Imagine you’re floating in the middle of the ocean, nothing but the deep blue waters in sight. Can you tell how far away you are from land? Can you determine how deep the waters under you are, or just what is happening within those murky depths? Exploring our galaxy is a lot more challenging, to put it mildly.
The Earth lies within a vast galaxy called the Milky Way, which is why it took decades of three-dimensional analyses for us to figure out that our galaxy was a spiral disc. Two prominent arms—Perseus arm and Scutum-Centaurus arm—make up the structure along with two less-pronounced arms—Sagittarius and the Local Arm. These arms, made up of stars and gases, are tightly wound about the galaxy.
But, now, there seems to be a new ‘break’ in this bond! Recently, scientists have spotted a significant inconsistency in the otherwise uniform spirals that form the arms of our galaxy.
Named the Cattail, this newly-detected arm-like structure is essentially a long curl of gas moving at nearly the same velocity and the same direction as the Sagittarius arm. The structure spans a whopping 3000 light-years (for scale, one light-year is equivalent to more than 9 trillion kilometres). It is the first of its kind to be discovered—sticking out from the Sagittarius arm like a splinter. It has left scientists wondering whether it is just a large gas filament or, in fact, another spiral arm.
Researchers used NASA’s Spitzer Space Telescope to find newly formed stars in the region near the Sagittarius arm to understand the structure better. They then used the most recent data release from the ESA (European Space Agency) Gaia mission to estimate the precise distances to the stars to obtain a 3D image of the arm section. Combining data from both sources, they could create a picture of how the Cattail looks.
A circle has a pitch angle of 0 degrees, and as a spiral arm forms and becomes more open, the pitch angle goes on increasing. “Most models of the Milky Way suggest that the Sagittarius Arm forms a spiral that has a pitch angle of about 12 degrees, but the structure we examined really stands out at an angle of nearly 60 degrees,” said Michael Kuhn, an astrophysicist at Caltech and lead author of the new paper, referring to the Cattail.
Researchers also used a catalogue of more than a hundred thousand young stars discovered by Spitzer in a scan of the galaxy known as the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire in the current study (GLIMPSE).
“When we put the Gaia and Spitzer data together and finally see this detailed, three-dimensional map, we can see that there’s quite a bit of complexity in this region that just hasn’t been apparent before,” said Kuhn.
We still don’t know what causes the formation of spiral arms or even the actual structure of the Milky Way, but this study has led to some interesting conclusions about our galaxy.
“Ultimately, this is a reminder that there are many uncertainties about the large-scale structure of the Milky Way, and we need to look at the details if we want to understand that bigger picture,” says Robert Benjamin, a co-author and an astrophysicist at the University of Wisconsin-Whitewater. “This structure is a small piece of the Milky Way, but it could tell us something significant about the Galaxy as a whole.”
The study has been published in the journal Astronomy & Astrophysics and can be accessed here.
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