Peanuts, brezels and bananas: food for thought on the orbital structure of the Galactic bulge

Matthieu Portail, Christopher Wegg, Ortwin Gerhard, 2015, MNRASL, 450, L66 (to the paper)

Recent observations have discovered the presence of a box/peanut or X-shape structure in the Galactic bulge. Such box/peanut structures are common in external disc galaxies, and are well known in N-body simulations where they form following the buckling instability of a bar. From studies of analytical potentials and N-body models, it has been claimed in the past that box/peanut bulges are supported by ‘bananas’, or x1v1 orbits. We present here a set of N-body models where instead the peanut bulge is mainly supported by brezel-like orbits, allowing strong peanuts to form with short extent relative to the bar length. This shows that stars in the X-shape do not necessarily stream along banana orbits which follow the arms of the X-shape. The brezel orbits are also found to be the main orbital component supporting the peanut shape in our recent made-to-measure dynamical models of the Galactic bulge. We also show that in these models the fraction of stellar orbits that contribute to the X-structure account for 40–45 per cent of the stellar mass.

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