X-ray twinkles and Population III stars

Population III stars are typically massive stars of primordial composition forming at the centres of the first collapsed dark matter structures. Here we estimate the optimal X-ray emission in the early universe for promoting the formation of Population III stars. This is important in determining the number of dwarf galaxies formed before reionization and their fossils in the local universe, as well as the number of intermediate-mass seed black holes. A mean X-ray emission per source above the optimal level reduces the number of Population III stars because of the increased Jeans mass of the intergalactic medium, while a lower emission suppresses the formation rate of H2 preventing or delaying star formation in dark matter minihaloes above the Jeans mass. The build-up of the H2 dissociating background is slower than the X-ray background due to the shielding effect of resonant hydrogen Lyman lines. Hence, the nearly unavoidable X-ray emission from supernova remnants of Population III stars is sufficient to boost their number to few tens per comoving Mpc3 by redshift z ∼ 15. We find that there is a critical X-ray to ultraviolet energy ratio emitted per source that produces a universe where the number of Population III stars is largest: 400 per comoving Mpc3. This critical ratio is very close to the one provided by 20–40 M Population III stars exploding as hypernovae. High-mass X-ray binaries in dwarf galaxies are far less effective at increasing the number of Population III stars than normal supernova remnants, we thus conclude that supernovae drove the formation of Population III stars.