Search arXiv⌕ Search
Home/Archive/arXiv

arXiv · 2610.40144

What kinds of strong star--black-hole encounters in AGN disks occur, and how often?

R. O'Shaughnessy·Z. Rosenberg·J. Nordhaus
View original source ↗Read source document ↗Export metadata ↓

Abstract

Disks surrounding active galactic nuclei contain many stars and black holes, which will inevitably encounter one another. The outcome of these encounters depends on how closely and how rapidly the bodies meet. Extending previous studies, we account for initial relative velocities seeded by stochastic effects in the disk; incorporate a strong-encounter cross-section accounting for single- and binary-mediated interactions computed via direct two-body (Hill) or three-body (isolated) interactions; and present a repeated-encounter model accounting both for energy loss due to stellar encounters as well as the SMBH tidal field. We convolve our encounter model with a parametric family of natal stellar and binary populations, focused on varying binary fraction and period distribution. In our fiducial disk model, even though single-single interactions supply most encounters, binary-mediated interactions supply 68% of collisions; most encounters are well-approximated as parabolic; and stochastically-driven relative velocities are comparatively substantial and driven by gravitational perturbations from disk fluctuations and neighboring bodies. Our tools are ready to use in downstream applications, including several easily-expressed semianalytic approximations to key interaction phenomena.

Explore related subjects

astro-ph.GAastro-ph.HE

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

R. O'Shaughnessy, Z. Rosenberg, J. Nordhaus. 2026-10-29. What kinds of strong star--black-hole encounters in AGN disks occur, and how often?. https://arxiv.org/abs/2610.40144

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related papers

arXiv2026

From ASTRID to BRAHMA -- The role of overmassive black holes in little red dots in cosmological simulations

We investigate the origin of Little Red Dots (LRDs) using a realization of the BRAHMA cosmological hydrodynamic simulation that produces overmassive black holes at $z \sim 4\text{-}8$. We generate mock JWST observations by modeling galaxy spectral energy distributions (including dust attenuation) together with AGN emission using cloudy-based templates, and test whether such systems reproduce the observed LRD population. We consider both a standard low-redshift AGN emission model and a gas-enshrouded AGN emission model, in which the AGN is embedded within unresolved dense gas that produces a strong Balmer break, and find that LRDs emerge in significantly larger numbers with the latter model. Assuming that all AGN reside in this gas-enshrouded phase, we recover LRD number densities ($Φ\sim 2 \times 10^{-4}~\mathrm{Mpc}^{-3}$), close to the observed abundance. This implies an effective gas-enshrouded duty cycle approaching unity, which is not predicted by current theoretical models. Current JWST constraints on the high-z black hole mass function suggest that this tension may arise from a deficit of low-mass ($M_{\rm BH}\lesssim10^7~M_\odot$) LRDs in BRAHMA. Applying the same gas-enshrouded model to the ASTRID simulation, which lacks overmassive black holes, we find that the LRD abundance remains $\gtrsim2~\rm dex$ below observations. Overall, while the overmassive black holes in this BRAHMA realization substantially enhance the production of LRD-like systems, reproducing the observed LRD abundance requires them to reside in dense gas environments with unrealistically high duty cycles. We therefore conclude that overmassive black holes are unlikely to be the sole origin of the observed JWST LRD population, and may instead account for only a subset of these systems.

astro-ph.GA↗
arXiv2026

Evolution of Binaries Under Stochastic Perturbations

We develop a general Fokker-Planck framework describing the dynamical evolution of Keplerian binaries subjected to stochastic perturbations. The formalism provides an algorithmic way to obtain the Fokker-Planck drift and diffusion coefficients of any set of orbital variables given the statistics of the perturbations. We apply the method to three physically distinct regimes: adiabatic tidal perturbations, white-noise tidal perturbations, and impulsive encounters with a third body of arbitrary density profile. In each regime we provide explicit drift and diffusion coefficients for all six orbital elements, derive the associated evolution timescale, and obtain analytic steady-state distribution functions. Our results extend previous treatments by including the evolution of the binary's orientation, retaining the complete tensor structure of tidal correlators, treating non-pointlike perturbers, and resolving the exact geometry of impulsive encounters. The latter correction leads to a steady-state eccentricity distribution that is slightly sub-thermal. We also show how these equations can be applied directly in several astrophysical scenarios, including binaries perturbed by dark matter subhaloes, ultralight dark matter, and the interstellar medium. This work delivers both a complete mathematical framework and a practical toolkit for stochastic binary evolution, providing ready-to-evaluate equations to be applied directly to binary population data.

astro-ph.GA↗
arXiv2026

Improved constraints on the Milky Way potential using the M68 stream and DESI spectroscopic data

We present a selection of stars belonging to the stellar stream of the M68 (NGC 4590) globular cluster, also known as Fjörm. This star selection is an improvement on previous ones that used only Gaia data, as it incorporates spectroscopic measurements from the DESI survey and photometric data from the DESI Legacy Surveys. The selection contains 96 stars, each with five phase-space parameters from Gaia-DR3 and radial velocity from DESI, covering the entire observed section of the stream. This constitutes the largest selection of M68 stream stars with measured radial velocities to date. The observed stream is wider than expected from N-body simulations, and the stars farthest from the centre of the stream appear to be correlated in radial velocity space. This suggests that these stars cannot have been stripped from the cluster in a static axisymmetric potential. By modelling a mock sample of stream stars created using an N-body simulation, we found that we could reliably constrain the disc mass $M_{\rm d}$ and the dark matter halo axis ratio $q_{\rm h}$ of the Milky Way. This is because the stream flows close to and almost parallel to the disc. Using the 44 stars that are consistent with having been stripped from the cluster, combined with measurements of the Milky Way's rotation curve, we constrain the Galactic potential, obtaining $M_{\rm d} = 5.34 \pm 0.57 \times 10^{10}$ M$_{\rm sun}$ and an oblate halo of $q_{\rm h} = 0.83^{+0.06}_{-0.05}$. Additionally, by fitting the stream track, we estimate the Heliocentric distance of M68 to be $r=10.55\pm0.09$ kpc.

astro-ph.GA↗