When you swim in a large body of water, it’s not easy to calculate its volume or locate distant floating objects. The same is true for our galaxy.
From our position inside the Milky Way, much about its size, content and three-dimensional structure is very difficult to understand. Much eludes us or cannot be computed, but despite this, from time to time there are discoveries that make us wonder, “How the hell did we miss this?
A recently discovered structure called the Cattail is just such a wonder. It’s a long curl of gas that’s so large that astronomers aren’t sure if it could be part of a galactic spiral arm that we haven’t noticed until now.
Even if it is not a sign of an uncharted spiral arm, Cat’s Tail may be the largest strand of gas in our galaxy detected to date. It is described in a paper accepted for publication in The Astrophysical Journal Letters and available on the arXiv preprints server.
The structure “appears to be the most distant and largest giant filament in the galaxy so far,” a team of astronomers from Nanjing University in China wrote in their paper.
“How such a huge flux forms at the most distant point in the galaxy remains an open question. Alternatively, the Cat’s Tail could be part of a new arm… although it is puzzling that the structure does not completely replicate the curvature of the galactic disk.”
There are several reasons why it is difficult to map the Milky Way in three dimensions. One is that it is very difficult to determine distances to cosmic objects. Another reason is that there are so many things out there, so it can be difficult to know if something is a significant cluster or just a random collection scattered along the line of sight.
To identify the Cattail, a team led by astronomer Chong Li of Nanjing University used a huge five-hundred-meter aperture spherical radio telescope (FAST) to search for neutral hydrogen atom (HI) clouds. Such clouds are usually found in the spiral arms of galaxies like ours; by studying the subtle differences in hydrogen light, it is possible to map the number and location of the Milky Way’s arms from within.
In August 2019, researchers used FAST to search for HI radio emission, and the data revealed what appeared to be a large structure. When they calculated the velocity of this structure, they got a surprise: Its speed corresponded to a distance of about 71,750 light-years from the galactic center-the outer regions of the galaxy.
Such a distance – farther than all known spiral arms in this region of the galaxy – means this structure is absolutely enormous, measuring about 3,590 light-years long and 675 light-years wide, according to FAST data.
But when the researchers combined their results with data from the HI4PI All-Galactic Survey, they found that it could be even larger — about 16,300 light-years long.
This makes it even more colossal than the gas structure known as the Gould Belt, which was recently found to be 9,000 light-years long.
The cattail raises some interesting questions. Most gas filaments arise much closer to the galactic center and are associated with spiral arms. If it is a filament, it is unclear how the Cattail could have formed and remained outside the known spiral arms of the Milky Way.
On the other hand, if it is a spiral arm, that is also unusual. The Milky Way’s galactic disk oscillates and deforms as a result of a long ago encounter with another galaxy. However, the shape of the Cattail is not completely consistent with this warp, which is what it should be if it is a spiral arm.
Even if this discovery wasn’t exciting, these features indicate that we might want to take a closer look at this amazing structure.
“Although these questions remain open with existing data,” the researchers write, “the observations provide new insights into our understanding of galactic structure.”