When you are swimming in a large body of water, it is 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 escapes us or cannot be calculated, but despite this, from time to time there are discoveries that make us think: “How the hell did we miss this ?!
A recently discovered structure called Cat's Tail is just such a miracle. It is a long swirl of gas that is so large that astronomers are unsure if it could be part of a galactic spiral arm that we have not yet noticed.
Even if it is not a sign of an uncharted spiral arm, Cat's Tail may be the largest filament of gas in our galaxy discovered to date. It is described in an article accepted for publication in The Astrophysical Journal Letters and available on the arXiv preprint server.
The structure "appears to be the most distant and largest giant filament in the galaxy so far," wrote a group of astronomers from Nanjing University in China.
"The question of how such a huge stream forms at the most distant point in the galaxy remains open. Alternatively, the Cat's Tail could be part of a new arm … although it is perplexing that the structure does not completely follow the curvature of the galactic disk."
There are several reasons why it is difficult to map the Milky Way in three dimensions. One of them is that it is very difficult to determine the distance to space objects. Another reason is that there are so many things there, so it can be difficult to know if something is a significant cluster or just a random aggregate scattered along the line of sight.
To identify the Cat's Tail, a team led by astronomer Chong Li of Nanjing University used a huge 500-meter aperture spherical radio telescope (FAST) to search for clouds of neutral atomic hydrogen (HI). Such clouds are usually found in the spiral arms of galaxies like ours; By examining the subtle differences in the light of hydrogen, one can map the number and location of the arms of the Milky Way from the inside.
In August 2019, researchers used FAST to search for HI radio emissions, and their findings revealed what appeared to be a large structure. When they calculated the speed of this structure, they got a surprise: its speed corresponded to the distance of about 71,750 light years from the galactic center - the outer regions of the galaxy.
This distance - farther than all known spiral arms in this region of the galaxy - means the structure is absolutely enormous, about 3,590 light years long and 675 light years wide, according to FAST data.
But when the researchers combined their findings with data from the all-galactic survey of HI4PI, they found that it could be even larger - about 16,300 light years in length.
This makes it even more colossal than the gaseous structure known as the Gould Belt, which was recently found to be 9,000 light years long.
The cat's tail raises some interesting questions. Most filaments of gas originate much closer to the galactic center and are associated with spiral arms.If it is a filament, then it is unclear how the Cat's Tail could have formed and stayed outside the known spiral arms of the Milky Way.
On the other hand, if it is a spiral arm, then this is also unusual. The galactic disk of the Milky Way is wobbling and deformed as a result of an old encounter with another galaxy. However, the shape of the Cat's Tail does not fully match this curvature, which it should be if it is a spiral sleeve.
Even if this discovery weren't exciting, these features indicate that we might want to take a closer look at this amazing structure.
"While these questions remain open with existing data," the researchers write, "observations provide a fresh look at our understanding of galactic structure."