Effects of Primordial Black Holes on IGM History

TL;DR

This work investigates how asteroid-mass primordial black holes, via Hawking radiation, could modify the intergalactic medium from the Dark Ages to cosmic dawn. By implementing a first-principles Hawking radiation framework and a detailed energy-deposition model, the authors quantify PBH-induced ionization, heating, and Lyα excitation and propagate these effects into the 21 cm signal. They find that the lightest PBHs can boost ionization and gas temperature by orders of magnitude by z ~ 25 and produce measurable shifts in the 21 cm brightness temperature, while heavier PBHs have progressively smaller impacts. The results highlight the importance of rigorously treating Hawking radiation in PBH cosmological observables and point to early-universe 21 cm signals as potential probes of ultra-light PBHs and their mass distributions.

Abstract

Currently the asteroid mass window (mass $\sim 10^{17}- 10^{21}$ grams) remains unconstrained for Primordial Black Holes (PBHs) to make up all of the dark matter content of the universe. Given these PBHs have very small masses, their Hawking temperature can be up to hundreds of keV. This study investigates the potential impacts of PBH Hawking radiation on the intergalactic medium from $z\sim 800-25$, namely studying the ionization history, kinetic gas temperature, and ultimately the 21 cm signature. We find that for masses on the low edge of the asteroid mass window, there are up two orders of magnitude increases in the ionization fraction and kinetic gas temperature by redshift 25, and the 21 cm spin temperature can differ from non-PBH cosmology by factors of a few. This analysis results in maximum differential brightness temperatures of +17 mK for our lightest PBH masses of $2.12\times 10^{16}$g. We also show maximal $53$ mK discrepancies in differential brightness temperatures between our PBH and non-PBH cosmologies for our lightest PBH mass, while our heaviest PBH mass of $1.65 \times 10^{17}$g shows only $0.5$ mK variations. We find the Hawking-radiated electrons and positrons are instrumental in driving these IGM modifications. This study shows the necessity for a rigorous treatment of Hawking radiation in PBH cosmological observables from the dark ages through cosmic dawn.

Figures (9)

• Figure 1: This flowchart describes the basic numerical engine that we utilize in this project. First, we generate our photon and fermion Hawking radiation spectra. We then compute the fraction of radiated photons then can photoionize and Compton scatter, and the rest are lost. If the photons Compton scatter, they must first undergo inverse Compton cooling. The fermions all inverse Compton cool and Compton scatter. From here, the radiation can contribute to ionizations, Lyman-$\alpha$ excitations, or heating the IGM. Based on which branching path the radiation falls under, we then use that computation to determine the ionization history. Next, we use this information to compute the kinetic gas temperature. The ionization history and gas temperature then also contribute to the Lyman-$\alpha$ coupling, and all of this information is used to compute the spin temperature.
• Figure 2: Rate of ionization produced by each ionization mechanism over time. Each panel corresponds do a different black hole mass (M1: top left, M2: top right, M3: bottom left, M4: bottom right). Here we see a slight decrease in all channels as mass increases. The most dominant ionization channel for smallest masses is through the Hawking radiated fermions, followed by the ionizations caused by 'tertiary' processes of Hawking radiated photons. The fermion contribution has the largest suppression as mass increases.
• Figure 3: Ionization history for PBH dark matter masses (M1 solid blue, M2 dotted orange, M3 dashed green, M4 dotdashed red) as well as a fiducial no PBH cosmological scenario (in tightly-dashed black). For the lightest black hole mass, we see over a two order of magnitude difference in the ionization history relative to a standard cosmology. The deviations diminish as we increase mass, and fully disappear at the largest black hole mass we test. M1 shows the largest changes at lowest redshifts, reaching factors over $10^2$, while M4 deviates maximally by a factors on the order of $10^{-2}$.
• Figure 4: Thermal History [in K] of gas with PBH dark matter (M1 solid blue, M2 dotted orange, M3 dashed green, M4 dotdashed red) and without (in black tightly-dashed line) the additional heating of PBHs. Also plotted is the CMB temperature in dash-dotted gray. We see that at the earliest times, there is very little deviation from the no-PBH kinetic gas temperature while it is tightly coupled to the CMB temperature. As we move to later times, we see the kinetic gas temperature for the lightest three PBH masses all begin to deviate from the no-PBH cosmology. The two lightest masses even overcome the CMB temperature at redshifts of $z\sim 80$ and $z\sim 45$. The most extreme deviation of IGM kinetic gas temperature is with M1, seeing over a two order of magnitude deviation from the no-PBH example. We again see factors well over 100 difference between the no-pbh and M1 cosmology, and see percent level changes in the M4 scenario at latest times.
• Figure 5: Rates of energy injection [in eV/s] from different heating channels (M1: top left, M2: top right, M3: bottom left, M4: bottom right). Here we see a wide spread in orders of magnitudes of heating rates. With M1, M2 and M3, we see the fermion contributions are most substantial, with rates near $10^{23}$ -$10^{22}$ eV/s that increase with redshift. The Compton scattering contribution with the Hydrogen in the IGM is then next important with rates of $10^{21}$ eV/s, and Helium Compton scattering contributes a few percent to the overall Compton scattering heating rates.