Population III stars: hidden or disappeared ?

TL;DR

This study uses simulations to explore the persistence and characteristics of Population III star formation, revealing ongoing low-level formation at redshifts as low as 2.5 and highlighting the challenge of detecting these primordial stars.

Contribution

It provides new insights into the late-time formation of Population III stars and their spatial distribution, emphasizing the importance of searching at moderate redshifts.

Findings

Pop III star formation persists down to z=2.5.

Formation occurs mainly in the periphery of structures.

Low peak formation rate at z~6 is about 10^-5 M_sun yr^-1 Mpc^-3.

Abstract

A PopIII/Pop II transition from massive to normal stars is predicted to occur when the metallicity of the star forming gas crosses the critical range Z_cr = 10^(-5 +/- 1) Z_sun. To investigate the cosmic implications of such process we use numerical simulations which follow the evolution, metal enrichment and energy deposition of both Pop III and Pop II stars. We find that: (i) due to inefficient heavy element transport by outflows and slow "genetic" transmission during hierarchical growth, large fluctuations around the average metallicity arise; as a result Pop III star formation continues down to z=2.5, but at a low peak rate of 10^-5 M_sun yr^-1 Mpc^-3 occurring at z~6 (about 10^-4 of the PopII one); (ii) Pop III star formation proceeds in a "inside-out" mode in which formation sites are progressively confined at the periphery of collapsed structures, where the low gas density and correspondingly long free-fall timescales result in a very inefficient astration. These conclusions strongly encourage deep searches for pristine star formation sites at moderate (2<z<5) redshifts where metal free stars are likely to be hidden.