Intriguing Vertical X-shaped Structure Confirmed in the Milky Way Bulge

Chinese astronomers have made important progress in the study of the Milky Way structures. Using a simple bulge model, astronomers in Shanghai Astronomical Observatory (SHAO) confirmed and explained the existence of an intriguing X-shaped structure in the Milky Way bulge. This research was led by Dr. Zhao-Yu Li under the supervision of Professor Juntai Shen at SHAO. The result is published in the latest volume of Astrophysical Journal Letters (ApJL) and is already cited by two research groups in the U.S. and Europe.

Understanding the Milky Way structures is non-trivial, mostly because we are located in the disk plane. As a spiral galaxy, the Milky Way is composed of a disk, a bulge and a dark matter halo. With high-resolution N-body simulations, Professor Juntai Shen has confirmed in previous research that the observed Galactic boxy bulge is actually a bar seen edge-on. Surprisingly, he found that the Milky Way does not seem to contain a significant merger-built classical bulge, which had long been expected as the dominant component of the Milky Way bulge.

Because of their roughly constant luminosity, red clump stars can be used as a good distance indicator, or the so-called "standard candle", to map out structures in the Milky Way. Recently two international research teams independently reported that along different line of sights toward the Milky Way bulge region the distance distributions of red clump stars are bimodal. This indicates that the stars in the Milky Way bulge region may lie on a vertical symmetric X-shaped structure. The X-shaped structure was puzzling, since it was hard to understand how it can co-exist with the bar/boxy bulge of our Milky Way.

Astronomers at SHAO carefully analyzed the Milky Way model published previously by Professor Juntai Shen. They found a significant X-shaped structure in the bulge region. The radial and vertical lengths of this X-shaped structure are about 10,000 and 6,000 light years, respectively, both of which agree quite well with the observational results. The model also predicts that the X-shaped structure probably contains ~7% of the total stellar mass of the Milky Way bulge. The X-shaped structure naturally arises from the dynamical buckling instability of the Milky Way bar after the bar formed. Their co-existence is not puzzling, but well expected instead. "Based on the symmetry of the observed X-shaped structure, we also infer that the intriguing X-shaped structure formed at least several billion years ago", Prof. Juntai Shen said, "Its existence further indicates that the formation of the Galactic bulge is dominated mainly by internal dynamical instabilities, instead of galaxy mergers as previously believed."


Figure 1 - Top panel shows the edge-on image of the model (with the major axis of the bar along the X-axis). Bottom panel highlights the X-shaped structure in the model, which agrees quite well with the observed distribution of red clump stars in our Milky Way bulge.

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