Impact of ultraluminous X-ray sources on photoabsorption in the first galaxies

In the local Universe, integrated X-ray emission from high-mass X-ray binaries (HMXBs) is dominated by the brightest ultraluminous X-ray sources (ULXs) with luminosity >10^40 erg/s. Such rare objects probably also dominated the production of X-rays in the early Universe. We demonstrate that a ULX with Lx~10^40-10^41 erg/s (isotropic-equivalent luminosity in the 0.1-10 keV energy band) shining for ~10^5 years (the expected duration of a supercritically accreting phase in HMXBs) can significantly ionise the ISM in its host dwarf galaxy of total mass M~10^7-10^8 Msun and thereby reduce its opacity to soft X-rays. As a result, the fraction of the soft X-ray (below 1 keV) radiation from the ULX escaping into the intergalactic medium (IGM) can increase from ~20-50% to ~30-80% over its lifetime. This implies that HMXBs can induce a stronger heating of the IGM at z>10 compared to estimates neglecting the ULX feedback on the ISM. However, larger galaxies with M>3 x 10^8 Msun could not be significantly ionised even by the brightest ULXs in the early Universe. Since such galaxies probably started to dominate the global star-formation rate at z<10, the overall escape fraction of soft X-rays from the HMXB population probably remained low, <30%, at these epochs.