X-ray Preionisation Powered by Accretion on the First Black Holes. II:   Cosmological Simulations and Observational Signatures
  We use cosmological simulations to study the X-ray ionisation and heating of the intergalactic medium by an early population of accreting black holes. By considering observational constraints from the X-ray background, we find an upper limit for the optical depth to Thompson scattering tau_e~0.17. The redshifted soft X-ray background from these early sources produces:(i) fully ionised atomic hydrogen in the low density intergalactic medium before redshift z~7 (consequently stellar reionisation is characterised by an instantaneous overlap phase of HII regions),(ii) a second HeII reionisation at z~3 and (iii) heats the intergalactic medium to near 10000 K at low redshifts. The typical luminosity in the soft X-ray band of the galaxies hosting the black holes is about one order of magnitude below the sensitivity limit of the Chandra deep field. About a thousand of these sources may be present per square arcmin of the sky, producing detectable fluctuations. Few rarer objects could be luminous enough to be visible in the Chandra deep field. XEUS and Con-X satellites will be able to detect more of these sources that, if radio loud, could be used to study the 21cm forest in absorption.A signature of an early X-ray preionisation is the production of secondary CMB anisotropies on small angular scales. We find that in these models the power spectrum of temperature fluctuations increases with decreasing angular scale (dT~16 muK at 1arcsec 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.[abridged] 
Massimo Ricotti, Jeremiah P. Ostriker and Nickolay Y. Gnedin