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

TL;DR

This paper models how primordial black holes, as early active galactic nuclei, influence the 21-cm signal at high redshift, revealing their significant impact on the cosmic dawn observables.

Contribution

It introduces an extended semi-numerical model incorporating PBH effects into 21-cm signal predictions, accounting for different PBH mass functions.

Findings

PBH X-ray heating makes the 21-cm global signal shallower.

PBH mass functions significantly alter the 21-cm power spectrum.

The model self-consistently predicts 21-cm signals from z ~ 30 to 5.

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.