. GWTC-4.0: Population Properties of Merging Compact Binaries

   https://doi.org/10.3847/2041-8213/ae771e  

   Abac, A G; Abouelfettouh, I; Acernese, F; Ackley, K; Adamcewicz, C; Adhicary, S; Adhikari, D; Adhikari, N; Adhikari, R X; Adkins, V K; et al (July 2026, The Astrophysical Journal Letters)

   Abstract We detail the population properties of binary neutron star, neutron star–black hole binary, and binary black hole mergers using 158 events from the cumulative Gravitational-Wave Transient Catalog 4.0. The black hole primary mass distribution consists of a power-law-like continuum that steepens above 35M_⊙with overdensities at 10M_⊙and 35M_⊙. Binary black holes with primary masses near 10M_⊙are more likely to have less massive secondaries, with a mass ratio distribution peaking at $q=0.71_{-0.11}^{+0.12}$ , potentially a signature of stable mass transfer during binary evolution. Black hole spins are inferred to be nonextremal, with 90% of black holes havingχ < 0.6, and preferentially aligned with binary orbits, implying many merging binaries form in isolation. However, we find that a significant fraction, 0.23–0.41, of binaries have negative effective inspiral spins, suggesting many could be formed dynamically in gas-free environments. We find evidence for correlation between effective inspiral spin and mass ratio, driven either by variation in the mode of the distribution or the width. The binary black hole merger rate increases with redshift, consistent with the cosmic star formation density. While there is no evidence of the mass spectrum evolving with redshift, the distribution of effective inspiral spin is found to broaden as redshift increases out toz ≈ 1. We infer the local merger rates (atz= 0) to be 8.8–250 Gpc⁻³yr⁻¹for binary neutron stars, 9.3–86 Gpc⁻³yr⁻¹for neutron star–black hole binaries, and 13–26 Gpc⁻³yr⁻¹for binary black holes; all values reflect central 90% credible intervals. 

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4. Trinity I: self-consistently modelling the dark matter halo–galaxy–supermassive black hole connection from z  = 0–10

   https://doi.org/10.1093/mnras/stac2633  

   Zhang; Behroozi, Peter; Volonteri, Marta; Silk, Joseph; Fan, Xiaohui; Hopkins, Philip F; Yang; Aird, James (November 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We present Trinity, a flexible empirical model that self-consistently infers the statistical connection between dark matter haloes, galaxies, and supermassive black holes (SMBHs). Trinity is constrained by galaxy observables from 0 < z < 10 [galaxies’ stellar mass functions, specific and cosmic star formation rates (SFRs), quenched fractions, and UV luminosity functions] and SMBH observables from 0 < z < 6.5 (quasar luminosity functions, quasar probability distribution functions, active black hole mass functions, local SMBH mass–bulge mass relations, and the observed SMBH mass distributions of high-redshift bright quasars). The model includes full treatment of observational systematics [e.g. active galactic nucleus (AGN) obscuration and errors in stellar masses]. From these data, Trinity infers the average SMBH mass, SMBH accretion rate, merger rate, and Eddington ratio distribution as functions of halo mass, galaxy stellar mass, and redshift. Key findings include: (1) the normalization and the slope of the SMBH mass–bulge mass relation increases mildly from z = 0 to z = 10; (2) The best-fitting AGN radiative+kinetic efficiency is ∼0.05–0.06, but can be in the range ∼0.035–0.07 with alternative input assumptions; (3) AGNs show downsizing, i.e. the Eddington ratios of more massive SMBHs start to decrease earlier than those of lower mass objects; (4) The average ratio between average SMBH accretion rate and SFR is ∼10−3 for low-mass galaxies, which are primarily star-forming. This ratio increases to ∼10−1 for the most massive haloes below z ∼ 1, where star formation is quenched but SMBHs continue to accrete. 

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5. On the mass distribution of the LIGO-Virgo-KAGRA events

   https://doi.org/10.1103/PhysRevD.111.043020  

   El_Bouhaddouti, Mehdi; Cholis, Ilias (February 2025, Physical Review D)

   The merging black hole binaries detected by the LIGO-Virgo-KAGRA (LVK) gravitational-wave observatories may help us shed light on how such binaries form. In addition, these detections can help us probe the hypothesized primordial black holes, a candidate for the observed abundance of dark matter. In this work, we study the black-hole mass distribution obtained from the LVK binary black hole merger events. In particular, we study the primary mass 𝑚1-distribution of the observed black hole binaries, and also the secondary to primary mass ratio 𝑞 =𝑚2/𝑚1 distribution. We obtain those distributions by first associating a skewed normal distribution to each event detected with a signal to noise ratio (SNR)>8 and then summing all such distributions. We also sample black hole binaries from two separate populations of merging binaries to which we associate a redshift from the redshift distribution. One of these is a stellar-origin population that follows a mass-distribution similar to the zero-age mass function of stars. The second population of black holes follows a Gaussian mass-distribution. Such a distribution could approximate a population of black hole binaries formed from earlier black hole mergers in dense stellar environments, or binaries of primordial black holes among other astrophysical processes. For those populations, we evaluate the number of detectable events and fit their combination to the LVK observations. In our work, we assume that stellar-origin binary black holes follow a similar mass-distribution to that of the initial mass-function of stars. We simulate a wide range of stellar-origin black-hole mass distributions. In agreement with the binary black hole merger rates-analysis of the LVK collaboration, we find that studying the observed LVK events can be fitted better by the combination of such a stellar-origin mass distribution and a Gaussian distribution, than by the stellar-origin mass distribution alone. As we demonstrate with some simple examples, our methodology allows for rapid testing of potential theoretical models for the binary black hole mergers to the observed events by LVK. 

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