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When swimming in a large body of water, it is not easy to calculate its volume or discern the location of distant floating objects. It is the same for our galaxy.
From our position inside the Milky Way, much of its size, content, and three-dimensional structure is really hard to understand. There are many things that escape us or that are impossible to calculate; even so, every now and then a discovery comes along that makes you wonder, how the hell did we miss this?!
A newly discovered structure named the Cattail is such a wonder. It’s a long loop of gas so large that astronomers aren’t sure whether or not it is part of a galactic spiral arm that we’ve never noticed until now.
While this is not a sign of an unmapped spiral arm, the Cattail may be our galaxy’s largest gas filament discovered to date. It was described in an accepted article Letters from the astrophysical journal, available on the arXiv preprint server.
The structure “appears to be the most distant and largest giant filament in the galaxy to date,” wrote a team of astronomers from Nanjing University in China in the journal.
“The question of how such a huge filament is produced at the extreme galactic location remains open. Alternatively, Cattail could be part of a new arm… although it is surprising that the structure does not completely follow the chain of the disc. galactic. “
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 calculate distances to cosmic objects. Another is that there is a lot going on out there, so it can be difficult to tell if something is a large grouping or just a random collection spread out along a line of sight.
To identify the distaff, a team led by Nanjing University astronomer Chong Li used the enormous five-hundred-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 studying the subtle differences in light from hydrogen, it is possible to map the number and arrangement of the Milky Way’s arms from within.
In August 2019, researchers used FAST to search for HI radio broadcasts, and the data revealed what appeared to be a large structure. When they calculated how fast the structure was moving, they got a surprise: its speed was consistent with a distance of about 71,750 light years from the galactic center – the outer regions of the galaxy.
This distance – farther than any known spiral arms in this region of the galaxy – would mean that the thing is absolutely huge, with a size of approximately 3,590 light years in length and 675 light years in width, based on FAST data.
But then, when the researchers combined their results with data from the HI4PI all-sky survey, they discovered that it could be even larger – up to about 16,300 light-years in length.
This would make 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 distaff raises some interesting questions. Most of the gas filaments occur much closer to the galactic center and are associated with spiral arms. If it is a filament, it is not known how the distaff may have formed and remained beyond the known spiral arms of the Milky Way.
On the other hand, if it is a spiral arm, it is also special. The Milky Way’s galactic disk is wobbly and warped from an encounter with another galaxy a long time ago. Still, the Cattail’s shape doesn’t fully conform to this chain – which it should if it were a spiral arm.
While the find wasn’t already fascinating, these peculiarities indicate that we might want to take a closer look at this astonishing structure.
“While these questions remain open with the existing data,” the researchers wrote, “the observations provide new information on our understanding of galactic structure.”
The research was accepted in Letters from the astrophysical journal, and is available on arXiv.
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