Stellar black holes at the dawn of the universe
  It is well established that between 380000 and 1 billion years after the Big
Bang the Inter Galactic Medium (IGM) underwent a "phase transformation" from
cold and fully neutral to warm (~10^4 K) and ionized. Whether this phase
transformation was fully driven and completed by photoionization by young hot
stars is a question of topical interest in cosmology. AIMS. We propose here
that besides the ultraviolet radiation from massive stars, feedback from
accreting black holes in high-mass X-ray binaries (BH-HMXBs) was an additional,
important source of heating and reionization of the IGM in regions of low gas
density at large distances from star-forming galaxies. METHODS. We use current
theoretical models on the formation and evolution of primitive massive stars of
low metallicity, and the observations of compact stellar remnants in the near
and distant universe, to infer that a significant fraction of the first
generations of massive stars end up as BH-HMXBs. The total number of energetic
ionizing photons from an accreting stellar black hole in an HMXB is comparable
to the total number of ionizing photons of its progenitor star. However, the
X-ray photons emitted by the accreting black hole are capable of producing
several secondary ionizations and the ionizing power of the resulting black
hole could be greater than that of its progenitor. Feedback by the large
populations of BH-HMXBs heats the IGM to temperatures of ~10^4 K and maintains
it ionized on large distance scales. BH-HMXBs determine the early thermal
history of the universe and mantain it as ionized over large volumes of space
in regions of low density. This has a direct impact on the properties of the
faintest galaxies at high redshifts, the smallest dwarf galaxies in the local
universe, and on the existing and future surveys at radio wavelengths of atomic
hydrogen in the early universe.
