Astronomers have identified a strange feature never before seen in our galaxy: a cluster of young stars and gas clouds protruding from Sagittarius’ arm like a splinter sticking out of a wooden plank. About three thousand light years long, it is the first large structure identified with an orientation so drastically different from that of the arm. All the details on A&A
You will know the saying Devil is in the details . Its origin is very ancient and is thought to derive from the form le bon Dieu est dans le détail , generally attributed to Gustave Flaubert. The sentence reminds us that the more you examine something, the more you look at the details, the more you appreciate its complexity and get closer to the truth. It also applies to its most negative meaning, the one that recalls the devil: something may seem simple at first or on the surface, but when you look at it in detail, a complexity can emerge that requires much more time and effort to be understood.
It seems that this is the case of the Milky Way , where scientists – studying its details with NASA’s Spitzer Space Telescope and ESA’s Gaia mission – have identified for the first time a strange feature: a “contingent” of young stars and clouds of gas protruding from one of its spiral arms, like a splinter sticking out of a wooden plank. About 3000 light years long , it is the first large structure identified with an orientation so drastically different from that of the arm.
Astronomers have a rough idea of the size and shape of the Milky Way’s arms, but much remains unknown as it is not possible to see the entire structure as the Earth is inside it. It’s like trying to draw a map of Rome from inside the Colosseum. It is not possible to measure distances accurately enough to know if two buildings were on the same block or a few streets away… or to hope to see as far as the ring road, with so many buildings along the line of sight.
To learn more, the authors of the new study focused on a nearby portion of one of the galaxy’s arms, the Sagittarius Arm . Using the Spitzer Space Telescope – prior to its retirement in January 2020 – they searched for newborn stars, nestled in the gas and dust nebulae where they form. Spitzer detected infrared light that can penetrate those clouds, while visible light is blocked.
Young stars and nebulae are thought to be closely aligned with the shape of the arms in which they reside. To get a 3D view of the arm segment, the scientists used the latest data released by the Gaia mission to accurately measure the distances to the stars. The combined data revealed that the long, thin structure associated with Sagittarius’ arm is made up of young stars moving at almost the same speed and in the same direction through space.
“A key property of spiral arms is how tightly they wrap around the galaxy,” says Michael Kuhn , Caltech astrophysicist and first author of the study. This characteristic is measured by the pitch angle of the arm, the angle between the tangent to the spiral arm at a point and the tangent to the circle passing through the same point. A circle has a pitch angle of 0 degrees and as the spiral becomes more open, the pitch angle increases. “Most of the models of the Milky Way suggest that the Sagittarius arm forms a spiral at an angle of pitchof about 12 degrees, but the structure we have examined stands out at an angle of almost 60 degrees ».
Similar structures – sometimes called spurs or feathers – have been found in the arms of other spiral galaxies. For decades, scientists have wondered if the spiral arms of our Milky Way also had them, and now one has been found.
The newly discovered feature contains four nebulae known for their breathtaking beauty: the Eagle Nebula (inside which are the famous Pillars of Creation ), the Omega Nebula , the Trifid Nebula and the Lagoon Nebula . In the 1950s, a team of astronomers made rough measurements of the distance of some of the stars in these nebulae and were able to deduce the existence of the Sagittarius arm. Their work provided some of the earliest evidence of our galaxy’s spiral structure.
“Distances are among the hardest things to measure in astronomy,” said co – author Alberto Krone-Martins , an astrophysicist and professor of computer science at the University of California, Irvine and a member of the Gaia Data Processing and Analysis Consortium (Dpac). “It’s only Gaia’s recent direct distance measurements that make the geometry of this new structure so obvious.”
In the new study, the researchers also drew on a catalog of more than 100,000 newborn stars discovered by Spitzer in an investigation of the galaxy called the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (Glimpse). “When we put together the data from Gaia and Spitzer and finally see this detailed three-dimensional map, we can see that this region is more complex than previously,” reports Kuhn.
Astronomers have not yet fully understood what causes the spiral arms to form in galaxies like ours. While we can’t see the complete structure of the Milky Way, the ability to measure the motion of individual stars is useful for understanding this phenomenon: the stars in the newly discovered structure probably formed around the same time, in the same area, and were affected. by the forces acting within the galaxy, including gravity and the shear due to the rotation of the galaxy.
“Ultimately, this reminds us that there are many uncertainties about the large-scale structure of the Milky Way, and we need to look at the details if we are to understand that bigger picture,” concludes one of the study’s co-authors, Robert Benjamin , an astrophysicist at the University of Wisconsin-Whitewater and principal investigator of the Glimpse survey. “This structure is a small piece of the Milky Way, but it could tell us something significant about the Galaxy as a whole.”
Featured image: A cluster of stars and star-forming clouds have been found protruding from the Sagittarius arm of the Milky Way. The inset shows the size of the structure and the distance from the Sun. Each orange star indicates star-forming regions that can contain dozens to thousands of stars. Credits: Nasa / Jpl-Caltech
To know more:
- Read on Astronomy & Astrophysics the article ” A high pitch angle structure in the Sagittarius Arm ” by A. Kuhn, RA Benjamin, C. Zucker, A. Krone-Martins, RS de Souza, A. Castro-Ginard, EEO Ishida, MS Povich and LA Hillenbrand for the COIN Collaboration
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