What Is This X-Shape at the Center of the Milky Way?

Twitter is normally a vehicle for adorable cat pictures or depressingly chronic trolling. But for University of Toronto astronomer Dustin Lang, it was the catalyst for new research into a remarkable X-shaped structure at the bright, bulging center of the Milky Way. Composed of stars that have been sculpted by galactic processes, this stellar X can be mined for clues about the history and evolution of our galaxy.

The discovery has its roots in May 2015, when Lang tweeted galaxy maps compiled from 150-gigapixels-worth of infrared images from NASA's Wide-field Infrared Survey Explorer (WISE) space telescope.

Dustin LangMay 7, 2015

The point of the project was to peer through the Milky Way to the galaxies beyond, an effort that is beautifully laid out in this interactive page. But some observers in the Twittersphere were drawn to a foreground image of the X-shaped bulge in Lang's map. Melissa Ness, a postdoctoral researcher at the Max Planck Institute for Astronomy, was among them.

Now, a little over a year later, Ness and Lang have authored a new paper, which appears in the July issue of the Astronomical Journal, confirming the presence of the structure.

"There was controversy about whether the X-shaped structure existed," Lang said in a statement. "But our paper gives a good view of the core of our own galaxy. I think it has provided pretty good evidence for the existence of the X-shaped structure."

The authors use stronger language in the paper itself, concluding that the "WISE image of the Milky Way bulge shows that the X-shaped nature of the Milky Way bulge is self-evident and irrefutable."

The new research suggests that the X-shape may have formed as an outgrowth from the Milky Way's central "bar," which is a structural feature that has been observed in about two thirds of nearby disk galaxies like our own. These bars are dense elongated strips of stars that closely orbit the galactic center.

The paper suggests that "dynamical instabilities" within the bar may have caused it to break apart, launching some of its clustered stars into orbits perpendicular to the main angle of the galactic disk. This is what creates the X alignment from our perspective in the outer limbs of the galaxy.

It also would have been possible for this shape to form from a galactic collision, which could displace the central bar. But according to Ness, it's much more likely that the Milky Way evolved the X-shape without any help from interloping galaxies.

"We see the boxy shape, and the X within it, clearly in the WISE image, which demonstrates that internal formation processes have driven the bulge formation," she said in a statement.

"This also reinforces the idea that our galaxy has led a fairly quiet life, without major merging events since the bulge was formed, as this shape would have been disrupted if we had any major interactions with other galaxies."

"The bulge is a key signature of formation of the Milky Way," Ness added. "If we understand the bulge we will understand the key processes that have formed and shaped our galaxy."