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

TL;DR

This study investigates black hole merger rates in the first billion years under a heavy-seed scenario guided by JWST AGN discoveries. It utilizes the Delphi semi-analytical model to simulate the coupled growth of gas, stars, and black holes within a hierarchical dark matter framework, calibrating to JWST galaxy and AGN observables. The authors explore instantaneous vs delayed BH mergers and three spin configurations, finding that instantaneous merging yields higher BH growth and a higher predicted merger rate (≈28 yr^-1 for z≥5) than delayed merging (≈19.6 yr^-1), with spin playing a major role in accretion efficiency and luminosity. The fiducial model matches the z≈6 bolometric LF but struggles with the bright end at z≈7 and with some JWST-derived BHMF points, indicating that additional physics (e.g., more efficient quenching, DC seeds, or spin evolution) may be needed; nonetheless, the predicted BH merger rates fall within ranges reported in the literature and imply LISA-scale detections for high-redshift BH mergers. The work highlights the potential of JWST data to constrain early BH assembly and informs expectations for future gravitational-wave observatories.

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.

Figures (9)

• Figure 1: Bolometric luminosity functions for AGNs for redshifts 5, 6, 7 and 10. The instantaneous models are shown as solid lines, while the delayed are dashed. The shaded areas for each model correspond to their Poisson errors. The fiducial model (ins) and delayed ( del) are shown in black, the extreme retrograde are shown in pink and the extreme prograde in blue (del-sp099). Observations pre-JWST are represented by the fits from Shen2020The07 (green dash-dot lines). JWST results are from Greene2023UNCOVERz5Kokorev2024AFieldsMatthee2023LittleSurveysKovacs2024Azapprox10Akins2024COSMOS-Web:AssemblyLin2024AWFSS.
• Figure 2: Black Hole mass functions at redshifts 5 and 6 for the fiducial (black solid lines), del (black dashed lines), ins-sn099 (pink solid lines), del-sp099 (blue dashed lines). The thin lines represent all BHs, while thick lines show BHs with $L_{Bol} \geq 10^{44}$ erg s$^{-1}$ and $f_{\rm Edd}^{\rm eff}>0.1$. Observational constraints from Pre-JWST come from Willott2010EDDINGTON-LIMITED6 and Shankar2009Self-consistentEfficiency, while JWST predictions are taken from Kokorev2024AFields, and Matthee2023LittleSurveys.
• Figure 3: Stellar to black hole (BH) mass relation for the fiducial (shaded contours) del (line contours), ins-sn099 and del-sp099 models at redshifts 5, 7 and 10. Here, we present the relation as contours from a smoothed 2d histogram, weighted by their associated number density. In all panels, we show the scaling relations for local Elliptical and Spheroidals (solid black line) Reines2015RelationsUniverse, high redshift galaxies ($z \sim 2.5$) Suh20202.5 (black dash-dotted line), and the relation for high-redshift (z=4-7) AGNs using only JWST sources Pacucci2023JWSTModels (black dashed lines). Observations pre-JWST come from works of Pensabene2020TheGalaxies, and the latest results from JWST from Maiolino2023JADES.Mighty, Harikane2023JWST/NIRSpecProperties, Yue2023EIGERzgtrsim6, Bogdan2023EvidenceQuasar, Rinaldi2024NotDots, Tripodi2024RedMyr, Larson2023AQuasars, Kokorev2023UNCOVER:8.50, Akins2025StrongFormation and Kovacs2024A10.
• Figure 4: In the upper panel, we show the cumulative merger events for BHs per year as a function of redshift, and in the bottom are the merger rates at a given redshift bin. Instantaneous models are shown as solid lines, while delayed are dashed.
• Figure 5: Merger rate events per year as a function of redshifted merged BH mass for all our models. Dashed lines correspond to the delayed models ( del, del-sp099, del-sn099), and the solid to the instantaneous (fid, ins-sp099, ins-sn099)