Exploring the origin of low-metallicity stars in Milky Way-like galaxies
  with the NIHAO-UHD simulations
  The kinematics of the most metal-poor stars provide a window into the early
formation and accretion history of the Milky Way. Here, we use
5~high-resolution cosmological zoom-in simulations ($\sim~5\times10^6$ star
particles) of Milky Way-like galaxies taken from the NIHAO-UHD project, to
investigate the origin of low-metallicity stars ([Fe/H]$\leq-2.5$). The
simulations show a prominent population of low-metallicity stars confined to
the disk plane, as recently discovered in the Milky Way. The ubiquity of this
finding suggests that the Milky Way is not unique in this respect.
Independently of the accretion history, we find that $\gtrsim~90$ per cent of
the retrograde stars in this population are brought in during the initial
build-up of the galaxies during the first few Gyrs after the Big Bang. Our
results therefore highlight the great potential of the retrograde population as
a tracer of the early build-up of the Milky Way. The prograde planar
population, on the other hand, is accreted during the later assembly phase and
samples the full galactic accretion history. In case of a quiet accretion
history, this prograde population is mainly brought in during the first half of
cosmic evolution ($t\lesssim7$~Gyr), while, in the case of an on-going active
accretion history, later mergers on prograde orbits are also able to contribute
to this population. Finally, we note that the Milky Way shows a rather large
population of eccentric, very metal-poor planar stars. This is a feature not
seen in most of our simulations, with the exception of one simulation with an
exceptionally active early building phase.
