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 greater than or similar to 10(40) erg s(-1). Such rare objects probably also dominated the production of X-rays in the early Universe. We demonstrate that a ULX with L-X similar to 10(40)-10(41) erg s(-1) (isotropic-equivalent luminosity in the 0.1-10 keV energy band) shining for similar to 10(5) yr (the expected duration of a supercritically accreting phase in HMXBs) can significantly ionize the ISM in its host dwarf galaxy of total mass M similar to 10(7)-10(8) M-circle dot 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 similar to 20-50 per cent to similar to 30-80 per cent over its lifetime. This implies that HMXBs can induce a stronger heating of the IGM at z greater than or similar to 10 compared to estimates neglecting the ULX feedback on the ISM. However, larger galaxies with M greater than or similar to 3 x 10(8) M-circle dot could not be significantly ionized even by the brightest ULXs in the early Universe. Since such galaxies probably started to dominate the global star formation rate at z less than or similar to 10, the overall escape fraction of soft X-rays from the HMXB population probably remained low, less than or similar to 30 per cent, at these epochs.