Energy Feedback from X-ray Binaries in the Early Universe. (arXiv:1306.1405v1 [astro-ph.CO])

Energy Feedback from X-ray Binaries in the Early Universe. (arXiv:1306.1405v1 [astro-ph.CO]):
X-ray photons, because of their long mean-free paths, can easily escape the
galactic environments where they are produced, and interact at long distances
with the inter-galactic medium, potentially having a significant contribution
to the heating and reionization of the early Universe. The two most important
sources of X-ray photons in the Universe are active galactic nuclei (AGN) and
X-ray binaries (XRBs). In this Letter we use results from detailed, large scale
population synthesis simulations to study the energy feedback of XRBs, from the
first galaxies (z~20) until today. We estimate that X-ray emission from XRBs
dominates over AGN at z>6-8. The shape of the spectral energy distribution of
the emission from XRBs shows no changes with redshift, in contrast to its
normalization which evolves by ~4 orders of magnitude, primarily due to the
evolution of the cosmic star-formation rate. However, the metallicity and the
mean stellar age of a given XRB population affect significantly its X-ray
output. Specifically, the X-ray luminosity from high-mass XRBs per unit of
star-formation rate varies by more than an order of magnitude going from solar
metallicity to less than 10% solar, and the X-ray luminosity from low-mass XRBs
per unit of stellar mass peaks at an age of ~300 Myr and then decreases
gradually at later times, showing little variation for mean stellar ages >3
Gyr. Finally, we provide analytical and tabulated prescriptions for the energy
output of XRBs, that can be directly incorporated in cosmological simulations.