Population III stars : hidden or disappeared ?

A Pop III/Pop 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 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 10M⊙yr Mpc occurring at z ≈ 6 (about 10 of the Pop II 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.