Beyond the first galaxies primordial black holes shine

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: 24 March 2025
Accepted: 14 July 2025

Abstract

The presence of nine candidate galaxies at z = 17 and z = 25 discovered by the James Webb Space Telescope in relatively small sky areas, if confirmed, is virtually impossible to reconcile with the predictions of the current galaxy formation model. We show here that the implied UV luminosity density can be produced by a population of primordial black holes (PBHs) of mass M_PBH = 10^4 − 5 M_⊙ residing in low-mass halos (M_h ≈ 10⁷ M_⊙), and accreting at a moderate fraction of the Eddington luminosity, λ_E ≃ 0.36. These sources precede the first significant episodes of cosmic star formation. At later times, as star formation is ignited, PBH emission becomes comparable to, or subdominant with respect to, the galactic emission. This PBH+galaxy scenario reconciles the evolution of the UV luminosity function (LF) from z = 25 to z = 11. If ultra-early sources are powered purely by accretion, this strongly disfavours seed production mechanisms requiring the presence of stars (massive stars, Pop III stars, or clusters), or their UV radiation (direct collapse BHs), leaving PBHs as the only alternative solution available so far. Alternative explanations, such as isolated, large clusters (≈10⁷ M_⊙) of massive (m_⋆ = 10³ M_⊙) Pop III stars are marginally viable, but require extreme and unlikely conditions that can be probed via UV and far-infrared (FIR) emission lines or gravitational waves.