The Vertical X-shaped Structure in the Milky Way: Evidence from a Simple Boxy Bulge Model
Abstract
A vertical X-shaped structure in the Galactic bulge was recently reported. 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°. 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- September 2012
- DOI:
- 10.1088/2041-8205/757/1/L7
- arXiv:
- arXiv:1206.3779
- Bibcode:
- 2012ApJ...757L...7L
- Keywords:
-
- galaxies: kinematics and dynamics;
- Galaxy: bulge;
- Galaxy: kinematics and dynamics;
- Astrophysics - Astrophysics of Galaxies;
- Astrophysics - Cosmology and Nongalactic Astrophysics
- E-Print:
- Accepted for publication in ApJL