Around the Milky Way, there is a region with a diffuse population of stars known as the halo. It is much bigger than the disk where the spiral arms, including the one with our Solar System in it, reside. This vast low-density region has been thought for a very long time to be a sphere, but new research shows that that idea is not exactly correct.
According to data collected as part of the H3 Survey and by the Gaia spacecraft, the halo is not a sphere. It is more oblate and tilted, resembling a kicked ball. The team tracked 5,559 stars that, while now part of the stellar halo of the Milky Way, were once another galaxy known as the Gaia-Sausage-Enceladus (GSE). This galaxy was cannibalized by our own, through two orbits and at an angle that created the peculiar shape: a triaxial ellipsoid.
The stellar halo is a tracer for the wider galactic halo, which is believed to be dominated by the gravitational pull of dark matter. Dark matter is invisible, so by studying the stellar halo, astronomers hope to gain insights into this hypothetical substance.
"The shape of the stellar halo is a very fundamental parameter that we've just measured to greater accuracy than was possible before," lead author Jiwon "Jesse" Han, from the Center for Astrophysics, Harvard & Smithsonian, said in a statement. "There are a lot of important implications of the stellar halo not being spherical but instead shaped like a football, rugby ball, or zeppelin – take your pick!"
"For decades, the general assumption has been that the stellar halo is more or less spherical and isotropic, or the same in every direction," added study co-author Charlie Conroy, a professor of astronomy at Harvard University and the Center for Astrophysics. "We now know that the textbook picture of our galaxy embedded within a spherical volume of stars has to be thrown out."
The collision between the Milky Way and the GSE took place around seven billion years ago. Given the elapsed time, researchers expected that the stellar halo would have settled into the assumed spherical configuration. The fact that it hasn’t, is very intriguing.
"The tilted stellar halo strongly suggests that the underlying dark matter halo is also tilted," said Conroy. "A tilt in the dark matter halo could have significant ramifications for our ability to detect dark matter particles in laboratories on Earth."
The insights into the shape of the galactic halo are published in The Astronomical Journal.