A GLIMPSE of Intermediate Mass Black holes in the epoch of reionization: Witnessing the Descendants of Direct Collapse?

Qinyue Fei, Seiji Fujimoto, Rohan P. Naidu, John Chisholm, Hakim Atek, Gabriel Brammer, Yoshihisa Asada, Volker Bromm, Lukas J. Furtak, Jenny E. Greene, Tiger Yu-Yang Hsiao, Junehyoung Jeon, Vasily Kokorev, Jorryt Matthee, Priyamvada Natarajan, Johan Richard, Alberto Saldana-Lopez, Daniel Schaerer, Marta Volonteri, Adi Zitrin

First listed 2025-09-24 | Last updated 2025-10-05

Abstract

JWST has revealed an abundance of supermassive black holes (BHs) in the early Universe, and yet the lowest mass seed black holes that gave rise to these populations remain elusive. Here we present a systematic search for broad-line Active Galactic Nuclei (AGNs) in some of the faintest high-$z$ galaxies surveyed yet by combining ultra-deep JWST/NIRSpec G395M spectroscopy with the strong lensing aid in Abell S1063. By employing the profile of the [OIII]$λ5007$ emission lines as a template for narrow-line components and carefully cross-validating with mock observations, we identify a sample of ten broad-line AGNs at $4.5<z<7.0$ (eight secure, two tentative). The inferred BH masses from the broad H$α$ line explore the intermediate BH mass regime down to $\sim 10^{5.5}\,M_\odot$. The stellar mass ($M_*$) is estimated with a galaxy+AGN composite model, and we find the BH to stellar mass ratio spans down to $M_{\rm BH}/M_*\lesssim 0.1\%$, unveiling populations on the empirical $M_{\rm BH}-M*$ relation observed in the local universe. We also derive the black hole mass function and investigate its low-mass end at this epoch. While we confirm the agreement of our results with previous studies at $M_{\rm BH}\gtrsim10^{6.5}M_{\odot}$, we find the mass range of $\sim 10^{5.5}\,M_\odot$ features an enhanced abundance with respect to the extrapolated best-fit Schechter function. Comparison with theoretical models suggests that a possible origin for this enhanced abundance is the direct-collapse BH formation, supporting the scenario that the direct collapse of massive gas clouds is a significant pathway for the earliest supermassive BHs.

Short digest

Ultra-deep JWST/NIRSpec G395M spectroscopy behind Abell S1063 enables a systematic search for faint broad-line nuclei, fitting narrow components with an [O III] λ5007 template and validating the method with mocks. The authors identify ten broad-line AGN at 4.5<z<7.0 (eight secure, two tentative) with broad Hα–based virial masses down to ~10^5.5 Msun; composite galaxy+AGN SEDs yield host masses implying M_BH/M_* ≲ 0.1% and placements consistent with the local M_BH–M_* relation. From these they build a BH mass function that agrees with prior results above ~10^6.5 Msun but shows an excess at ~10^5.5 Msun relative to a Schechter extrapolation. They interpret the low-mass excess as evidence for descendants of direct-collapse seeds in the reionization era.

Key figures to inspect

• Figure 1 (footprint and magnification): Verify where each BLAGN sits relative to the Abell S1063 magnification contours (μ), clarifying which detections rely on the highest lensing boost and the survey’s effective sensitivity across the field.
• Figure 2 (2D/1D spectra and fits): Inspect the [O III] profile used as the narrow template and the broad Hα wings, along with FWHM, S/N, ΔBIC, and χ2_red for each source; pay special attention to tentative IDs 38548 and 7404 to see why they are classified as less secure.
• Figure 3 (composite SED, ID13131): Check the CIGALE decomposition (stellar, nebular, AGN) that sets M_* and the AGN fraction, illustrating how the analysis reaches M_BH/M_* ≲ 0.1% while retaining a good fit to NIRCam photometry.
• Figure 4 (M_BH vs UV magnitude): Locate the ~10^5.5–10^6.5 Msun regime relative to literature samples (Harikane+, Maiolino+), to see how these sources extend into IMBH territory at faint UV luminosities.