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. We predict that the MDF of [Fe/H]≤ −2 stars varies only very weakly within the central 50 kpc, where the stellar distribution of −2 <[Fe/H]≤ −1 stars follows a power-law in radius, r with γ = 2.2, while that of [Fe/H]≤ −2 stars is well fitted by a β-profile [1 + (r/rc) ] , with β = 1 and rc = 35 kpc. As a consequence, 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 makes the outer halo, 20 kpc < r < 40 kpc, 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 starforming haloes, and (ii) the metal enrichment history of the Milky Way environment.