Stars behind bars II: A cosmological formation scenario for the Milky
  Way's central stellar structure
  The stellar populations in the inner kiloparsecs of the Milky Way (MW) show
complex kinematical and chemical structures. The origin and evolution of these
structures is still under debate. Here we study the central region of a fully
cosmological hydrodynamical simulation of a disk galaxy that reproduces key
properties of the inner kiloparsecs of the MW: it has a boxy morphology and
shows an overall rotation and dispersion profile in agreement with
observations. We use a clustering algorithm on stellar kinematics to identify a
number of discrete kinematic components: a high- and low-spin disk, a stellar
halo and two bulge components; one fast rotating and one slow-rotating. We
focus on the two bulge components and show that the slow rotating one is
spherically symmetric while the fast rotating component shows a boxy/peanut
morphology. Although the two bulge components are kinematically discrete
populations at present-day, they are both mostly formed over similar time
scales, from disk material. We find that stellar particles with lower initial
birth angular momentum (most likely thick disc stars) end up in the
slow-rotating low-spin bulge, while stars with higher birth angular momentum
(most likely thin disc stars) are found in the high-spin bulge. This has the
important consequence that a bulge population with a spheroidal morphology does
not necessarily indicate a merger origin. In fact, we do find that only
$\sim2.3$\% of the stars in the bulge components are ex-situ stars brought in
by accreted dwarf galaxies early on. We identify these ex-situ stars as the
oldest and most metal-poor stars on highly radial orbits with large vertical
excursions from the disk.
