Formation of Merging Black Hole Binaries Inside Massive AGN Stars
Abstract
Stars embedded in the disks of active galactic nuclei (AGN) can grow to hundreds of solar masses, and the same disks are expected to host a population of stellar-mass black holes. A star may therefore capture passing black holes, turning its interior into a potential factory for forming compact binary black hole systems and, ultimately, for the gravitational-wave events seen by our detectors. We test how readily this channel operates using three-dimensional hydrodynamic simulations of black hole--star encounters. As a proof of principle, we adopt a star-to-black-hole mass ratio of 34 : 1, and find that the capture does not disrupt the structure of the star: it heats the star by $10−−30\%$, and the star loses only ≲1% of its mass throughout. The captured black hole loses its orbital angular momentum rapidly to dynamical friction, sinking to the stellar center within a stellar dynamical time ≲104s. If the star already harbors a black hole at its center, the two form a bound binary whose gravitational-wave coalescence time falls below 104yr. Depending on the geometry and speed of the encounter, the resulting orbit may be nearly circular or retain substantial eccentricity. Our calculations confirm that black hole--star encounters in AGN disks are indeed a viable channel for assembling stellar-mass black hole binaries and supplying sources for gravitational-wave detectors.
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BibTeX
@misc{ma2026formation,
title = {{Formation of Merging Black Hole Binaries Inside Massive AGN Stars}},
author = {Sizheng Ma and Douglas N. C. Lin and David A. Velasco-Romero},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.16290},
eprint = {2609.16290},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.16290},
}
RIS
TY - UNPB
TI - Formation of Merging Black Hole Binaries Inside Massive AGN Stars
AU - Sizheng Ma
AU - Douglas N. C. Lin
AU - David A. Velasco-Romero
PY - 2026
DA - 2026/09/14
PB - arXiv
DO - 10.48550/arxiv.2609.16290
UR - https://arxiv.org/abs/2609.16290
AB - Stars embedded in the disks of active galactic nuclei (AGN) can grow to hundreds of solar masses, and the same disks are expected to host a population of stellar-mass black holes. A star may therefore capture passing black holes, turning its interior into a potential factory for forming compact binary black hole systems and, ultimately, for the gravitational-wave events seen by our detectors. We test how readily this channel operates using three-dimensional hydrodynamic simulations of black hole--star encounters. As a proof of principle, we adopt a star-to-black-hole mass ratio of 34 : 1, and find that the capture does not disrupt the structure of the star: it heats the star by $10−−30\%$, and the star loses only ≲1% of its mass throughout. The captured black hole loses its orbital angular momentum rapidly to dynamical friction, sinking to the stellar center within a stellar dynamical time ≲10^4s. If the star already harbors a black hole at its center, the two form a bound binary whose gravitational-wave coalescence time falls below 10^4yr. Depending on the geometry and speed of the encounter, the resulting orbit may be nearly circular or retain substantial eccentricity. Our calculations confirm that black hole--star encounters in AGN disks are indeed a viable channel for assembling stellar-mass black hole binaries and supplying sources for gravitational-wave detectors.
ER -