Illuminating the dark ages: Cosmic backgrounds from accretion onto primordial black hole dark matter

Abstract

Cold dark matter as the sum of different mass Primordial Black Holes (PBH) can explain a number of unsolved astrophysical mysteries. Here I assume a broad PBH mass distribution providing the bulk of the dark matter, consistent with all constraints and estimate the baryon accretion onto PBH contributing to cosmic background radiations, mainly the correlation signal between the Cosmic X-ray and the Cosmic infrared background fluctuations in deep Chandra and Spitzer surveys. I assume Bondi capture and advection dominated disk accretion with reasonable baryon densities and effective relative velocities between baryons and PBH, as well as appropriate accretion and radiation efficiencies, and integrate over the PBH mass spectrum and cosmic time. The X-ray contribution is indeed consistent with the residual X-ray background signal and the X-ray/infrared fluctuations. The flux peaks at z~17-30, consistent with constraints requiring the signal to come from high redshifts. The PBH contribution to the infrared background fluctuations is only about 1%. Magnetic fields are an essential ingredient in the Bondi capture process, and PBH can play a role in amplifying early seed fields and maintaining them until the galactic dynamo processes set in. The contribution of the assumed PBH population to the re-ionization history of the universe does not conflict with the Planck ionization limits. X-ray heating from PBH can provide a contribution to the entropy floor in groups of galaxies. The tantalizing EDGES redshifted 21-cm absorption line feature can be connected to the radio contribution to the cosmic background radiation. Finally, the number of intermediate-mass black holes and the diffuse X-ray emission in the Galactic Center are consistent with the assumed PBH dark matter. Some of the discrete sources in the deepest Chandra observations of the Galactic Ridge could indeed be accreting PBH.