The origin of accreted stellar halo populations in the Milky Way using
  APOGEE, $\textit{Gaia}$, and the EAGLE simulations
  Recent work indicates that the nearby Galactic halo is dominated by the
debris from a major accretion event. We confirm that result from an analysis of
APOGEE-DR14 element abundances and $\textit{Gaia}$-DR2 kinematics of halo
stars. We show that $\sim$2/3 of nearby halo stars have high orbital
eccentricities ($e \gtrsim 0.8$), and abundance patterns typical of massive
Milky Way dwarf galaxy satellites today, characterised by relatively low
[Fe/H], [Mg/Fe], [Al/Fe], and [Ni/Fe]. The trend followed by high $e$ stars in
the [Mg/Fe]-[Fe/H] plane shows a change of slope at [Fe/H]$\sim-1.3$, which is
also typical of stellar populations from relatively massive dwarf galaxies. Low
$e$ stars exhibit no such change of slope within the observed [Fe/H] range and
show slightly higher abundances of Mg, Al and Ni. Unlike their low $e$
counterparts, high $e$ stars show slightly retrograde motion, make higher
vertical excursions and reach larger apocentre radii. By comparing the position
in [Mg/Fe]-[Fe/H] space of high $e$ stars with those of accreted galaxies from
the EAGLE suite of cosmological simulations we constrain the mass of the
accreted satellite to be in the range $10^{8.5}\lesssim M_*\lesssim
10^{9}\mathrm{M_\odot}$. We show that the median orbital eccentricities of
debris are largely unchanged since merger time, implying that this accretion
event likely happened at $z\lesssim1.5$. The exact nature of the low $e$
population is unclear, but we hypothesise that it is a combination of
$\textit{in situ}$ star formation, high $|z|$ disc stars, lower mass accretion
events, and contamination by the low $e$ tail of the high $e$ population.
Finally, our results imply that the accretion history of the Milky Way was
quite unusual.
