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Effect of Primordial Black Holes on the global 21-cm signal

Atrideb Chatterjee

TL;DR

The study addresses how primordial black holes seeding early galaxies could modify the global 21-cm signal. It develops a two-component semi-analytic framework combining standard star-forming galaxies with PBH-seeded systems, using a log-normal PBH mass function calibrated to observational constraints. The results show that PBH-driven X-ray heating can noticeably soften the absorption trough, while PBHs mainly affect the very early Ly-α and have limited impact on reionization, with the magnitude depending on the PBH X-ray escape fraction. These findings inform the interpretation of current and upcoming global 21-cm experiments and provide constraints on PBH contributions in the early universe.

Abstract

The 21-cm global signal, a treasure trove of information about the nature of the first luminous sources of the Universe, has traditionally been modelled assuming that these early sources were predominantly star-forming galaxies. However, recent observations by the James Webb Space Telescope (JWST) have revealed several AGNs as early as z ~ 10 - 10.4 . In light of this, it is important to investigate the contribution of such AGNs to the 21-cm signal. Assuming that these AGNs are seeded by Primordial Black Holes (PBHs) and employing an analytical PBH model, consistent with existing cosmological and astrophysical constraints, we show that these exotic objects can have a significant impact on the redshift evolution of the global signal.

Effect of Primordial Black Holes on the global 21-cm signal

TL;DR

The study addresses how primordial black holes seeding early galaxies could modify the global 21-cm signal. It develops a two-component semi-analytic framework combining standard star-forming galaxies with PBH-seeded systems, using a log-normal PBH mass function calibrated to observational constraints. The results show that PBH-driven X-ray heating can noticeably soften the absorption trough, while PBHs mainly affect the very early Ly-α and have limited impact on reionization, with the magnitude depending on the PBH X-ray escape fraction. These findings inform the interpretation of current and upcoming global 21-cm experiments and provide constraints on PBH contributions in the early universe.

Abstract

The 21-cm global signal, a treasure trove of information about the nature of the first luminous sources of the Universe, has traditionally been modelled assuming that these early sources were predominantly star-forming galaxies. However, recent observations by the James Webb Space Telescope (JWST) have revealed several AGNs as early as z ~ 10 - 10.4 . In light of this, it is important to investigate the contribution of such AGNs to the 21-cm signal. Assuming that these AGNs are seeded by Primordial Black Holes (PBHs) and employing an analytical PBH model, consistent with existing cosmological and astrophysical constraints, we show that these exotic objects can have a significant impact on the redshift evolution of the global signal.
Paper Structure (11 sections, 9 equations, 3 figures)

This paper contains 11 sections, 9 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Theoretical Model
  3. The semi-analytic model for star-forming galaxies
  4. The formation and evolution of PBH-seeded galaxies
  5. The 21-cm global signal
  6. Kinetic temperature of the IGM
  7. Ly-$\alpha$ coupling
  8. Reionization
  9. Effect of PBHs on 21-cm signal
  10. Conclusions and Discussion
  11. Redshift evolution of $Q_{\rm HII}$ for different models

Figures (3)

  • Figure 1: Redshift evolution of key quantities that affect the 21-cm signal. The orange, blue, and green curves correspond to contributions from PBH-only, SF-only, and SF+PBH galaxies, respectively. Panel (a): Intrinsic X-ray emissivity as a function of redshift. In this case, PBH-only dominates over SF-only across the entire redshift range. Panel (b): Evolution of the Ly-$\alpha$ background flux. While dominated by PBH-seeded sources for the very early redshift around $z = 30 - 20$, the SF-only completely dominates the redshift $z=20-7$. Panel (c): Ionizing photon emissivity, which mimics the Ly-$\alpha$ background trends.
  • Figure 2: Redshift evolution of the global 21-cm signal for different values of $f^{\rm PBH}_{X, \rm esc}$, as indicated in the plots. The blue and green curves correspond to the SF-only and SF+PBH scenarios, respectively. In the leftmost panel, with $f^{\rm PBH}_{X, \rm esc} = 1$, the SF-only case exhibits a much deeper absorption trough compared to the SF+PBH scenario. This difference arises because the additional X-ray emission from PBH-seeded galaxies heats the intergalactic medium (IGM) more efficiently, thereby reducing the depth of the absorption feature. As $f^{\rm PBH}_{X, \rm esc}$ decreases (moving towards the right), the X-ray output from PBH-seeded galaxies diminishes, leading to a progressively smaller difference between the two signals. We also note the difference in the signals around $25 \gtrsim z \gtrsim 20$, and this can be attributed to the fact that SF+PBH produces significantly more Ly-$\alpha$ photons compared to SF-only model, as is also evident in Panel (b) of \ref{['fig:all_quan']}.
  • Figure 3: The redshift evolution of the volume filling factor of HII region, $Q_{\rm HII}$. The data points are collected from different observations as mentioned in the text.