The Milky Way's star halo isn't the neat sphere astronomers expected

The Milky Way’s star halo isn’t the neat sphere astronomers expected

Step out of the Milky Way for a moment and you might notice that the luminous disk of stars we call home has a strange distortion. Now, it looks like the rest of our galaxy is a little off as well.

A new map of stars above and below the galactic plane shows that its galactic halo – the diffuse globe of gas, dark matter and stars that surrounds spiral galaxies – is also wobbly. Rather than the beautiful round sphere expected by astronomers, the Milky Way’s halo is an oscillating ellipsoid whose three axes are all 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 all directions,” says astronomer Charlie Conroy of the Harvard & Smithsonian Center for Astrophysics (CfA).

“We now know that the classic image of our galaxy embedded in a spherical volume of stars must be discarded.”

Determining the shape of our galaxy is really hard to do. Imagine trying to determine the shape of a large lake as you walk through the middle of it. It is only in recent years, with the launch of the European Space Agency’s Gaia telescope in 2013, that we have gained a detailed understanding of the three-dimensional shape of our galaxy.

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

Thanks to this data, we now know that the disk of the Milky Way is distorted and bent. We also know that the Milky Way has repeatedly engaged in acts of galactic cannibalism, one of the most significant of which appears to have been a collision with a galaxy we call Gaia Sausage, or Gaia Enceladus, about 7 to 10 billion years old.

This collision, according to scientists, created the stellar halo of the Milky Way. The Gaia sausage split apart when it encountered our galaxy, its distinct population of stars scattering 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 tracer of the galactic halo,” says Han. “In order to learn more about galactic halos in general, and in particular about the galactic halo and the history of our own galaxy, the stellar halo is an excellent starting point.”

Unfortunately, Gaia’s data on the chemical abundance of halo stars beyond certain distances is not very reliable. Stellar populations can be related by their chemical abundance, making this important information for mapping the relationship between stars in the halo.

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

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

The dimensions deduced from the halo. (Han et al., AJ, 2022)

This matches previous studies that have found stars in the Milky Way’s halo to occupy a triaxial ellipsoid formation (although the specifics vary a bit). It also fits with the theory that the Gaia sausage created, or at least played a huge role in creating, the Milky Way’s halo. The oblique shape of the halo suggests that the two galaxies collided at an angle.

The researchers also found two star stacks at significant distances from the galactic center. These collections, they found, represent the apocenters of the initial stellar orbits around the galactic center – the furthest distance that stars travel in their elongated, elliptical orbits.

Just as a body in orbit accelerates on reach the point closest to its center of attraction, or “pericenter”, the apocenter is a slowing point. When the Gaia sausage encountered the Milky Way, its stars were thrown into two wild orbits, slowing down at the apocenters – to the point of stopping and making this place their new home.

However, that was a long time ago, long enough for the strange shape to have resolved long ago, settling back into a sphere. The steep tilt suggests that the halo of dark matter that binds the Milky Way – a mysterious mass responsible for excess gravity in the Universe – is also steeply tilted.

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

“These are intuitively interesting questions to ask about our galaxy: ‘What does the galaxy look like?’ and ‘What does the stellar halo look like?’ “, says Han.

“With this line of research and study in particular, we are finally answering these questions.”

The research has been published in The Astronomical Journal.

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