Impact of Primordial Black Hole population on 21 cm observables at high redshift

Atrideb Chatterjee, Barun Maity, Koushiki

First listed 2026-04-23 | Last updated 2026-04-23

Abstract

The 21-cm signal, one of the most promising probes of the high-redshift Universe, has traditionally been modelled without accounting for the effects of active galactic nuclei (AGN) in the pre-JWST era, primarily due to the lack of observational evidence for AGNs at z > 6. However, following the discovery of several AGNs at redshifts as high as z ~ 10 by JWST, it has become imperative to incorporate the impact of these early AGNs when predicting the 21-cm signal. Supposing that these AGNs are seeded by primordial black holes (PBHs), we study their impact with a semi-numerical model setup. Specifically, we extended the explicitly photon-conserving reionization framework, SCRIPT, including essential cosmic dawn physics and PBH contributions. This enables us to compute both the global signal and the power spectrum of the 21-cm line over the redshift range z ~ 30 - 5 within a self-consistent framework. Building on this setup, we then investigate the impact of different PBH mass functions (obeying current observational constraints) on the resulting signal. The X-ray heating from PBHs can substantially make the depth of the global 21-cm signal shallower and suppress the expected power amplitude during cosmic dawn. We also find that the choice of mass function plays a crucial role in shaping the 21-cm signal, and can, in fact, lead to significantly different predictions.

Short digest

This paper studies how a population of primordial-black-hole-seeded AGN would change high-redshift 21-cm observables. The main result is that the predicted global signal and power spectrum depend strongly on the adopted PBH mass function, but the same models still underproduce the JWST-inferred AGN abundances unless extra sources are added. The paper matters because it links the emerging JWST AGN population to a completely different observational probe and clarifies where PBH scenarios are already under pressure.

Key figures to inspect

• Figure 3 is the must-see plot: it shows the global 21-cm signal and power spectrum directly, so this is the bottom-line figure for whether the PBH population leaves an observable imprint.
• Figure 1 is worth checking early: it compares the PBH-seeded mass functions against the JWST-inferred abundances and shows the main tension, namely that the model underproduces the observed population by orders of magnitude unless extra AGN sources are added.
• Figure 2 explains the physics behind the later 21-cm signals by tracking how the X-ray, Ly-alpha, and ionizing backgrounds change across the PBH scenarios.
• Figure 4 is useful for sensitivity: it shows how much the 21-cm prediction moves when the width of the lognormal PBH mass function is changed.