We examine the spatial distribution of the oldest and most metal poor stellar populations of Milky Way-sized galaxies using the APOSTLE cosmological hydrodynamical simulations of the Local Group .
In agreement with earlier work , we find strong radial gradients in the fraction of the oldest ( t _ { form } < 0.8 Gyr ) and most metal poor ( [ Fe/H ] < -2.5 ) stars , both of which increase outwards .
The most metal poor stars form over an extended period of time ; half of them form after z = 5.3 , and the last 10 % after z = 2.8 .
The age of the metal poor stellar population also shows significant variation with environment ; a high fraction of them are old in the galaxy ’ s central regions and an even higher fraction in some individual dwarf galaxies , with substantial scatter from dwarf to dwarf .
Overall , over half of the stars that belong to both the oldest and most metal poor population are found outside the solar circle .
Somewhat counter-intuitively , we find that dwarf galaxies with a large fraction of metal poor stars that are very old are systems where metal-poor stars are relatively rare , but where a substantial old population is present .
Our results provide guidance for interpreting the results of surveys designed to hunt for the earliest and most pristine stellar component of our Milky Way .