The Milky Way's stellar halo is not the neat sphere astronomers expected

The Milky Way’s stellar halo is not the neat sphere astronomers expected

If you step out of the Milky Way for a moment, you might notice that the bright disk of stars we call home has an odd curvature. Now it seems that the rest of our galaxy is a bit out of whack as well.

A new map of stars above and below the galactic plane shows that their galactic halo — the diffuse sphere of gas, dark matter, and stars that surrounds spiral galaxies — is also wonky. Instead of the beautiful round sphere astronomers expected, the Milky Way’s halo is a wobbly ellipsoid with three axes 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 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 jettisoned.”

Determining the shape of our galaxy is really difficult. Imagine trying to figure out the shape of a huge lake while bobbing around in its middle. 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 the parallax of objects in the Milky Way, giving the most accurate measurements yet used to calculate the positions and motions of thousands of distant stars.

Thanks to this data, we now know that the Milky Way’s disk is warped and curved. We also know that the Milky Way has repeatedly been involved in acts of galactic cannibalism, one of the most prominent of which seems to have been a collision with a galaxy we call the Gaia Sausage, or Gaia Enceladus, some 7 to 10 billion years ago.

Scientists believe this collision created the Milky Way’s stellar halo. The Gaia sausage ripped apart as it impacted our galaxy, its distinct stellar population scattering through the Milky Way’s halo.

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

“The stellar halo is a dynamic tracer of the galactic halo,” says Han. “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, beyond certain distances, Gaia’s data on the chemical abundance of halo stars are not very reliable. Star populations can be linked by their chemical abundances, making them important information for mapping the relationship between the stars of the halo.

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

From this data, the researchers inferred the density profile of the stellar population in the halo of the Milky Way. They found that the best fit for their data was a football-shaped halo tilted 25 degrees with respect to the galactic plane.

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

This ties in with previous studies that found that the stars in the Milky Way’s halo form a triaxial ellipsoid formation (although the specifics vary a bit). It also fits with the theory that the Gaia Sausage created the Milky Way’s halo, or at least played a large role in its creation. The oblique shape of the halo suggests the two galaxies collided at an angle.

The researchers also found two clusters of stars at a considerable distance from the galactic center. They found that these clusters represent the apocenters of the initial stellar orbits around the galactic center — the furthest distance stars travel in their elongated, elliptical orbits.

Just like a rotating body accelerates Reaching the point closest to its center of attraction or “pericenter”., the apocenter is a point of deceleration. When the Gaia Sausage hit the Milky Way, its stars were thrown into two wild orbits, slowing down at the apocenters – to the point where they stopped and simply making that place their new home.

However, that was a very long time ago, long enough that the strange shape should have dissipated and settled back into a sphere long ago. The strong tilt suggests that the dark matter halo surrounding the Milky Way – a mysterious mass responsible for the excessive gravity in the universe – is also strongly tilted.

So while it seems 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 uncover how our galaxy evolved.

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

“Especially with this research and study direction, we are finally answering these questions.”

The research was published in The Astronomical Journal.

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