Chinese astronomers have discovered a new spiral arm at the outer reaches of the Milky Way. But it might be an extension of a distant arm discovered back in 2011, suggesting that a majestic spiral arm encircles the entire galaxy.


Our understanding of what the Milky Way looks like is inhibited by the fact that we live directly inside it; nearby stars tend to obscure more distant ones, making direct observations extremely challenging for astronomers. It wasn't until 1852 that we even realized our galaxy was a spiral. Today, some 163 years later, there's still no consensus on the exact configuration of the Milky Way's spiral arms.

That said, we are fairly certain that the Milky Way boasts several prominent arms that all start near the galaxy's center, including the Perseus Arm, the Sagittarius Arm, and the Scutum-Centaurus Arm. As for our Sun, it's located in the Local Arm, which is a prominent "offshoot" of the Perseus Arm.

That's a Wrap

Of relevance to the new discovery is the Scutum-Centaurus Arm, an immensely long spiral that extends from the galaxy's bar. Back in 2011, astronomers Thomas Dame and Patrick Thaddeus discovered an extension to this arm (or at the very least, a new arm altogether). Assuming it's an extension, Scutum-Centaurus winds outward in a counterclockwise direction, stretching all the way to the galaxy's opposite side.

But as the new study suggests, it may actually curl even further, making a complete 360-degree revolution around the Milky Way.



By combining data from the Canadian Galactic Plane Survey and data from the Milky Way Imaging Scroll Painting project, Chinese astronomer Yan Sun of the Purple Mountain Observatory in Nanjing, along with his colleagues, have discovered a completely new spiral arm at the extreme outer reaches of the Milky Way. And as the researchers note in their study, "The consistent fitting results confirm that the new arm can be interpreted as the far-extension of the distant arm recently discovered by Dame & Thaddeus."

The new segment is located at a radius of somewhere between 15 to 19 kpc (50,000-62,000 light-years). For context, our Sun is located about 27,000 light-years from the galactic center. Across most most of its length the arm is about 400 to 600 pc (1,300-2,000 light-years) thick.

By using radio waves, the astronomers were able to identify the new band by detecting sizable chunks of carbon monoxide gas, which is an extremely abundant interstellar molecule. In total, their survey identified 72 molecular clouds with substantive masses that probably lie within the newly discovered arm.

Ken Croswell from Scientific American highlights some reactions to the discovery:


What is most remarkable, the astronomers say, is that the segment may extend from the outermost part of Scutum–Centaurus, making this arm even longer. If so, the arm actually makes a full 360-degree turn around the galaxy. "That's amazing," says Robert Benjamin of the University of Wisconsin–Whitewater, an astronomer who was not involved with the discovery. "It's rare," notes Thomas Dame, an astronomer at the Harvard–Smithsonian Center for Astrophysics. "I bet that you would have to look through dozens of face-on spiral galaxy images to find one where you could convince yourself you could track one arm 360 degrees around." Dame helped discover the 2011 extension of the Scutum–Centaurus Arm. "My impression was that we had found the end of it," he says. "So I was very surprised to see this."

Mind the Gap

All this said, there is a rather important caveat to the apparent discovery: a 40,000 light-year gap between the end of Dame & Thaddeus's arm to where Yan's new arm begins. The new band could be a lone arm, or a spur from another prominent arm. But this shouldn't be too difficult to prove one way or another; astronomers should start to look for similar molecular clouds in the gap. Crosswell quotes Benjamin as saying, "It should be easy in the next few years to confirm or refute their hypothesis."


Should it be confirmed, the discovery suggests the Milky Way is far more spectacular than we thought; very few spiral galaxies feature arms that wrap completely around them.

Read the entire study at The Astrophysical Journal: "A Possible Extension of the Scutum-Centaurus Arm into the Outer Second Quadrant." A pre-print version is available here.

Images: Yan et. al.

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