The Metallicity Gradient and Complex Formation History of the Outermost
  Halo of the Milky Way
  We present an examination of the metallicity distribution function of the
outermost stellar halo of the Galaxy based on an analysis of both local (within
4 kpc of the Sun, ~16,500 stars) and non-local (~21,700 stars) samples. These
samples were compiled using spectroscopic metallicities from the Sloan Digital
Sky Survey and photometric metallicities from the SkyMapper Southern Survey. We
detect a negative metallicity gradient in the outermost halo (r > 35 kpc from
the Galactic center), and find that the frequency of very metal-poor ([Fe/H] <
-2.0) stars in the outer-halo region reaches up to ~60% in our most distant
sample, commensurate with previous theoretical predictions. This result
provides clear evidence that the outer-halo formed hierarchically. The
retrograde stars in the outermost halo exhibit a roughly constant metallicity,
which may be linked to the accretion of the Sequoia progenitor. In contrast,
prograde stars in the outermost halo exhibit a strong metallicity-distance
dependence, indicating that they likely originated from the accretion of
galaxies less massive than the Sequoia progenitor galaxy.
