Step out of the Milky Way for a moment, and you just might get a glimpse of the brilliant disk of stars we call home. it has a strange deformation. 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 its galactic halo: the diffuse globe of gas, dark matter, and the stars surrounding 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 from 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. Changes in the position of the telescope in the Solar System allow it to measure the parallax of objects in the Milky Way, obtaining the most precise measurements to date to calculate the positions and movements of thousands of distant stars.
Thanks to these data, we now know that the disk of the Milky Way is warped and bent. We also know that the Milky Way has been repeatedly involved in acts of galactic cannibalismone of the most prominent of which appears to have been a collision with a galaxy we call the Gaia sausageor 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”, says han. “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.
this fits previous studies who found that the stars in the Milky Way’s halo occupy a triaxial ellipsoid formation (although the details vary somewhat). He also fits 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.
Like a body in orbit speed up in reaching the point closest to its center of attraction, or ‘pericentre’, the apocenter is a point of deceleration. When Gaia Sausage encountered the Milky Way, her stars were thrown into two wild orbits, slowing at the apocenters, to the point of stopping, and simply making that place her 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.
So while it looks like we have some exciting new answers, we also have some exciting new questions. Ongoing and future surveys, the researchers said, should provide even stronger constraints on the shape of the halo to help figure out how our galaxy evolved.
“These are intuitively interesting questions to ask about our galaxy: ‘What is the galaxy like?’ and ‘What does the stellar halo look like?'”, have says.
“With this particular line of research and study, we are finally answering those questions.”
The research has been published in the astronomical journal.
