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arXiv · 2511.20414

Black Hole merger rates in the first billion years in light of JWST data

P. F. V. Cáceres-Burgos·P. Dayal·P. Lira·V. Mauerhofer·F. P. Pratama·M. Trebitsch
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Abstract

Context. Recent James Webb Space Telescope (JWST) discoveries have unveiled an abundance of faint and massive Active Galactic Nuclei (AGNs) at high redshifts (z=4-9), that surpass by 10 to 100 times the extrapolated bolometric (Bol) and ultraviolet (UV) luminosity functions (LF) from previous AGN campaigns. The two main models that are put forward to explain these observations correspond to light seeds (150 Msol) accreting in episodes of super Eddington, and heavy seeds ($10^3$ - $10^5$ Msol) growing at the Eddington limit. Future gravitational observatories like the Laser Interferometer Satellite Antenna (LISA) will help disentangle these models by reporting the BH-BH merger events from mid to high redshifts. Aims. This work aims to report the predicted merger rates in the heavy seed scenario in light of recent JWST data. In our models we explore (i) instantaneous merging between BHs, (ii) delayed merging after a dynamical timescale, as well as extreme spin configurations (a=0.99, a=-0.99) to bracket BH mass growth. Methods. We use Delphi, a semi-analytical model that tracks baryonic physics over a hierarchical evolution of dark matter halos through cosmic time within the first billion years of the Universe. We calibrate this model for it to simultaneously reproduce galaxy and JWST-AGNs observables. Results. We show reasonable agreement with the Bolometric Luminosity function at z=6, where BHs must accrete 10-100 times more gas than in previous works calibrated to pre-JWST data. However, we underpredict (overpredict) the bright end $10^45.5$ erg s$^-1$ (all luminosity range) at z=7 (z=5) by 1-3.2 dex (0.22-1.6 dex). Regarding BH-BH merger events, the instantaneous (delayed) models predict a total of 28.06 (19.61) yr$^-1$ for BHs at z>=5, which is within the range of merger rates reported in previous literature.

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P. F. V. Cáceres-Burgos, P. Dayal, P. Lira, V. Mauerhofer, F. P. Pratama, M. Trebitsch. 2025-11-25. Black Hole merger rates in the first billion years in light of JWST data. https://doi.org/10.1051/0004-6361%2F202556153

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