The Spatial Distribution of the Galactic First Stars. I. High‐ResolutionN‐Body Approach

We study the spatial distribution of Galactic metal-free stars by combiningan extremely high-resolution (7.8 X 10^5 solar masses per particle) Cold DarkMatter N-body simulation of the Milky-Way with a semi-analytic model of metalenrichment. This approach allows us to resolve halos with virial temperaturesdown to the 10^4K atomic cooling limit, and it is sufficiently flexible to makea number of robust conclusions, despite the extremely uncertain properties ofthe first stars. Galactic metal-free stars are formed over a large redshiftrange, which peaks at z~10, but continues down to z~5, contributing stars atwide range of Galactocentric radii. Stars containing only metals generated byprimordial stars are similarly widespread. Neither changing the efficiency ofmetal dispersal by two orders of magnitude, nor drastically changing theapproximations in our semi-analytical model can affect these result. Thus, ifthey have sufficiently long lifetimes, a significant number of stars formed ininitially primordial star clusters should be found in the nearby Galactic haloregardless of the specifics of metal-free star formation. Observations of metalabundances in Galactic halo stars should be taken as directly constraining theproperties of primordial stars, and the lack of metal-free halo stars todayshould be taken as strongly suggesting a 0.8 solar mass lower limit on theprimordial initial mass function.