6. THE MILKY WAY BULGE IN A NUTSHELL
The properties of the Galactic bulge described above can be summarised
as follows:
- The Bulge stellar population properties show
independent evidence for a multiple component scenario, with different
morphological and dynamical characteristics. The metallicity
distribution can be separated in, at least, a main metal-poor and a
metal-rich components. Their different contributions across the Bulge
produces a vertical metallicity gradient of ∼ 0.6 dex/kpc which
seems to flatten in the inner 700 pc. From a morphological/structural
viewpoint, there is strong evidence of a box/peanut, while more recent
work points out the existence of a component with different geometrical
properties, as seen in the spatial distribution of RR Lyrae stars, which
trace the oldest Galactic population
(Fig. 1). The next two bullet
points connect these structural components to differences observed also
in chemical content, age and dynamics.
- There is a metal-rich (mean [Fe/H] ∼ 0.3),
alpha-poor population of stars in the bulge, composed mostly by old
stars but with a fraction of young stars. This population of stars
rotates cylindrically and shows a large vertex deviation consistent with
the bar structure traced with a position angle of ∼ 27 deg. At
latitudes |b|>5 the inner parts of the bar have grown out of
the disc plane, originating the box/peanut. These metal-rich stars
further show the split red-clump in the luminosity function tracing the
X-shape of the Bulge.
- The metal-poor bulge population (mean [Fe/H]
∼ -0.3) is composed predominantly by old stars and it shows an
alpha-enhancement similar to that of the local thick disc. The
kinematics of these stars follow a more spheroidally distributed
population than the one traced by the metal-rich stars, consistent with
the structure traced by the bulge RR Lyrae stars. Furthermore, these
stars do not trace the X-shape morphology of the Bulge.