  I present a new Galactic chemical evolution model motivated by and grounded
in the hierarchical theory of galaxy formation, as expressed by a halo merger
history of the Galaxy. This model accurately reproduces the "metallicity
distribution function" (MDF) for Population II stars residing today in the
Galactic halo. The observed MDF and the apparent absence of true Population III
stars from the halo strongly imply that there is some critical metallicity,
Z_crit <~ 10^-4 Z_sun, below which low-mass star formation is inhibited, and
perhaps impossible. The observed constraints from the halo MDF, relative metal
abundances from Galactic halo stars, and the ionizing photon budget needed to
reionize the IGM together imply a stellar IMF that is peaked in the range of
massive stars that experience core-collapse supernovae, with mean mass <M> = 8
- 42 Msun. This mass range is similar to the masses predicted by models of
primordial star formation that account for formation feedback. The model also
implies that 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 paper is the first part of a long term
project to connect the high-redshift in situ indicators of early star formation
with the low-z, old remnants of the first stars.
