Just how massive is the Milky Way?

Led by Ekta Patel, a fourth-year graduate at the University of Arizona, the study is the first of its kind to utilize the 3D motions of satellite galaxies and compare their angular momenta to a simulated universe, ultimately concluding the Milky Way tips the scales at 0.96 trillion times the mass of the Sun. Unlike previous studies that have used the positions and velocities of satellite galaxies, this new study exploits the lack of net change between the two. Since the angular momentum of a system stays constant over time, this novel method allows the researchers to remove some of the uncertainty that plagues other approaches, paving the way for more reliable results.

“Think of a figure skater doing a pirouette,” says Patel in a press release. “As she draws in her arms, she spins faster. In other words, her velocity changes, but her angular momentum stays the same over the whole duration of her act.”
 

Presented at the 232nd American Astronomical Society in Denver, the study uses satellite galaxies’ data from the Hubble Space Telescope. In order to arrive at the 0.96-trillion-solar-mass estimate, the study compared angular momenta of nine satellite galaxies to those of a simulated universe of 20,000 galaxies just like our own. This comparison helped chart nine probability distributions — possible ranges of values for our galaxy’s mass — whose ensemble resulted in the estimate of 0.96 trillion solar masses.

“Our method allows us to take advantage of measurements of the speed of multiple satellite galaxies simultaneously to get an answer for what cold dark matter theory would predict for the mass of the Milky Way’s halo in a robust way,” says co-author Gurtina Besla in a press release..

It is not uncommon for researchers to use information from satellite galaxies to measure the mass of the Milky Way. Since we are unable to see the entire galaxy, we rely on its interactions with neighboring galaxies. The Milky Way is the proud owner of at least 50 such galaxies — called the Local Group — each encompassing its own abundance of stars.

However, not all of these are well understood. Except for the Magellanic Clouds, which are clearly visible to the naked eye, all other satellite galaxies are extremely hard to detect even with telescopes, making it difficult to determine if they exist at all. A satellite galaxy’s luminosity is often used to estimate its mass, but the orbital motions do not always comply with the results obtained from the former method. In order to explain this imbalance between what we can detect and the invisible mass in our universe, researchers turn to the cold dark matter theory.

The theory proposes that dark matter is made of heavy, slow-moving particles that account for roughly 85% of the universe’s matter. This type of dark matter weakly interacts with visible matter to form small clumps, which are later drawn together to form larger bodies.