The boxy appearance of the Galactic bulge is typical of galactic bars seen edge-on. These bar/bulges are very common: about 2/3 of spiral galaxies show some kind of central bar structure in the infra-red. Where do these bar/bulges come from ?
Bars can arise naturally from the instabilities of the disk. A rotating disk is often unstable to forming a flat bar structure at its center. This flat bar in turn is often unstable to vertical buckling which generates the boxy appearance. This kind of bar/bulge is not generated by mergers but follows simply from the dynamics of a flat rotating disk of stars. The maximum vertical extent of boxy or peanut-shaped bulges occurs near the radius of the vertical and horizontal Lindblad resonances, i.e. where
Here Ω is the circular angular velocity, Ωb is the pattern speed of the bar, κ is the epicyclic frequency and νz is the vertical frequency of oscillation. We note that the frequencies κ and particularly νz depend on the amplitude of the oscillation. Stars in this zone oscillate on 3D orbits which support the peanut shape.
We can test whether the Galactic bulge formed through this kind of bar-buckling instability of the inner disk, by comparing the structure and kinematics of the bulge with those of N-body simulations that generate a boxy/bar bulge (e.g. Athanassoula 2005). The simulations show an exponential structure and near-cylindrical rotation: do these simulations match the properties of the Galactic bar/bulge?
The stars of the Galactic bulge appear to be old and enhanced in α-elements. This implies a rapid history of star formation. If the bar formed from the inner disk, then it would be interesting to know whether the bulge stars and the stars of the adjacent disk have similar chemical properties. This is not yet clear. There do appear to be similarities in the α-element properties between the bulge and the thick disk in the solar neighborhood (e.g. Meléndez et al. 2008).
The bar-forming and bar-buckling process takes 2-3 Gyr to act after the disk settles. In the bar-buckling instability scenario, the bulge structure is probably younger than the bulge stars, which were originally part of the inner disk. The alpha-enrichment of the bulge and thick disk comes from the rapid chemical evolution which took place in the inner disk before the instability acted. In this scenario, the stars of the bulge and adjacent disk should have similar ages: accurate asteroseismology ages for giants of the bulge and inner disk would be a very useful test of the scenario.
We are doing a survey of about 28,000 clump giants in the Galactic bulge and the adjacent disk, to measure the chemical properties (Fe, Mg, Ca, Ti, Al, O) of stars in the bulge and adjacent disk: are they similar, as we would expect if the bar/bulge grew out of the disk? We use the AAOmega fiber spectrometer on the AAT, to acquire medium-resolution spectra of about 350 stars at a time, at a resolution R ∼ 12,000.
The central regions of our Galaxy are not only the location of the bulge and inner disk, but also include the central regions of the Galactic stellar halo. Recent simulations (e.g. Diemand et al. 2005, Moore et al. 2006, Brook et al. 2007) indicate that the metal-free (population III) stars formed until redshift z ∼ 4, in chemically isolated subsystems far away from the largest progenitor. If its stars survive, they are spread throughout the Galactic halo. If they are not found, then it would be likely that their lifetimes are less than a Hubble time which in turn implies a truncated IMF. On the other hand, the oldest stars form in the early rare high density peaks that lie near the highest density peak of the final system. They are not necessarily the most metal-poor stars in the Galaxy. Now, these oldest stars are predicted to lie in the central bulge region of the Galaxy. Accurate asteroseismology ages for metal-poor stars in the inner Galaxy would provide a great way to tell if they are the oldest stars or just stars of the inner Galactic halo. This test would require a ∼ 10% precision in age.
Our data so far indicate that the rotation of the Galactic bulge is close to cylindrical (see also Howard et al. 2009). Detailed analysis will be needed to see if there is any evidence for a small classical merger generated bulge component, in addition to the boxy/peanut bar/bulge which probably formed from the disk. We also see a more slowly rotating metal-poor component in the bulge region. The problem now is to identify the first stars from among the expected metal-poor stars of the inner halo.