Our galaxy’s stellar halo is giving astronomers new food for thought. It turns out everyone thought the halo was spherical. But it is not. That’s news to anyone who said it was spherical. It has a tilted, elongated football shape, according to a new measurement made by a team at the Harvard-Smithsonian Center for Astrophysics. All of this tells astronomers an interesting story about the ancient history of our galaxy.
“The shape of the stellar halo is a very fundamental parameter that we’ve just measured with greater accuracy than was previously possible,” said the study’s lead author, Jiwon “Jesse” Han, a graduate student at the Center for Astrophysics | Harvard & Smithsonian. “There are many important implications for the star halo not being spherical but shaped like a football, rugby ball or blimp – take your pick!”
So is it a big deal if it’s not like a beach ball like astronomers have been predicting for so many years? Well yes it is. The odd shape turns out to be an important clue to the 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 to 10 billion years ago,” said Hans co-author Charlie Conroy.
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Anatomy of an Ancient Collision
The evolutionary history of the halo around the Milky Way includes some interesting characters. First, there’s a strange, lonely dwarf galaxy that astronomers have dubbed Gaia-Sausage-Enceladus, or GSE. The name comes from the European Space Agency’s Gaia spacecraft. The second part arises from the form of GSE in its datasets. Finally, Enceladus comes to us from Greek mythology. He was a buried, mysterious giant similar to how GSE was hidden in Gaia data.
Billions of years ago, GSE collided with the Milky Way. Collisions are a natural way to build large galaxies from small ones, and our galaxy was formed that way. This mashup tore GSE to shreds and scattered stars from both galaxies in a scattered halo surrounding the galaxy. The interactions between the two also caused clusters of stars in the halo. This changed the shape of the halo significantly. And since GSE came in at an angle, the collision also tipped it. The amazing thing is that the shape is still weird and offset.
One would think that the halo would have made itself “spherical” after billions of years. However, the stars remain in this strange three-axis “cloud”. According to Charlie Conroy, there is something else at play that turns out to be dark matter. “The tilted star halo strongly suggests that the underlying dark matter halo is also tilted,” he said. “A tilt of the dark matter halo could have significant implications for our ability to detect dark matter particles in laboratories on Earth.”
This is interesting for scientists looking for dark matter. If there really is a tilt in the dark matter halo, there could be areas where this mysterious material is more concentrated. Finding these regions could give astronomers the opportunity to detect interactions with dark matter. This would be particularly interesting as the Earth moves through them in the future.
More about the Stellar Halo
The Milky Way isn’t the only one with a halo. Every galaxy has one that is dominated by dark matter. While we don’t see dark matter, it provides a framework for the distribution of ordinary, visible matter. These include stars, clusters and nebulae in the galaxy’s body, and stars in the halo.
According to Han, the starry outer “hulls” are important targets for observation. “The stellar halo is a dynamic tracer of the galactic halo,” he said. “To learn more about galactic halos in general, and the galactic halo in particular and the history of our own galaxy, the stellar halo is a great place to start.”
To tell the story of the Milky Way’s collision with GSE, the team examined two large astronomical datasets. That allowed them to create a computer model of what had happened. One is from the GAIA spacecraft, which is measuring the positions, motions and distances of millions of Milky Way and halo stars. The other dataset is from a ground-based survey called H3 (short for Hectochelle in the Halo at High Resolution). The combined results showed that the oddly sloping football shape emerged almost immediately.
The team’s work will eventually help them address astrophysical questions about the Milky Way. “These are such intuitively interesting questions about our galaxy: ‘What does the galaxy look like?’ and ‘What does the star halo look like?’” Han said. “Especially with this research and study direction, we are finally answering these questions.”
The Tilt of Our Stars: The shape of the Milky Way’s halo of stars is realized
The galaxy’s stellar halo is tilted and doubly refracted
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