X-ray pre-ionization 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 ionization and heating of the intergalactic medium (IGM) by an early population of accreting black holes (BHs). 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 which can be produced using these models is τ_e~= 0.17, in agreement with previous semi-analytic results. The redshifted soft X-ray background produced by these early sources is important in producing a fully ionized atomic hydrogen in the low-density intergalactic medium before stellar reionization at redshift z~ 6-7. As a result, stellar re-ionization is characterized by an almost instantaneous `overlap phase' of HII regions. The background also produces a second HeII re-ionization at about redshift 3 and maintains the temperature of the IGM at about 10000K 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 BHs 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. The 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 pre-ionization is the production of secondary cosmic microwave background (CMB) anisotropies on small angular scales (<1arcmin). We find that in these models the power spectrum of temperature fluctuations increases with decreasing angular scale (ΔT~ 16μK at ~1-arcsec scales), while for stellar re-ionization 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.