Hierarchical Black Hole Mergers at High Redshift: Predictions for LISA and LGWA from SEEDZ

@misc{mehta2026hierarchical,
  title = {{Hierarchical Black Hole Mergers at High Redshift: Predictions for LISA and LGWA from SEEDZ}},
  author = {Daxal H. Mehta and Lewis R. Prole and John A. Regan and Rüdiger Pakmor and Sophie Koudmani and Ricarda S. Beckmann and Michael Tremmel and John H. Wise and Martin A. Bourne and Ralf S. Klessen and Simon C. O. Glover and Martin G. Haehnelt and John Brennan and Pelle Van De Bor and Debora Sijacki and Paul C. Clark},
  howpublished = {arXiv},
  year = {2026},
  month = oct,
  doi = {10.48550/arxiv.2610.07183},
  eprint = {2610.07183},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2610.07183},
  note = {CC BY 4.0},
}

TY  - UNPB
TI  - Hierarchical Black Hole Mergers at High Redshift: Predictions for LISA and LGWA from SEEDZ
AU  - Daxal H. Mehta
AU  - Lewis R. Prole
AU  - John A. Regan
AU  - Rüdiger Pakmor
AU  - Sophie Koudmani
AU  - Ricarda S. Beckmann
AU  - Michael Tremmel
AU  - John H. Wise
AU  - Martin A. Bourne
AU  - Ralf S. Klessen
AU  - Simon C. O. Glover
AU  - Martin G. Haehnelt
AU  - John Brennan
AU  - Pelle Van De Bor
AU  - Debora Sijacki
AU  - Paul C. Clark
PY  - 2026
DA  - 2026/10/05
PB  - arXiv
DO  - 10.48550/arxiv.2610.07183
UR  - https://arxiv.org/abs/2610.07183
AB  - We investigate the hierarchical assembly of black holes (BHs) in the early Universe using the next-generation SEEDZ cosmological zoom-in simulations. Our suite follows two independent regions (\textsc{Rarepeak} and \textsc{Normal1}) evolved with three BH feedback prescriptions: no feedback (NoFB), weak feedback (WeakFB), and standard thermal isotropic feedback (FidBH). Focusing on BH mergers at z ≥ 10, we explore merger-driven BH growth and detectability with next-generation gravitational-wave observatories. We find that stronger AGN feedback models dramatically suppress hierarchical BH assembly compared to weaker feedback implementations. In the NoFB simulations, BHs frequently undergo repeated mergers, with some experiencing more than 50 merger events within a few hundred Myr and gaining over 10^8,M_⊙ through mergers alone. In contrast, BHs in the FidBH simulations rarely undergo more than three mergers. NoFB therefore represents an upper limit to BH merger-driven growth beyond what is expected in the Universe. The suppression of BH mergers in the FidBH case is not driven by differences in galaxy assembly, BH occupation fractions, or BH seed abundances, which remain broadly similar across the simulation suites. Instead, feedback disrupts the dense gas surrounding newly formed BH seeds, suppressing their early accretion-driven growth and preventing them from entering runaway hierarchical merger pathways. Our results demonstrate that early-Universe feedback prescriptions strongly influence hierarchical BH growth through both mergers and accretion, producing distinct high-redshift merger populations. Coupled with seeding choices and other BH subgrid prescriptions, this introduces multiple degeneracies that will need to be broken in advance of LISA and other next-generation GW facilities.
ER  -