Step outside the Milky Way for a moment and you can see the bright disk of stars we call home has an amazing path. Now it seems that our entire galaxy is small again.
A new map of the stars above and below the plane of the galaxy shows its galactic halo — the sprawling world of gas, dark matter, and stars surrounding the surrounding galaxies — is also wonky. Instead of having the perfect round shape that astronomers expected, the Milky Way halo is an elongated ellipsoid whose three axes are of different lengths.
“For many years, many theories have been that the star’s halo is more or less circular, or the same on all sides,” says astronomer Charlie Conroy of the Harvard & Smithsonian Center for Astrophysics (CfA).
“Now we know that the image of our galaxy inside the orbiting stars has to be discarded.”
Determining the shape of our galaxy is very difficult to do. Imagine that you are trying to see what a large lake looks like while you are walking around in the middle of the lake. It is only in recent years, with the launch of the European Space Agency’s Gaia telescope in 2013, that we have gained a more detailed understanding of the three-dimensional structure of our galaxy.
Gaia shares the Earth’s orbit around the Sun. The adjustment of the telescope’s position in the Solar System makes it possible to measure the parallax of objects in the Milky Way, obtaining the most accurate measurements to date of calculating the positions and movements of thousands of distant stars.
Thanks to this data, we now know that the Milky Way’s disk is curved and curved. We also know that the Milky Way has repeatedly engaged in bouts of galactic cannibalism, one of the most notable of which appears to have collided with the galaxy we call Gaia Sausage, or Gaia Enceladus, 7 to 10 billion years ago.
Scientists believe that this collision caused the light from the stars in the Milky Way. Gaia’s sausage was torn apart when it collided with our galaxy, its separate stars scattered in the light of the Milky Way.
Led by astronomer and PhD student Jiwon “Jesse” Han of CfA, the team of scientists set out to better understand the galactic halo and Gaia Sausage’s role in it.
“The star reflects the light of the star,” says Han. “To learn more about all galaxies, especially our galaxy and its history, stargazing is a good place to start.”
Unfortunately Gaia’s findings on the chemical composition of halo stars beyond a certain distance are not very reliable. Clusters of stars can be linked together by their chemical abundances, making them useful information for mapping the relationships between halo stars.
So the researchers added data from a probe called Hectochelle in the Halo at High Resolution, or H3; ground-based research that has collected, among other indicators, a large amount of chemical data on thousands of stars in the Milky Way galaxy.
With this information, the researchers mapped the star density history of the Milky Way’s halo. They found that the best fit to their data was a ball-shaped halo, tilted 25 degrees relative to the galactic plane.
This is consistent with previous studies that found that the stars in the Milky Way’s halo have a triaxial ellipsoid shape (although the reality is slightly different). It also supports the theory that Gaia’s Sausage created, or was instrumental in creating, the Milky Way’s halo. The skewiff shape of the halo suggests that the two galaxies collided at an angle.
The researchers also found two clusters of stars far away from the center of the galaxy. These clusters, they found, represent the apocenters of the stars’ primary orbits around the galactic center — the most distant distances through which the stars travel in their long, circular paths.
As a rotating body moves get very close to its center of gravity, or ‘pericenter’, the apocenter is the lowest point. When the Sausage of Gaia met the Milky Way, its stars were thrown into two wild paths, moving slowly on the apocenters – until they stopped, and only made that place their new home.
However, this was a very long time, long enough that the odd shape had to settle already, settle back into the line. The strong tilt indicates that the light from the dark matter that makes up the Milky Way – the mysterious mass that causes the universe’s gravity – is also highly tilted.
So, while it seems we have new and exciting answers, we also have new and exciting questions. Ongoing and future studies, the researchers said, should provide stronger constraints on the shape of the halo to help determine how our galaxy evolved.
“These are fascinating questions about our galaxy: ‘What does this galaxy look like?’ and ‘What does the star’s halo look like?’,” says Han.
“With this line of research and study in particular, we’re answering those questions.”
Research has been published in The Astronomical Journal.
