The origin of metal-poor stars on prograde disk orbits in FIRE
  simulations of Milky Way-mass galaxies
  In hierarchical structure formation, metal-poor stars in and around the Milky
Way (MW) originate primarily from mergers of lower-mass galaxies. A common
expectation is therefore that metal-poor stars should have isotropic,
dispersion-dominated orbits that do not correlate strongly with the MW disk.
However, recent observations of stars in the MW show that metal-poor ([Fe/H] <
-2) stars are preferentially on prograde orbits with respect to the disk. Using
the FIRE-2 suite of cosmological zoom-in simulations of MW/M31-mass galaxies,
we investigate the prevalence and origin of prograde metal-poor stars. Almost
all (11 of 12) of our simulations have metal-poor stars on preferentially
prograde orbits today and throughout most of their history: we thus predict
that this is a generic feature of MW/M31-mass galaxies. The typical
prograde-to-retrograde ratio is ~2:1, which depends weakly on stellar
metallicity at Fe/H] < -1. These trends predicted by our simulations agree well
with MW observations. Prograde metal-poor stars originate largely from a single
LMC/SMC-mass gas-rich merger 7 - 12.5 Gyr ago, which deposited existing
metal-poor stars and significant gas on an orbital vector that sparked the
formation of and/or shaped the orientation of a long-lived stellar disk, giving
rise to a prograde bias for all low-metallicity stars. We find sub-dominant
contributions from in-situ stars formed in the host galaxy before this merger,
and in some cases, additional massive mergers. We find few clear correlations
between any properties of our MW/M31-mass galaxies at z = 0 and the degree of
this prograde bias as a result of diverse merger scenarios.
