Cosmic stellar relics in the Galactic halo

We study the stellar population history and chemical evolution of the Milky Way in a hierarchical ΛCDM model for structure formation. Using a Monte Carlo method based on the semi-analytical extended Press & Schechter formalism, we develop a new code GAMETE (GAlaxy MErger Tree & Evolution) to reconstruct the merger tree of the Galaxy and follow the evolution of gas and stars along the hierarchical tree. Our approach allows us to compare the observational properties of the Milky Way with model results, exploring different properties of primordial stars, such as their Initial Mass Function (IMF) and the critical metallicity for low-mass star formation, Zcr. In particular, by matching our predictions to the metallicity distribution function of metal-poor stars in the Galactic halo we find that: (i) a strong supernova feedback is required to reproduce the observed properties of the Milky Way; (ii) stars with [Fe/H]< −2.5 form in halos accreting Galactic Medium (GM) enriched by earlier supernova explosions; (iii) the fiducial model (Zcr = 10 Z⊙, mPopIII = 200M⊙) provides an overall good fit to the MDF, but cannot account for the two HMP stars with [Fe/H]< −5; the latter can be accommodated if Zcr ≤ 10 Z⊙ but such model overpopulates the “metallicity desert”, i.e. the range −5.3 < [Fe/H] < −4 in which no stars have been detected; (iv) the current non-detection of metal-free stars robustly constrains either Zcr > 0 or the masses of the first stars mPopIII > 0.9M⊙; (v) the statistical impact of truly second generation stars, i.e. stars forming out of gas polluted only by metal-free stars, is negligible in current samples; (vi) independently of Zcr, 60% of metals in the Galactic Medium are ejected through winds by halos with masses M < 6 × 109M⊙, thus showing that low-mass halos are the dominant population contributing to cosmic metal enrichment. We discuss the limitations of our study and comparison with previous work.