X-ray Twinkles and Pop III Stars

TL;DR

This paper estimates the optimal X-ray emission level in the early universe that maximizes Pop III star formation, highlighting the role of supernova remnants over X-ray binaries in driving early star formation.

Contribution

It identifies the critical X-ray to UV energy ratio that maximizes Pop III star formation, emphasizing supernova remnants as the primary X-ray sources influencing early star formation.

Findings

Optimal X-ray emission boosts Pop III star numbers to ~40 per Mpc^3 by redshift 15.

Critical X-ray/UV ratio aligns with hypernovae from 20-40 M_sun Pop III stars.

Supernova remnants are more effective than X-ray binaries in promoting Pop III star formation.

Abstract

Pop III stars are typically massive stars of primordial composition forming at the centers of the first collapsed dark matter structures. Here we estimate the optimal X-ray emission in the early universe for promoting the formation of Pop 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 Pop III stars because of the increased Jeans mass of the intergalactic medium (IGM), while a lower emission suppresses the formation rate of H2 preventing or delaying star formation in dark matter minihalos 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 Pop III stars is sufficient to boost their number to few tens per comoving Mpc^3 by redshift z~15. We find that there is a critical X-ray to UV energy ratio emitted per source that produces a universe where the number of Pop III stars is largest: 400 per comoving- Mpc^3. This critical ratio is very close to the one provided by 20-40 M_sun Pop III stars exploding as hypernovae. High mass X-ray binaries in dwarf galaxies are far less effective at increasing the number of Pop III stars than normal supernova remnants, we thus conclude that supernovae drove the formation of Pop III stars.