Mapping a galaxy isn't easy when you live inside it. It took astronomers a century after the discovery of the first celestial spiral to prove that the Milky Way itself looks like a giant spiral. Its spiral arms squeeze interstellar gas and dust, causing gas clouds to grow dense, collapse and create new stars; the brightest newborn stars illuminate the arms so gloriously that spiral galaxies resemble glowing cosmic hurricanes. The Milky Way has several of these arms. Now astronomers in China have discovered that one of them may wrap around the entire galaxy, putting our galactic home in an elite group among its spiral neighbors.
The spiral arm is named Scutum–Centaurus, after two of the constellations seen from Earth through which it threads. Even before the new discovery many astronomers regarded Scutum–Centaurus as one of the greatest spiral arms in the Milky Way. It emerges from the near end of the Milky Way's bar, a long cigar-shaped structure at the galaxy's center. The arm winds outward in a counterclockwise direction, passing between us and the galactic center before stretching all the way to the other side of the Milky Way. In 2011 astronomers discovered that this arm reaches across the galaxy's far side and begins to approach our side of the galaxy again.
Now astronomer Yan Sun of the Purple Mountain Observatory in Nanjing, China, and colleagues suggest the Scutum–Centaurus Arm may extend even farther. Using a large radio telescope with a dish 13.7 meters across the astronomers sought the dense interstellar gas clouds that mark spiral arms. Such gas is made mostly of molecular hydrogen, which is difficult to detect. Instead, Sun's team searched for radio waves from the next most abundant interstellar molecule, carbon monoxide gas.
The astronomers detected 48 new molecular clouds as well as 24 others that earlier observers had seen in the outer galaxy. The clouds are about twice as far from the center of the galaxy as our solar system is: Whereas the sun is located about 27,000 light-years from the galactic center the new clouds are 46,000 to 67,000 light-years out. As the astronomers report in the January 10, 2015, issue of The Astrophysical Journal Letters, the 72 clouds line up along a previously unknown spiral-arm segment that is around 30,000 light-years long.
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."
There is a problem, however: a 40,000-light-year-long gap between the end of the segment astronomers discovered in 2011 and the start of the new one. So although Benjamin and Dame say the clouds almost certainly represent the discovery of a new spiral-arm segment, it may not truly be part of the Scutum–Centaurus Arm. Fortunately, scientists know how to test the new claim: Look for molecular clouds in the gap. "It should be easy in the next few years to confirm or refute their hypothesis," Benjamin says.
If the proposal holds up, our galaxy viewed from afar may be more striking than previously thought. Most spirals are modest, but a prestigious few galaxies, known as grand-design spirals, flaunt their beauty. The prototype is the incredible Whirlpool Galaxy, one of the most beautiful galaxies in the universe. “I don't think we're as spectacular as the Whirlpool Galaxy," Benjamin says. The Whirlpool probably owes its stunning looks to an orbiting galaxy that stirs up its disk and intensifies its spiral. In our galaxy the rotating bar may play a similar role, and the tentative discovery of a 360-degree spiral arm, Benjamin says, certainly strengthens the case that we, too, live in a grand-design spiral—a galaxy so attractive that it may be the envy of its spiral neighbors for many millions of light-years around.