An Immense Spiral Arm Could Encircle The Entire Milky Way

Lev Savitskiy, via Shutterstock.

Although we’ve known our galaxy is spiral-shaped for almost a century and a half, our idea of what it looks like as a whole often changes thanks to new discoveries made possible by advances in observational techniques.

It was believed for some time that the Milky Way consisted of four arms, packed with clouds of dust, gas and stars, which emanate outwards in an elegant twist from a central region. But back in 2008, Spitzer data suggested that our galaxy actually only has two arms, extending from a large central bar. Now, it seems that our portrait of the Milky Way has changed once again with the discovery of a new segment of a spiral arm. And, tantalizingly, it could be an extension of a distant arm discovered a couple of years ago, which would mean that one of the arms may wrap around our entire galaxy.

This mighty arm, Scutum-Centaurus, winds outward in a counter-clockwise direction from one end of the Milky Way’s bar, passes between us and the galactic center, and extends all the way to the other side of the galaxy. In 2011, two astronomers from the Harvard-Smithsonian Center for Astrophysics, Thomas Dame and Patrick Thaddeus, spotted an extension of this arm, which placed it outside of our solar system. But now, new data from the Purple Mountain Observatory, China, could suggest that the incredible arm is even longer than that.

For the study, astronomer Yan Sun and colleagues used radio telescope data to look for interstellar gas clouds that mark spiral arms. Rather than looking for the most common ingredient, hydrogen, which is difficult to detect, they hunted radio waves emitted by the second most abundant molecule in interstellar space—carbon monoxide gas.

The researchers focused on clouds located between 46,000 and 67,000 light-years from the galactic center; for some context, our sun is around 27,000 light-years out. As described in The Astrophysical Journal Letters, the scientists found a total of 72 clouds, including 42 that were previously unknown, which line up along a 30,000-light-year-long spiral arm segment. But that’s not the most interesting part. If it turns out to be an extension of the distant segment discovered back in 2011, Scutum-Centaurus may actually make a full 360o turn around the Milky Way. Considering something like that has never been observed in nearby spiral galaxies, it would be pretty incredible if the proposal holds up.

Illustration of the Milky Way, showing the possible extension of Scutum-Centaurus. Credit: Yan Sun/The Astrophysical Journal Letters/NASA/JPL-Caltech/SSC

“It’s rare,” said Dame. “I bet that you would have to look through dozens of face-on spiral galaxy images to find out where you could convince yourself you could track one arm 360 degrees round.”

While the prospect is certainly exciting, there is still the possibility that this arm is not actually part of Scutum-Centaurus at all, but instead a newly discovered lone segment. There exists a problematic 40,000-light-year-long gap between the start of the new arm and the tail of the 2011 extension. However, it shouldn’t take long for scientists to work out whether the arms do indeed join up, as they can start looking for the existence of molecular clouds within this region. If the segments do indeed connect, then we reside in one truly magnificent and unusual galaxy.

[Via Universe Today, Smithsonian, io9 and The Astrophysical Journal Letters]

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