Milky Way’s halo of stars isn’t the pure sphere astronomers expected it to be: ScienceAlert

Milky Way’s halo of stars isn’t the pure sphere astronomers expected it to be: ScienceAlert

Step outside the Milky Way for a moment and you might spot the bright disc of stars we call home has a strange deformity. Now it seems that the rest of our galaxy is also a little more unstable.

A new map of the stars above and below the galactic plane shows its galactic halo – the diffuse ball of gas, dark matter, and stars that surround spiral galaxies – is also strange. Instead of the nice round sphere that astronomers expected, the Milky Way’s halo is a wobbly ellipsoid whose three axes are of different lengths.

“For decades, the general assumption has been that the stellar halo is more or less spherical and isotropic, or the same in every direction,” says astronomer Charlie Conroy of the Harvard-Smithsonian Center for Astrophysics (CfA).

“We now know that the textbook picture of our galaxy embedded in a spherical volume of stars must be thrown out.”

Determining the shape of our galaxy is really difficult. Imagine trying to figure out the shape of a huge lake while you’re bobbing in the middle of it. Only in recent years, with the launch of the European Space Agency’s Gaia telescope in 2013, have we gained a detailed understanding of the three-dimensional shape of our galaxy.

Gaia shares Earth’s orbit around the Sun. Changes in the telescope’s position in the solar system allow it to measure parallax of objects in the Milky Way, obtaining the most accurate measurements yet to calculate the positions and motions of thousands of distant stars.

Thanks to this data, we now know that the disk of the Milky Way is twisted and bent. We also know that the Milky Way has repeatedly dealt with acts of galactic cannibalismone of the most famous of which seems to have been a collision with a galaxy we call Gaia Sausageor Gaia Enceladus, about 7 to 10 billion years ago.

This collision, according to scientists, created the star halo of the Milky Way. The Gaia sausage broke apart when it encountered our galaxy, its own population of stars scattered across the Milky Way’s halo.

Led by CfA astronomer and PhD student Jiwon “Jesse” Han, a team of scientists set out to better understand the galactic halo and Gaia Sausage’s role in it.

“The stellar halo is a dynamic marker of the galactic halo,” Khan says. “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.”

Unfortunately, Gaia’s data on the chemical abundances of halo stars beyond certain distances are not very reliable. Stellar populations can be linked together by their chemical abundances, making it important information for mapping the relationship between stars in the halo.

So the researchers added data from a study called Hectochelle in the Halo at High Resolution, or H3; a ground-based survey that has collected, among other features, data on the chemical abundances of thousands of stars in the Milky Way’s stellar halo.

With these data, the researchers inferred the stellar population density profile of the Milky Way’s halo. They found that the best fit for their data was a football-shaped halo tilted 25 degrees to the galactic plane.

The supposed dimensions of the halo. (Han et al., AJ, 2022)

This corresponds to previous studies who found that the stars in the halo of the Milky Way occupy a triaxial ellipsoidal formation (although the specifics vary slightly). It also fits the theory that the Gaia sausage created, or at least played a huge role in creating, the Milky Way halo. The distorted shape of the halo suggests that the two galaxies collided at an angle.

The researchers also found two clusters of stars at significant distances from the galactic center. They found that these collections represent the apocenters of the stars’ initial orbits around the galactic center—the farthest distance the stars travel in their elongated elliptical orbits.

Just like an orbital body accelerates On reaching the point closest to the center of gravity or “pericenter”, the apocenter is a deceleration point. When the Gaia sausage met the Milky Way, its stars were thrown into two wild orbits, slowing down in the apocenters—to the point of stopping and simply making that place its new home.

However, that was a long time ago, long enough that the strange shape should have long since resolved itself, settling back into a sphere. The strong tilt suggests that the dark matter halo bounding the Milky Way—a mysterious mass responsible for the excess gravity in the universe—is also highly tilted.

So while it looks like we have some new and exciting answers, we also have some new and exciting questions. Ongoing and future studies, the researchers said, should provide even stronger constraints on the shape of the halo to help understand how our galaxy evolved.

“These are such intuitively interesting questions about our galaxy: ‘What does the galaxy look like?’ and ‘What does the star halo look like?'” Khan says.

“With this line of research, and a study in particular, we’re finally answering those questions.”

The study was published in The Astronomical Journal.

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