The halo of stars in the Milky Way is not the neat ball that astronomers expected

Step outside the Milky Way for a moment and you might notice that the bright disk of stars we call home has a strange curve to it. Now it appears that the rest of our galaxy is a bit far away.

A new map of stars above and below the galactic plane shows that the galaxy’s halo — the diffuse globe of gas, dark matter and stars that surround spiral galaxies — is also wonky. Instead of the nice round ball that astronomers had expected, the Milky Way’s halo is a like ellipsoid with all three axes of different lengths.

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

“Now we know that the textbook image of our galaxy embedded in a spherical volume of stars must be discarded.”

It’s really hard to say what our galaxy looks like. Imagine trying to figure out what a vast lake looks like while 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 participates in the Earth’s orbit around the Sun. Changes in the telescope’s position in the solar system allow it to measure the divergence 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 warped and curved. We also know that the Milky Way has repeatedly engaged in acts of galactic cannibalism, one of the most notable of which seems to have been a collision with a galaxy we call Gaia Sausage, or Gaia Enceladus, about 7 to 10 billion years ago.

Scientists believe this collision created the Milky Way’s stellar halo. Gaia Sausage was torn as it encountered our galaxy, its distinct population of stars scattered across the Milky Way’s halo.

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

“The stellar halo is a galactic halo dynamic tracker,” Hahn says. “In order to learn more about galactic halos in general, and especially our galaxy’s halo and its history, 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 overly reliable. Star clusters can be linked to each other by their chemical abundances, which makes them important information for mapping the relationship between halo stars.

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, among other properties, chemical abundance data for thousands of stars in the Milky Way’s stellar halo.

Using this data, the researchers inferred the stellar cluster 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 with respect to the galactic plane.

Inferred halo dimensions. (Han et al., AJ, 2022)

This fits with previous studies that found stars in the Milky Way’s halo to occupy a triaxial elliptical configuration (although the details are slightly different). It also fits with the theory that Gaia’s sausage created, or at least played a large role in, the formation of the Milky Way’s halo. The halo’s skewed shape indicates that the two galaxies collided at an angle.

The researchers also found a heap of stars at great distances from the galactic center. They found that these clusters represent the central constellations of the protostellar orbits around the galactic center – the furthest distance traveled by the stars in their elongated, elliptical orbits.

Just as a body in orbit accelerates Reaching the point closest to its center of gravity, or “circumference”, the apocenter is the slowing point. When Gaia Sausage met the Milky Way, its stars darted into two wild orbits, slowed at the central constellations – almost to a halt, and made this location their new home.

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

So while we seem to have some new and exciting answers, we also have some new and exciting questions. The researchers said that ongoing and future surveys should provide stronger constraints on the shape of the halo to help figure out how our galaxy has evolved.

These are interesting intuitive questions to ask about our galaxy: “What does the galaxy look like?” and “What does a stellar halo look like?”, Hahn says.

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

Research published in The Astronomical Journal.

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