Huge Structure Detected in the Milky Way Maybe a New Spiral Arm

Like stumbling across a room in your home, astronomers have found a huge path of gas hidden at the outer edge of our galaxy. Named Cattail, the newly discovered behemoth was 22 kiloparsecs – 7,1754 light-years from the center of the galaxy, and astronomers say it could be a previously spotted spiral arm. In a new study published in The Astrophysical Journal Letters, researchers explained how they used a 500-meter aperture circular radio telescope (4,566 light-years) to detect clouds of neutral atomic hydrogen (HI) in an area of ​​1.4 kiloparsecs. 

Such clouds exist within the spiral arm of the Milky Way, although no large gas filament has been observed outside the outer spiral arm, known as the outer Scutum-Centaurus (OSC) arm. Surprisingly, however, the study authors identified a gas traveling at an average speed of 150 kilometers per second (93.2 miles per second), which is “located far behind the OSC”. The researchers wrote, “The length of the H I structure is 1.1 kpc [3,588 light-years], which seems to be the farthest and largest giant filament in the galaxy so far, and we named it Cattail.”

However, after cross-referencing their observations with HI4PI All-Sky H I survey data; they calculated that the giant entity was much larger than the initial estimate, with a length of about 5 kiloparsecs (16,310 light-years). While it is possible that the kettle is simply a large-sized gas filament, it does not follow many of the rules that govern such structures. For example, except for its unusual size and extreme distance from the galactic center, it does not appear to be physically connected to the spiral arms of the Milky Way.

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

According to their calculations, the Cattail is roughly five times as long as it is wide; one aspect ratio is similar to the other spiral arm. On the other hand, the structure lacks the correct shape for a spiral arm, which is not consistent with the distorted axis of the rest of the galaxy. For example, researchers seem to have more questions than answers at this stage and are unable to make definitive decisions about Cattle’s true nature and identity.