Mining the Galactic Halo for Very Metal-Poor Stars
  We study the age and metallicity distribution function (MDF) of metal-poor
stars in the Milky Way halo as a function of galactocentric radius by combining
N-body simulations and semi-analytical methods. We find that the oldest stars
populate the innermost region, while extremely metal-poor stars are more
concentrated within r < 60 kpc. The MDF of [Fe/H] < -2 stars varies only very
weakly within the central 50 kpc, while the relative contribution of [Fe/H] <
-2 stars strongly increases with r, varying from 16% within 7 kpc < r < 20 kpc
up to > 40% for r > 20 kpc. This is due to the faster descent of the spatial
distribution (as seen from Earth) of the more enriched population. This implies
that the outer halo < 40 kpc is the best region to search for very metal-poor
stars. Beyond ~ 60 kpc the density of [Fe/H] < -2 stars is maximum within dwarf
galaxies. All these features are imprinted by a combination of (i) the
virialization epoch of the star-forming haloes, and (ii) the metal enrichment
history of the Milky Way environment.
