X-ray Background at High Redshifts from Pop III Remnants: Results from Pop III star formation rates in the Renaissance Simulations

TL;DR

This study models the X-ray background from Pop III star remnants during reionization, showing it significantly impacts IGM heating and ionization, with implications for 21-cm signals and cosmic evolution.

Contribution

It provides the first detailed calculation of the high-redshift X-ray background from Pop III binaries using cosmological simulations, highlighting their role in early universe heating and ionization.

Findings

Pop III binaries produce ~6 eV per hydrogen atom in X-rays.

X-ray background extends up to ~500 eV, heating IGM to ~1000 K.

Pop III X-rays ionize a few percent of neutral hydrogen.

Abstract

Due to their long mean free paths, X-rays are expected to have many significant impacts globally on the properties of the intergalactic medium (IGM) by their heating and ionizing processes on large scales. At high redshifts, X-rays from Population (Pop) III binaries might have important effects on cosmic reionization and the Lyman alpha forest. As a continuation of our previous work on Pop III binary X-rays (Xu et al. 2014), we use the Pop III distribution and evolution from the Renaissance Simulations, a suite of self-consistent cosmological radiation hydrodynamics simulations of the formation of the first galaxies, to calculate the X-ray luminosity density and background over the redshift range 20 > z > 7.6. As we find that Pop III star formation continues at a low, nearly constant rate to the end of reionization, X-rays are being continuously produced at significant rates compared to other possible X-ray sources, such as AGNs and normal X-ray binaries during the same period of time. We estimate that Pop III binaries produce approximately 6 eV of energy in the X-rays per hydrogen atom. We calculate the X-ray background for different monochromatic photon energies. KeV X-rays redshift and accumulate to produce a strong X-ray background spectrum extending to roughly 500 eV. The X-ray background is strong enough to heat the IGM to ~ 1000 K and to ionize a few percent of the neutral hydrogen. These effects are important for an understanding of the neutral hydrogen hyperfine transition 21-cm line signatures, the Ly alpha forest, and optical depth of the CMB to Thomson scattering.