Population III star foation in an X-ray background - I. Critical halo mass of foation and total mass in stars

Abstract

The first luminous objects foing in the universe produce radiation backgrounds in the far-ultraviolet and X-ray bands that affect the foation of Population III stars. Using a grid of cosmological hydrodynamics zoom-in silations, we explore the impact of the Lyman-Warner (LW) and X-ray radiation backgrounds on the critical dark matter (DM) halo mass for Population III star foation and the total mass in stars per halo. We find that the LW radiation background lowers the H2 fraction and delays the foation of the Population III stars. On the other hand, X-ray irradiation anticipates the redshift of collapse and reduces the critical halo mass, unless the X-ray background is too strong and gas heating shuts down gas collapse into the haloes and prevents star foation. Therefore, an X-ray background can increase the number of DM haloes foing Population III stars by about a factor of 10, but the total mass in stars foing in each halo is reduced. This is because X-ray radiation increases the molecular fraction and lowers the minim temperature of the collapsing gas (or equivalently the mass of the quasi-hydrostatic core) and therefore slows down the accretion of the gas on to the central protostar.