Population III stars: hidden or disappeared?

A Population III/Population II transition from massive to normal stars is predicted to occur when the metallicity of the star-forming gas crosses the critical range Zcr= 10−5±1 Z⊙. To investigate the cosmic implications of such a process, we use numerical simulations which follow the evolution, metal enrichment and energy deposition of both Population II and Population III 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, Population III star formation continues down to z= 2.5, but at a low peak rate of 10−5 M⊙ yr−1 Mpc−3 occurring at z≈ 6 (about 10−4 of the Population II one); and (ii) Population III star formation proceeds in an ‘inside–out’ mode in which formation sites are progressively confined to the periphery of collapsed structures, where the low gas density and correspondingly long free-fall time-scales 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.