THE GENESIS OF THE MILKY WAY'S THICK DISK VIA STELLAR MIGRATION
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abstract
The separation of the Milky Way disk into a thin and thick component is
supported by differences in the spatial, kinematic and metallicity
distributions of their stars. These differences have led to the view that the
thick disk formed early via a cataclysmic event and constitutes fossil evidence
of the hierarchical growth of the Milky Way. We show here, using N-body
simulations, how a double-exponential vertical structure, with stellar
populations displaying similar dichotomies can arise purely through internal
evolution. In this picture, stars migrate radially, while retaining nearly
circular orbits, as described by Sellwood & Binney (2002). As stars move
outwards they populate a thickened component. Such stars found at the present
time in the solar neighborhood formed early in the disk's history at smaller
radii where stars are more metal-poor and alpha-enhanced, leading to the
properties observed for thick disk stars. Classifying stars as members of the
thin or thick disk by either velocity or metallicity leads to an apparent
separation in the other property as observed. This scenario is supported by the
SDSS observation that stars in the transition region do not show any
correlation between rotational velocity and metallicity. The good qualitative
agreement between our simulation and observations in the Milky Way hint that
the thick disk may be a ubiquitous galaxy feature generated by stellar
migration. Nonetheless, we cannot exclude that some fraction of the thick disk
is a fossil of a past more violent history, nor can this scenario explain thick
disks in all galaxies.
