Cosmic stellar relics in the Galactic halo

Abstract

We study the stellar population history and chemical evolution of the Milky Way (MW) in a hierarchical LCDM model for structure formation. Using a Monte Carlo method based on the semi-analytical EPS formalism, we reconstruct the merger tree of our Galaxy and follow the evolution of gas and stars along the hierarchy. Our approach allows us to compare the observational properties of the MW with model results, exploring different properties of primordial stars, such as their IMF and the critical metallicity for low-mass star formation, Zcr. By matching our predictions to the Metallicity Distribution Function (MDF) of metal-poor stars in the Galactic halo we find that: (i) supernova feedback is required to reproduce the observed properties of the MW; (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^-4Zsun,m_PopIII=200Msun) provides an overall good fit to the MDF but cannot account for the two stars with [Fe/H]<-5; the latter can be accommodated if Zcr<10^-6Zsun but such model overpopulates 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 m_PopIII>0.9Msun; (v) the statistical impact of 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 GM are ejected through winds by halos with masses M<6x10^9 Msun, showing that low-mass halos are the dominant population contributing to cosmic metal enrichment.