Primordial black holes and the velocity acoustic oscillations features in 21 cm signals from the cosmic Dark Ages

Abstract

Astrophysical luminous objects such as the first stars have not yet formed in the Dark Ages. However, primordial black holes (PBHs) always exist throughout cosmic history since the inflation epoch. During the Dark Ages, PBHs may accrete the ambient gas and release radiation like astrophysical luminous objects, change the cosmic radiation field, the thermal status of the intergalactic medium (IGM), and the hydrogen spin temperature. The accretion rate is modulated by the relic supersonic relative streaming velocities between dark matter (DM) and baryons, imprinting Velocity Acoustic Oscillations (VAOs) features in the 21 cm power spectrum. Such VAOs features could be a promising probe for detecting the PBHs in Dark Ages. We find that even if PBHs comprise only a small fraction of DM, they can generate VAOs wiggles with a relative amplitude up to about 30% in Dark Ages. For example, for PBHs with a mass at recombination of 200 solar masses and mass fraction in the total DM f_PBH,rec around 1e-13 at the recombination era, VAOs features appear at redshift around 20; if f_PBH,rec is around 3e-10, then VAOs features could appear as early as redshift around 40. Moreover, the redshift evolution of the VAOs features exhibits clearly separated stages dominated by inhomogeneous Ly-alpha scattering, and inhomogeneous X-ray heating, respectively. It reflects the characteristics of PBHs (mass and fraction in total DM) and their interactions with the IGM. We also estimate that, the VAOs wiggles at redshift around 20 are detectable for the upcoming SKA-low AA*, while wiggles at redshift around 40 are detectable for an hypothetic lunar surface-based interferometer array in the future.

Figures (14)

• Figure 1: $Top$: A slice of the DM-baryon relative streaming velocity field at $z=20$. $Bottom$: The power spectrum of the DM-baryon relative streaming velocity field, $\Delta_{v_{\rm db}}^2(k)=k^3/(2\pi^2)P_{v_{\rm db}}(k)$, at $z=20$ (top sub-panel). We fit the power spectrum by a 5th degree polynomial $\Delta^2_{v_{\rm db},\rm poly}(k)$, then show the relative amplitude of the VAOs wiggles, $[\Delta_{v_{\rm db}}^2(k) - \Delta^2_{v_{\rm db},\rm poly}(k)]/\Delta^2_{v_{\rm db},\rm poly}(k)$, in the bottom sub-panel. Throughout this paper, errorbars on the power spectrum curves and relative amplitude curves refer to the 1$\sigma$ sample variance of the signal calculated from the generated fields.
• Figure 2: The redshift evolution of the dimensionless accretion rate for a PBH with $M_{\rm PBH,rec} = 200~M_{\odot}$. Both panels adopt $\delta_{\rm b}=0$ and $v_{\rm db}=0$ as the reference case (black solid line). $Left$: The $\dot{m}$ for varying $v_{\rm db}$. $Right$: The $\dot{m}$ for varying $\delta_{\rm b}$. Obviously, $\dot{m}$ is more sensitive to $v_{\rm db}$ than $\delta_{\rm b}$.
• Figure 3: $Top$: A slice of the dimensionless PBH accretion rate field at $z=20$, for $M_{\rm PBH,rec}=200~M_{\odot}$. Here we set $T_{\rm K}=5$ K and $x_{\rm e}=10^{-4}$. $Bottom$: The power spectrum of the accretion field, $\Delta_{\dot{m}}^2(k)$ (top sub-panel), and the relative amplitude of the VAOs wiggles, $[\Delta_{\dot{m}}^2(k)-\Delta_{\dot{m},\rm poly}^2(k)]/\Delta_{\dot{m},\rm poly}^2(k)$ (bottom sub-panel).
• Figure 4: $Top$: A slice of the PBH masses field $M_{\rm PBH}$ at $z=20$. $Bottom$: The growth of the mean PBH mass and fraction in the total DM. Here we set the mass $M_{\rm PBH,rec}=200~M_{\odot}$.
• Figure 5: The evolution of the mean and scattering of the IGM temperature under the influence of PBHs radiation, assuming $M_{\rm PBH,rec}=200~M_{\odot}$ and $f_{\rm PBH,rec}=10^{-8}$. The black dashed line is the CMB temperature.