A new study suggests the Milky Way doesn’t need a makeover: It’s already just about perfect.

Astronomers base that assertion on their discovery of a vast section of a spiral, star-forming arm at the Milky Way’s outskirts. The finding suggests that the galaxy is a rare beauty with an uncommon symmetry — one half of the Milky Way is essentially the mirror image of the other half.

Thomas Dame and Patrick Thaddeus of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., say the structure they’ve discovered is most likely the outer extension of the Scutum-Centaurus arm from the inner galaxy. The finding suggests that Scutum-Centaurus wraps all the way around the Milky Way, making it a symmetric counterpart to the galaxy’s other major star-forming arm, Perseus.

The two arms appear to extend from opposite ends of the galaxy’s central, bar-shaped cluster of stars, each winding around the galaxy, the researchers note in an upcoming issue of Astrophysical Journal Letters.

Dame found evidence for the new structure while reviewing galactic data on atomic hydrogen gas, which radiates at a radio wavelength of 21 centimeters. After tracing the extension of the arm in the 21-centimeter radio emission, “I was in the unique position of being able to walk up two flights of stairs to the roof of my building [at Harvard] and search for carbon monoxide emissions from molecular clouds using the CfA 1.2-meter radio telescope,” says Dame. Molecular gas clouds contain the raw material for making stars.

“This is a major new discovery,” comments Robert Benjamin of the University of Wisconsin-Whitewater. “Dame and Thaddeus have found evidence for a large-scale coherent structure, spanning 60 degrees in the sky … which contains giant molecular gas clouds very far from the galactic center.” The newfound structure lies about 49,000 light-years from the galaxy’s center, and one of the arm’s many large molecular clouds contains an amount of molecular hydrogen equivalent to that of 50,000 suns.

Virtually every spiral arm in the Milky Way has been found in sections, Dame notes. When astronomers realized that the Sagittarius arm, found in the northern sky, and the Carina Arm, in the south, were part of a single, larger structure, they became known as the Sagittarius-Carina Arm. Similarly, since Dame and Thaddeus believe the new arm is an extension of Scutum-Centaurus, “we suggested ‘Outer Scutum-Centaurus’ as a more logical name,” Dame says. The structure is longer than the known parts of the Scutum-Centaurus arm, he adds.

The new feature was previously overlooked because it tilts out of the plane of the galaxy, following the outer galaxy’s warp. Most studies examining spiral arms focus on the galaxy’s plane, says Dame.

The team’s “identification of the feature as a discrete structure is new, and the discovery that it contains molecular gas makes a very strong case for this being a spiral arm,” Benjamin notes.

Mapping all the carbon monoxide in the newfound feature will take several years, says Dame. Nonetheless, the galactic symmetry revealed by the new observations, along with previous evidence, suggests that the Milky Way’s spiral structure is both simpler and easier to study than had long been assumed, he and Thaddeus note. “We’ve not proven that, but it’s evidence in that direction,” says Dame.

Image: A newly discovered star-forming arm at the fringes of the Milky Way may be a vast, outer extension of the arm Scutum-Centaurus. The finding suggests that the Milky Way has a rare symmetry, with one half of the galaxy the mirror image of the other half. (T. Dame, Robert Hurt)

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