Stars made in outflows may populate the stellar halo of the Milky Way
  We study stellar-halo formation using six Milky Way-mass galaxies in FIRE-2
cosmological zoom simulations. We find that $5-40\%$ of the outer ($50-300$
kpc) stellar halo in each system consists of $\textit{in-situ}$ stars that were
born in outflows from the main galaxy. Outflow stars originate from gas
accelerated by super-bubble winds, which can be compressed, cool, and form
co-moving stars. The majority of these stars remain bound to the halo and fall
back with orbital properties similar to the rest of the stellar halo at
$z=0$.In the outer halo, outflow stars are more spatially homogeneous, metal
rich, and alpha-element-enhanced than the accreted stellar halo. At the solar
location, up to $\sim 10 \%$ of our kinematically-identified halo stars were
born in outflows; the fraction rises to as high as $\sim 40\%$ for the most
metal-rich local halo stars ([Fe/H] $> -0.5$). We conclude that the Milky Way
stellar halo could contain local counterparts to stars that are observed to
form in molecular outflows in distant galaxies. Searches for such a population
may provide a new, near-field approach to constraining feedback and outflow
physics. A stellar halo contribution from outflows is a phase-reversal of the
classic halo formation scenario of Eggen, Lynden-Bell $\&$ Sandange, who
suggested that halo stars formed in rapidly $\textit{infalling}$ gas clouds.
Stellar outflows may be observable in direct imaging of external galaxies and
could provide a source for metal-rich, extreme velocity stars in the Milky Way.
