The oldest and most metal poor stars in the APOSTLE Local Group
  simulations
  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 (tform < 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.
