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Huge Structure Detected In The Milky Way May Be A New Spiral Arm


Ben Taub

Freelance Writer

clockAug 6 2021, 11:56 UTC
Milky Way

The huge trail of gas lies far beyond the galaxy's outermost spiral arm. Image: Alex Mit/

Like stumbling upon a room in your house, astronomers have just detected an enormous trail of gas lurking in the outer reaches of our home galaxy. Named Cattail, the newly discovered behemoth was located 22 kiloparsecs – 7,1754 light-years – from the center of the Milky Way, and astronomers say it might be a previously unspotted spiral arm.

In a new study due for publication in The Astrophysical Journal Letters, researchers explain how they used the Five-hundred-meter Aperture Spherical radio Telescope (FAST) to search for clouds of neutral atomic hydrogen (H I) in a region of sky located 1.4 kiloparsecs (4,566 light-years) from our sun. Such clouds tend to exist within the Milky Way’s spiral arms, though no major gas filaments have ever been seen beyond the outermost spiral arm, known as the Outer Scutum-Centaurus (OSC) arm.


Surprisingly, however, the study authors detected a body of gas traveling at an average speed of 150 kilometers per second (93.2 miles per second), suggesting it is “located far behind the OSC.”

“The H I structure has a length of 1.1 kpc [3,588 lightyears], which appears to be so far the furthest and largest giant filament in the Galaxy and we name it Cattail,” write the researchers.

However, after cross-referencing their observations with HI4PI all-sky H I survey data, they calculated that the colossal entity is acually much larger than initial estimates, with a length of around 5 kiloparsecs (16,310 light-years).


While it’s possible that Cattail is simply an oversized gas filament, it doesn't comply with many rules that such structures tend to obey. For instance, aside from its unusual size and extreme distance from the galactic center, it does not appear to be physically connected to any of the Milky Way’s spiral arms.

Based on these observations, the researchers propose “two possible explanations for Cattail: it is a giant filament with a length of ∼5 kpc [16,308 light-years], or part of a new arm in the EOG [extreme outer galaxy].”

According to their calculations, Cattail is roughly five times as long as it is wide, an aspect ratio similar to other spiral arms.


On the other hand, the structure lacks the correct shape for a spiral arm, not aligning with the warped axis of the rest of the galaxy. As such, the researchers appear to have more questions than answers at this stage, and are unable to draw firm conclusions as to Cattail’s true nature and identity.

“If Cattail is a gas filament located beyond the OSC, how is such a huge structure formed?” they ask. “Alternatively, if Cattail is part of a new spiral arm in the EOG, it is also puzzling that the new arm does not fully follow the Galactic warp.”



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spaceSpace and PhysicsspaceAstronomy
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