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 ] \leq - 2 stars varies only very weakly within the central 50 kpc , while the relative contribution of [ Fe/H ] \leq - 2 stars strongly increases with r , varying from 16 \% within 7 kpc < r < 20 kpc up to \geq 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 \sim 60 kpc the density of [ Fe/H ] \leq - 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 .