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

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.

5 pages, 5 figures, accepted by ApJ Letter

References in corpus (11)

• The Structure of the Milky Way's Bar Outside the Bulge
• Made-to-Measure models of the Galactic Box/Peanut bulge: stellar and total mass in the bulge region
• Modelling the Galactic bar using OGLE-II Red Clump Giant Stars
• The Long Bar in the Milky Way. Corroboration of an old hypothesis
• Milky Way mass galaxies with X-shaped bulges are not rare in the local Universe
• Peanuts, brezels and bananas: food for thought on the orbital structure of the Galactic bulge
• The phase-space of boxy-peanut and X-shaped bulges in galaxies I. Properties of non-periodic orbits
• The X-Shaped Milky Way Bulge in OGLE-III Photometry and in N-Body Models
• A Kiloparsec-Scale Nuclear Stellar Disk in the Milky Way as a Possible Explanation of the High Velocity Peaks in the Galactic Bulge
• Resonant Orbits and the High Velocity Peaks Towards the Bulge
• Kinematics of the X-shaped Milky Way Bulge: Expectations from a Self-consistent N-body Model

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• What determines the flatness of X-shaped structures in edge-on galaxies
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• Bar Classification based on the Potential Map
• Galactic Orbits of Globular Clusters in the Region of the Galactic Bulge
• Distribution of red clump stars does not support the X-shaped Galactic bulge
• Testing for relics of past strong buckling events in edge-on galaxies: Simulation predictions and data from S${}^4$G
• Metallicity Properties of the Galactic Bulge Stars Near and Far: Expectations from the Auriga Simulation
• Early Growing Supermassive Black Holes Strengthen Bars and Boxy/Peanut Bulges
• Influence of Barlens on the Bulge Parameters in the 2D Image Decomposition
• Blanco DECam Bulge Survey (BDBS) IV: Metallicity Distributions and Bulge Structure from 2.6 Million Red Clump Stars
• Age distribution of stars in boxy/peanut/X-shaped bulges formed without bar buckling