Mapping the Three-Dimensional "X-Shaped Structure" in Models of the Galactic Bulge

TL;DR

This study uses numerical simulations to analyze the three-dimensional structure of the Milky Way's bulge, revealing that the X-shape is more accurately described as a peanut-shaped structure influenced by buckling instabilities.

Contribution

The paper provides a detailed analysis of the 3D morphology of the Galactic bulge, challenging the traditional X-shape interpretation and highlighting the prominence of peanut-shaped structures.

Findings

The X-shaped structure is more peanut-like than a true X-shape.

The central boxy core extends about 500 pc above and below the Galactic plane.

Lines of sight through the core do not show bimodal distributions, consistent with observations.

Abstract

Numerical simulations have shown that the X-shaped structure in the Milky Way bulge can naturally arise from the bar instability and buckling instability. To understand the influence of the buckling amplitude on the morphology of the X-shape, we analyze three self-consistent numerical simulations of barred galaxies with different buckling amplitudes (strong, intermediate and weak). We derive the three-dimensional density with an adaptive kernel smoothing technique. The face-on iso-density surfaces are all elliptical, while in the edge-on view, the morphology of buckled bars transitions with increasing radius, from a central boxy core to a peanut bulge and then to an extended thin bar. Based on these iso-density surfaces at different density levels, we find no clear evidence for a well-defined structure shaped like a letter X. The X-shaped structure is more peanut-like, whose visual perception is probably enhanced by the pinched inner concave iso-density contours. The peanut bulge can reproduce qualitatively the observed bimodal distributions which were used as evidence for the discovery of the X-shape. The central boxy core is shaped like an oblong tablet, extending to $\sim$ 500 pc above and below the Galactic plane ($|b| \sim 4^\circ$). From the solar perspective, lines of sight passing through the central boxy core do not show bimodal distributions. This generally agrees with the observations that the double peaks merge at $|b| \sim 4^\circ - 5^\circ$ from the Galactic plane, indicating the presence of a possibly similar structure in the Galactic bulge.