Observational Evidence from SDSS for a Merger Origin of the Milky Way's
  Thick Disk
  We test competing models that aim at explaining the nature of stars in the
Milky Way that are well away (|z|$\gtrsim$ 1kpc) from the midplane, the
so-called thick disk: the stars may have gotten there through orbital
migration, through satellite mergers and accretion, or through heating of
pre-existing thin disk stars. Sales et al. (2009) proposed the eccentricity
distribution of thick disk stars as a diagnostic to differentiate between these
mechanisms. Drawing on SDSS DR7, we have assembled a sample of 34,223 G-dwarfs
with 6-D phase-space information and metallicities, and have derived orbital
eccentricities for them. Comparing the resulting eccentricity distributions,
p(e|z), with the models, we find that: a) the observed p(e|z) is inconsistent
with that predicted by orbital migration only, as there are more observed stars
of high and of very low eccentricity; b) scenarios where the thick disk is made
predominantly through abrupt heating of a pre-existing thin disk are also
inconsistent, as they predict more high-eccentricity stars than observed; c)
the observed p(e|z) fits well with a "gas-rich merger" scenario, where most
thick disk stars were born from unsettled gas in situ.
