Galactic Archaeology with RAVE: Clues to the Formation of the Thick Disk
  I present an analysis of ~13000 stars from RAVE Data Release 4 (DR4) to
better understand the formation of the thick disk. The stars I consider are
mostly within 1 kpc of the Sun. Based on a Monte-Carlo analysis of the data and
comparison with a toy model, I suggest the thick disk formed fairly rapidly
when [Mg/Fe] ${\approx}$0.2-0.3. We consider an origin via direct accretion
unlikely as the thick disk is fast-rotating and fairly metal-rich for its age.
As internal disk heating can not easily reach the high observed velocity
dispersions and would likely be more gradual, it also appears implausible. A
very early formation of the thick disk (e.g. from star formation in the
collapsing primordial gas cloud of the Milky Way) appears unlikely given it
would require [Mg/Fe] ${\geq}$ 0.4 at the time the thick disk formed. An origin
via tidal interaction with another galaxy is one of the few remaining
possibilities. My proposed explanation for the nature of the event which formed
the thick disk is a close flyby of Andromeda, whose orbital dynamics in MOND
are consistent with my estimate that the event occurred 7-11 Gyr ago. This
leads to reasonable thick disk velocity dispersions. It will be important to
determine whether the event heated the outer regions of the thin disk uniformly
or if substantial portions of it were left largely unaffected. This may
distinguish between models (like mine) involving a single close encounter and
models with multiple encounters (as common in mergers). The main limiting
factor at present seems to be the accuracy of elemental abundances and lack of
direct observations at different galactocentric radii to the Sun.
