Step out of the Milky Way for a moment, and you’ll notice that the bright disk of stars we call home has a strange warp. Now it looks like the rest of our galaxy is a bit off as well.
A new map of the stars above and below the galactic plane shows that their galactic halo, the diffuse globe of gas, dark matter and stars that surrounds spiral galaxies, is also unstable. Instead of the nice round sphere astronomers expected, the Milky Way’s halo is an oscillating ellipsoid whose three axes have different lengths.
“For decades, the general assumption has been that the stellar halo is roughly 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 textbook image of our galaxy embedded within a spherical volume of stars must be discarded.”
Determining the shape of our galaxy is really hard to do. Imagine trying to figure out the shape of a large lake while bobbing in the middle of it. Only in the last few 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. The telescope’s position changes in the Solar System allowing it to measure the parallax of objects in the Milky Way, obtaining the most accurate measurements to date for calculating the positions and motions of thousands of distant stars.
Thanks to these data, we now know that the Milky Way’s disk is warped and bent. We also know that the Milky Way has repeatedly engaged in acts of galactic cannibalism, one of the most prominent seems to have been a collision with a galaxy we call Gaia Sausage, or Gaia Enceladus, around 7 to 10 billion years ago.
This collision, scientists believe, created the stellar halo of the Milky Way. The Gaia Sausage was torn apart when it encountered our galaxy, its distinct 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 tracer of the galactic halo,” Han says. “To learn more about galactic halos in general, and especially about the galactic halo and the history of our own galaxy, the stellar halo is a great place to start.”
Unfortunately, Gaia’s data on the chemical abundance of halo stars beyond certain distances are not very reliable. Stellar populations can be linked by their chemical abundance, making 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 terrestrial study that has collected, among other characteristics, chemical abundance data on thousands of stars in the stellar halo of the Milky Way.
Using these data, the researchers inferred the stellar population density profile of the Milky Way halo. They found that the best fit for their data was a football-shaped halo, tilted 25 degrees to the galactic plane.
It also fits with the theory that Gaia Sausage created, or at least played a huge role in creating, the halo of the Milky Way. The skewed shape of the halo suggests that the two galaxies collided at an angle.
The researchers also found two clumps of stars at significant distances from the galactic center. They found that these collections represent the apocenters of the initial stellar orbits around the galactic center, the farthest distance the stars travel in their elongated elliptical orbits.
Just as an orbiting body accelerates towards the point closest to its center of attraction, or ‘pericentre’, the apocenter is a point of deceleration. When 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 simply making that place their new home.
However, this was a long time ago, long enough that the strange shape should 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 highly tilted.