The discovery of a newly identified structure in the Milky Way called the Cattail has intrigued astronomers. This structure is a long curl of gas, and its size and origin remain a mystery. It could be part of an unmapped galactic spiral arm or the largest gas filament ever discovered in our galaxy.
The astronomers note that the Cattail appears to be the furthest and largest giant filament observed in the galaxy. However, they are uncertain about how such a massive filament could form in the extreme outer regions of the galactic location. It is also puzzling that the structure does not align completely with the warp of the galactic disk, indicating that it might not be a spiral arm.
Mapping the Milky Way in three dimensions is challenging due to the difficulty in determining distances to cosmic objects and the vast amount of matter present. To identify the Cattail, the astronomers used the Five-hundred-meter Aperture Spherical radio Telescope (FAST) to search for clouds of neutral atomic hydrogen. These clouds are typically found in the spiral arms of galaxies like ours. By analyzing the light emitted by the hydrogen, they could map the number and arrangement of the Milky Way's arms.
The researchers discovered the Cattail when they observed radio emissions from hydrogen using FAST in August 2019. Surprisingly, the structure's velocity indicated a distance of approximately 71,750 light-years from the galactic centre, placing it in the galaxy's outer regions. Based on the FAST data, the Cattail's estimated size is around 3,590 light-years in length and 675 light-years in width. When combined with data from the HI4PI all-sky HI survey, the structure's size could be even larger, possibly reaching 16,300 light-years in length, surpassing the length of the known gas structure called Gould's Belt.
The peculiarities of the Cattail raise intriguing questions. Most gas filaments are located closer to the galactic centre and are associated with spiral arms. If the Cattail is indeed a filament, how it formed and remained beyond the known spiral arms is unclear. On the other hand, if it is a spiral arm, its shape does not align completely with the warp of the galactic disk caused by a past encounter with another galaxy.
While the researchers acknowledge that many questions remain unanswered with the current data, the discovery provides valuable insights into understanding the structure of our galaxy. Further investigation is needed to unravel the mysteries of the Cattail and its significance in the Milky Way's complex structure.
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