The Vertical X-shaped Structure in the Milky Way: Evidence from a Simple Boxy Bulge Model

TL;DR

This paper presents a model demonstrating that the Milky Way's bulge contains a vertical X-shaped structure formed through internal disk instabilities, aligning well with recent observations and providing insights into galactic bulge formation.

Contribution

The study introduces a simple boxy bulge model that naturally produces an X-shaped structure, matching observed features and offering a new explanation for bulge formation via internal dynamics.

Findings

The X-shaped structure in the model matches observed size and orientation.

It contains about 7% of the bulge light, making it observable.

The structure's symmetry suggests formation several billion years ago.

Abstract

A vertical X-shaped structure was recently reported in the Galactic bulge. Here we present evidence of a similar X-shaped structure in the Shen et al. (2010) bar/boxy bulge model that simultaneously matches the stellar kinematics successfully. The X-shaped structure is found in the central region of our bar/boxy bulge model, and is qualitatively consistent with the observed one in many aspects. End-to-end separations of the X-shaped structure in the radial and vertical directions are roughly 3 kpc and 1.8 kpc, respectively. The X-shaped structure contains about 7% of light in the boxy bulge region, but it is significant enough to be identified in observations. An X-shaped structure naturally arises in the formation of bar/boxy bulges, and is mainly associated with orbits trapped around the vertically-extended x_1 family. Like the bar in our model, the X-shaped structure tilts away from the Sun--Galactic center line by 20 degrees. The X-shaped structure becomes increasingly symmetric about the disk plane, so the observed symmetry may indicate that it formed at least a few billion years ago. The existence of the vertical X-shaped structure suggests that the formation of the Milky Way bulge is shaped mainly by internal disk dynamical instabilities.