The spatial distribution of the Galactic first stars. I. High-resolution N-body approach

We study the spatial distribution of Galactic metal-free stars by combining an extremely high-resolution (7.8 × 10⁵ M_⊙ per particle) cold dark matter N-body simulation of the Milky Way with a semianalytic model of metal enrichment. This approach allows us to resolve halos with virial temperatures down to the 10⁴K atomic cooling limit, and it is sufficiently flexible to make a number of robust conclusions, despite the extremely uncertain properties of the first stars. Galactic metal-free stars are formed over a large redshift range, which peaks at z ≈ 10, but continues down to z ≈ 5, contributing stars at a wide range of Galactocentric radii. Stars containing only metals from primordial stars are similarly widespread. Neither changing the efficiency of metal dispersal by 2 orders of magnitude, nor drastically changing the approximations in our semianalytical model can affect these result. Thus, if they have sufficiently long lifetimes, a significant number of stars formed in initially primordial star clusters should be found in the nearby Galactic halo. Observations of metal abundances in Galactic halo stars should be taken as directly constraining the properties of primordial stars, and the lack of metal-free halo stars today should be taken as strongly suggesting a 0.8 M_⊙ lower limit on the primordial initial mass function.