Near-field cosmology and the first stars: Insights from chemical evolution

I present a new Galactic chemical evolution model motivated by and grounded in the hierarchical theory of galaxy formation. This model accurately reproduces the “metallicity distribution function” (MDF) for Population II stars residing today in the Galactic halo, and implies that the IMF of the first stars was peaked in the range of massive stars that experience core-collapse supernovae (mean mass 〈M〉 = 8-42 M⊙), similar to the masses predicted by models of primordial star formation that account for formation feedback. The observed MDF and the apparent absence of true Population III stars from the Galactic halo strongly imply that there is some critical metallicity, Zcrit ≲ 10−4 Z⊙, below which low-mass star formation was inhibited. Metal-poor halo stars below [Fe/H] ≲ −3 had only 1-10 metal-free stars as their supernova precursors, such that the relative abundances in these halo stars exhibit IMF-weighted averages over the intrinsic yields of the first supernovae. This model is the first component of a long-term program to relate near-field indicators of early star formation to the high-redshift universe, an approach termed “near-field cosmology”.