- 3760 - MILKY WAY - and galaxies beyond? We used to think the Milky Way Galaxy was our entire universe. Then we discovered some other galaxies further away. The Milky Way has a halo of stars. Today this stellar halo is giving astronomers some new food for thought. It turns out everyone thought the halo was spherical. But, it’s not.
--------------------- 3760 - - MILKY WAY - and galaxies beyond?
- According to a new measurement done by a team at Harvard-Smithsonian Center for Astrophysics, it has a tilted, oblong football shape. This new shape tells astronomers an interesting tale about our galaxy’s ancient history.
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- The shape of the stellar halo is a very fundamental parameter that we’ve just measured to greater accuracy than was possible before (2022). There are a lot of important implications of the stellar halo not being spherical but instead shaped like a football, rugby ball, or zeppelin.
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- So, is it a big deal if it’s not like a beach ball, as astronomers predicted for so many years? It turns out the odd shape is an important clue to Milky Way’s early history. The tilt and distribution of stars in the stellar halo provide dramatic confirmation that our galaxy collided with another smaller galaxy 7-10 billion years ago.
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- The evolutionary history of the halo around the Milky Way involves some interesting characters. First, there’s a strange, lonely dwarf galaxy that astronomers dubbed “Gaia-Sausage-Enceladus” or GSE.
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- Billions of years ago GSE galaxy collided with the Milky Way. Collisions are a natural way to build big galaxies out of small ones, and our galaxy formed that way. That mash up tore GSE to shreds and scattered stars from both galaxies into a dispersed halo surrounding the galaxy. The interactions between the two also caused pileups of stars in the halo. That changed the shape of the halo significantly. And, since GSE came in at an angle, the collision also tilted it. The shape is still odd and offset today.
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- Gaia-Enceladus merged with our Milky Way galaxy during its early formation stages, nearly 10,000,000,000 years ago. Its debris can now be found throughout the galaxy and in the stellar halo. You would think that after billions of years, the halo would have “sphericized” itself. Yet, the stars remain in this weird triaxially shaped “cloud”.
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- Something else, which turns out to be dark matter, is at play. The tilted stellar halo strongly suggests that the underlying dark matter halo is also tilted. A tilt in the dark matter halo could have significant ramifications for our ability to detect dark matter particles in laboratories on Earth.
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- That’s interesting to scientists searching for dark matter. If there really is a tilt in the dark matter halo, there could be areas where this mysterious stuff is more concentrated. Finding those regions could give astronomers a chance to detect interactions with dark matter.
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- The Milky Way isn’t alone in having a halo. Every galaxy has one dominated by dark matter. While we don’t see dark matter, it provides a framework for the distribution of ordinary, visible matter. That includes stars, clusters, and nebulae in the body of the galaxy, plus stars in the halo.
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- The astronomers studied two major astronomy data sets that allowed them to make a computer model of what happened. One came from the GAIA spacecraft, which measures the positions, motions, and distances of millions of Milky Way and halo stars. The other data set came from a ground-based survey called H3 (short for Hectochelle in the Halo at High Resolution). The combined results showed the weirdly tilted football shape that emerged.
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- Astronomers have long theorized that a nucleus of stars almost certainly exists at the center of the Milky Way galaxy, but until now, they have been unable to find proof. In this new effort, the researchers took up the challenge by sifting through data from the Gaia space telescope.
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- Theory has also suggested that if there is a nucleus of stars at the center of the galaxy, they very likely contain much less metal than other stars because they would have been formed before such metals were scattered across the area where the Milky Way formed 12.5 billion years ago.
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- Thus, they would likely be made up mostly of helium and hydrogen. Looking only for stars that fit into this category narrowed things down somewhat, and, so did theories that have long suggested that if there is a nucleus of stars at the center of the galaxy, it would most likely be located in the constellation Sagittarius, as it is situated at what appears to be the center of the disk that makes up the galaxy.
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- That still left the team sifting through approximately 2 million stars, a daunting task. Astronomers were looking for, a cluster of approximately 18,000 stars at the center of the Milky Way galaxy. The stars in the cluster have less than 3% of the metal concentration of stars farther away.
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- Astronomers pinpointed the location of what should be the center of the galactic disk. They accounted for stars blocked by dust or other objects. Convinced that they had found the heart of the Milky Way, they took measurements and found the cluster makes up just 0.2% of the mass of the galaxy. So now the Milky Way has a center to its oblong shape.
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November 27, 2022 3760
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