Can primordial black holes explain the overabundance of bright super-early galaxies?

¹ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
² Dipartimento di Fisica “Enrico Fermi”, Università di Pisa, Largo Bruno Pontecorvo 3, Pisa I-56127, Italy

^⋆ Corresponding author.

Received: 8 January 2025
Accepted: 26 February 2025

Abstract

The James Webb Space Telescope (JWST) is detecting an excess of high-redshift (z ≳ 10) bright galaxies that challenge most theoretical predictions. To address this issue, we investigated the impact of primordial black holes (PBHs) on the halo mass function and UV luminosity function (LF) of super-early galaxies. We explored two key effects: (i) The enhancement of the massive halo abundance due to the compact nature and spatial distribution of PBHs, and (ii) the luminosity boost, characterized by the Eddington ratio λ_E, caused by active galactic nuclei (AGN) that are powered by matter accretion onto PBHs. We built an effective model, calibrated using data at lower redshifts (z ≈ 4 − 9), to derive the evolution of the LF, including the additional PBH contribution. A Bayesian analysis yielded the following results: (a) Although a small fraction (log f_PBH ≈ −5.42) of massive (log M_PBH/M_⊙ ≈ 8.37) nonemitting (λ_E = 0) PBHs can explain the galaxy excess via the halo abundance enhancement, this solution is excluded by cosmic microwave background μ-distortion constraints on monochromatic PBHs. (b) If PBHs power an AGN that emits at super-Eddington luminosity (λ_E ≈ 10), the observed LF can be reproduced by a PBH population with a characteristic mass log M_PBH/M_⊙ ≈ 3.69, constituting a tiny (log f_PBH ≈ −8.16) fraction of the cosmic dark matter content. In the AGN scenario, about 75% of the observed galaxies with M_UV = −21 at z = 11 should host a PBH-powered AGN and typically reside in low-mass halos, M_h = 10^8 − 9 M_⊙. These predictions can be tested with available and forthcoming JWST spectroscopic data. We note that our analysis considered a lognormal PBH mass function and compared its parameters with monochromatic limits on the PBH abundance. Further work is required to relax these limitations.