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 < 60kpc. The MDF of [Fe/H] <= -2 stars varies only very weakly within the central 50kpc, while the relative contribution of [Fe/H] <= -2 stars strongly increases with r, varying from 16 per cent within 7 < r < 20kpc up to >=40 per cent for r > 20kpc. 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 < 40kpc is the best region to search for very metal-poor stars. Beyond ~60kpc 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.