If you’re swimming in a large volume of water, i’s difficult to judge the properties of distant floating objects with exacting precision, and the same is true for our star system as it swirls around the galaxy.
This is perhaps why scientists have just discovered a new structure encompassing a long curl of gas so gigantic that no one can say whether or not this is a section of a galactic spiral arm we simply hadn’t noticed until now, according to a recent study shared on a preprint server and accepted for publication in The Astrophysical Journal Letters.
This could change our understanding of how the Milky Way behaves, pending more research.
A newly discovered colossal filament of gas in the outer regions of our Galaxy
The gas filament in the Milky Way, known as Cattail, may be the largest ever discovered and “appears to be so far the furthest and largest giant filament in the galaxy,” according to a team of astronomers from Nanjing University in China in a recent paper. “The question about how such a huge filament is produced at the extreme galactic location remains open,” they continued. “Alternatively, Cattail might be part of a new arm … though it is puzzling that the structure does not fully follow the warp of the galactic disk.”While the find is surprising, that it wasn’t made until now is more understandable, since reasons abound for why mapping our galaxy in three dimensions is no easy feat.
One reason is the inherent difficulty in calculating the distance between celestial objects. Second, the galaxy is full of material and distracting signals, making it difficult to distinguish between objects that happen to be aligned from our unique perspective and those that are in fact part of a grouping of related objects. In the case of Cattail, an astronomical team led by Chong Li from Nanjing used the massive Five-hundred-meter Aperture Spherical radio Telescope (FAST) to identify clouds of neutral atomic hydrogen (HI). These clouds are typically found in the spiral arms of galaxies like the Milky Way, and by analyzing the subtle varying patterns of hydrogen light, astronomers can map the number and dispersal of the Milky Way’s arms from our position within one of them.
New ‘galactic filament’ appears larger than Gould’s Belt
Back in August 2019, the astronomers used FAST to search for HI radio emissions, which produced data that described a colossal structure. After calculating its velocity, they discovered that it corresponded to a distance of approximately 71,750 light-years from the galaxy’s center. That’s way out in the outer regions! This is significant not only because it is much further out than any previously identified spiral arm of our galaxy, but also because it would have to be enormous in scope; an arm roughly 3,590 light-years long and 675 light-years wide, according to FAST data. However, this was quickly surpassed: When the researchers combined their findings with additional data from the HI4PI all-sky HI survey, they discovered that this potential spiral arm could be up to 16,300 light-years long!
This would make it an even more mind-bogglingly giant gas structure, larger than Gould’s Belt, which was recently discovered to be 9,000 light-years long. While this is exciting, the discovery raises some significant follow-up questions for astronomers around the world to discuss. For example, how did such a massive gas filament end up so far away from the galactic center? Furthermore, it appears to lack a “wobbly” feature that other spiral arms in our galaxy have (the traces of an ancient intergalactic collision). For now, “these questions remain open with the existing data,” said the researchers in the study. But “The observations provide new insights into our understanding of the galactic structure.” Hear, hear.
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