Halos of stars in the Milky Way aren’t the neat orb astronomers expected

Step outside the Milky Way and you might notice that the bright disk of stars we call home has a weird curvature. Now it seems that the rest of our galaxy is also a bit out of balance.

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

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

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

Determining the shape of our galaxy is really hard to do. Imagine trying to figure out the shape of a vast lake while floating 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 the parallax of objects in the Milky Way, providing the most accurate measurements yet for calculating the positions and motions 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 galactic cannibalism, one of the most prominent of which appears to have been a collision with a galaxy we call the Gaia Sausage or Gaia Enceladus about 7 to 10 billion years ago.

Scientists believe this collision created the Milky Way’s stellar halo. The Gaia sausage tore apart when it encountered our galaxy, its separate population of stars scattered throughout 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 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 and history of our own galaxy in particular, the stellar halo is a great place to start.”

Unfortunately, Gaia’s data on the chemical quantities of halo stars beyond certain distances isn’t too reliable. Stellar populations can be linked by their chemical abundance, making it important information for mapping the relationship between the halo’s stars.

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

With this data, the researchers derived 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 derived dimensions of the halo. (Han et al., AJ, 2022)

This is consistent with previous studies that found the stars in the Milky Way’s halo occupy a triaxial ellipsoidal formation (although the details vary a bit). It also fits the theory that the Gaia Sausage created the halo of the Milky Way, or at least played a major role in creating it. The oblique shape of the halo suggests that the two galaxies collided at an angle.

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

Just like a body going faster in orbit reaching the point closest to the point of attraction, or “pericenter”, the apocenter is a point of delay. When the Gaia Sausage met the Milky Way, the stars were flung in two wild trajectories, slowed down at the apocenters – to the point of stopping, and simply made that location their new home.

This was a long time ago, though, long enough that the odd shape should have dissolved itself and settled back into a sphere long ago. The strong tilt suggests that the halo of dark matter binding the Milky Way — a mysterious mass responsible for excess gravity in the Universe — is also strongly tilted.

So while it may seem 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 halo’s shape to figure out 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.

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

The research has been published in The astronomical magazine.

Leave a Reply

Your email address will not be published. Required fields are marked *