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

TL;DR

This paper presents N-body models of the Galactic bulge showing that brezel-like orbits, rather than traditional banana orbits, primarily support the X-shaped structure, challenging previous assumptions about bulge orbital dynamics.

Contribution

It introduces a new orbital perspective for the Galactic bulge, demonstrating brezel-like orbits as the main support for the X-shape, contrasting with prior banana orbit models.

Findings

Brezel-like orbits support strong peanut bulges with short extent.

Approximately 40-45% of stellar mass contributes to the X-structure.

Brezel orbits are dominant in recent dynamical models of the Galactic bulge.

Abstract

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% of the stellar mass.