What is the Milky Way outer halo made of? High resolution spectroscopy
  of distant red giants
  In a framework where galaxies form hierarchically, extended stellar haloes
are predicted to be an ubiquitous feature around Milky Way-like galaxies and to
consist mainly of the shredded stellar component of smaller galactic systems.
The type of accreted stellar systems are expected to vary according to the
specific accretion and merging history of a given galaxy, and so is the
fraction of stars formed in-situ versus accreted. Analysis of the chemical
properties of Milky Way halo stars out to large Galactocentric radii can
provide important insights into the properties of the environment in which the
stars that contributed to the build-up of different regions of the Milky Way
stellar halo formed. In this work we focus on the outer regions of the Milky
Way stellar halo, by determining chemical abundances of halo stars with large
present-day Galactocentric distances, $>$15 kpc. The data-set we acquired
consists of high resolution HET/HRS, Magellan/MIKE and VLT/UVES spectra for 28
red giant branch stars covering a wide metallicity range, $-3.1
\lesssim$[Fe/H]$\lesssim -0.6$. We show that the ratio of $\alpha$-elements
over Fe as a function of [Fe/H] for our sample of outer halo stars is not
dissimilar from the pattern shown by MW halo stars from solar neighborhood
samples. On the other hand, significant differences appear at [Fe/H]$\gtrsim
-1.5$ when considering chemical abundance ratios such as [Ba/Fe], [Na/Fe],
[Ni/Fe], [Eu/Fe], [Ba/Y]. Qualitatively, this type of chemical abundance trends
are observed in massive dwarf galaxies, such as Sagittarius and the Large
Magellanic Cloud. This appears to suggest a larger contribution in the outer
halo of stars formed in an environment with high initial star formation rate
and already polluted by asymptotic giant branch stars with respect to inner
halo samples.
