The properties of primordially-seeded black holes and their hosts in the first billion years: implications for JWST

¹ Kapteyn Astronomical Institute, University of Groningen PO Box 800 9700 AV Groningen, The Netherlands
² Canadian Institute for Theoretical Astrophysics, 60 St George St University of Toronto Toronto ON M5S 3H8, Canada
³ David A. Dunlap Department of Astronomy and Astrophysics, University of Toronto 50 St George St Toronto ON M5S 3H8, Canada
⁴ Department of Physics, 60 St George St University of Toronto Toronto ON M5S 3H8, Canada
⁵ Kavli Institute for Cosmology, University of Cambridge Madingley Road Cambridge CB3 0HA, UK
⁶ Cavendish Laboratory, University of Cambridge 19 JJ Thomson Avenue Cambridge CB3 0HE, UK
⁷ Department of Physics and Astronomy, University College London Gower Street London WC1E 6BT, UK

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 16 June 2025
Accepted: 20 November 2025

Abstract

Aims. James Webb Space Telescope (JWST) observations have opened a tantalising new window onto possible black holes as early as redshifts of z ∼ 10.4. Using local relations for calibration, these systems show a number of puzzling properties including black holes as massive as M_BH ≳ 10⁸ M_⊙ in place at z ∼ 10, unexpectedly high black hole-to-stellar mass ratios of M_BH/M_* ≳ 0.1 and, for some of them, extremely low metallicities. These pose a serious challenge for “astrophysical” seeding and growth models that we aim to explain with “cosmological” primordial black holes (PBHs) in this work.

Methods. We present PHANES (Primordial black holes accelerating the assembly of nascent early structures), an analytic framework that follows the evolution of dark matter halos, and their baryons in the first billion years, seeded by a population of PBHs with seed masses between 10^0.5 − 10⁶ M_⊙. In addition to a fiducial model where black holes are considered non-spinning (s = 0), we also explore two “maximal” scenarios where black holes show maximally prograde (s = +1) or retrograde (s = −1) spins.

Results. PBH seeded models yield a black hole mass function that extends between 10^1.25−11.25 (10^0.75−7.25) M_⊙ at z ∼ 5(15) for the different models considered in this work. Interestingly, PBH-seeded models (with s = 0 or −1) naturally result in extremely high values of M_BH/M_* ≳ 0.25 at z ∼ 5 − 15. For a typical stellar mass of M_∗ = 10⁹ M_⊙, we find an average value of M_BH/M_* ∼ 0.4(1.6) for s = 0(−1) at z = 5, providing a smoking gun for PBH-seeded models. Showing Eddington accretion fractions that range over two orders of magnitude (f_Edd ∼ 0.01 − 1), another particularity of PBH-seeded models is their ability of producing systems with high black hole-to-stellar mass ratios that are extremely metal poor (Z ≲ 10⁻² Z_⊙). Yielding a PBH-to-dark matter fraction ≲10⁻⁹ and a stellar mass function that lies four orders of magnitude below observations, our model is in accord with all current cosmological and astrophysical bounds.