X-ray Preionisation Powered by Accretion on the First Black Holes. II: Cosmological Simulations and Observational Signatures

We use numerical simulations of a cosmological volume to study the X-ray ionisation and heating of the intergalactic medium by an early population of accreting black holes. By considering theoretical limits on the accretion rate and observational constraints from the X-ray background and faint X-ray source counts, we find that the maximum value of the optical depth to Thompson scattering that can be produced using these models is τe ≃ 0.17, in agreement with previous semianalytic results. The redshifted X-ray background produced by these early sources, independent of the their assumed spectra, is important in producing a fully ionised atomic hydrogen in the low density intergalactic medium before redshift z ∼ 7. The background also produces a second He II reionisation at about redshift three and maintains the temperature of the intergalactic medium at about 10,000 K even at low redshifts. If the spectral energy distribution of these sources has a non-negligible high energy power-law component, the luminosity in the soft X-ray band of the “typical” galaxies hosting intermediate-mass accreting black holes is maximum at z ∼ 15 and is about one or two orders of magnitude below the sensitivity limit of the Chandra deep field. We find that about a thousand of these sources may be present per square arcmin of the sky, producing potentially detectable fluctuations. We also estimate that a few rare objects, not present in our small simulated volume, could be luminous enough to be visible in the Chandra deep field. XEUS and Constellation-X satellites will be able to detect more of these sources that, if radio loud, could be used to study the 21 cm forest in absorption. A signature of an early X-ray preionisation is the production of secondary CMB anisotropies on small angular scales (< 1 arcmin). We find that in these models the power spectrum of temperature fluctuations increases with decreasing angular scale (∆T ∼ 16μK at ∼ 1 arcsec scales), while for stellar reionisation scenarios the power decreases on smaller scales. We also show that the redshifted 21 cm radiation from neutral hydrogen can be marginally detected in emission at redshifts 7 < z < 12. At a redshift of about z ∼ 30 a stronger and narrower (in redshift space) signal in absorption against the CMB, that is peculiar to these models, could be detectable.