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A whole new way to weigh the Milky Way

A first-of-its-kind study has used the angular momentum of satellite galaxies to weigh the Milky Way, providing a test for cold dark matter theory along the way.
In this image, taken near Roma, Queensland, Australia, The Milky Way streaks up from the horizon while comet C/2011 W3 (Lovejoy) frames it on the left. To the right are the two best-known satellite galaxies of the Milky Way, the Large and Small Magellanic Clouds
Naskies/Wikimedia Commons
Estimating the Milky Way's mass is quite tedious, especially since we live within it. While seeing the entire galaxy would help tremendously, our vantage point on the outskirts of the galactic disk does not allow for that. Most of the Milky Way's luminous matter resides in the galaxy's core, where dense clouds of gas and dust make any kind of accurate measurement cumbersome. Orbital motions of the stars have to be calculated, the motion, growth, and distortion of orbiting satellite galaxies accounted for, and our galaxy’s own evolution has to be factored in. And that doesn't even consider the influence of elusive dark matter yet.

From utilizing gravitational lensing to studying hypervelocity stars, researchers have experimented with a number of methods to pin down our galaxy’s mass. But given our limited knowledge, it's not surprising that the results have varied from a few hundred billion solar masses to a monstrous two trillion solar masses. Now, a new study published in the Astrophysical Journal taps into the angular momenta of the Milky Way’s satellite galaxies to weigh our home galaxy.
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.
This visualization, created as part of the Illustris project, shows how dark matter (blue) permeates the universe and causes regular matter (red) to clump together.
Dark matter is thought to have played an important role in galaxy formation, influencing how they evolved into their present-day structures, and why they tend to form clusters. Though the theory is widely accepted, it still lacks substantial experimental evidence; and a precise measurement of Milky Way’s mass will provide a testing ground for this theory.

“Currently, we know of about 50 satellite galaxies, but simulations implementing the cold dark matter theory suggest that there could be [roughly] 100-200 satellite galaxies depending on the exact mass. The gap between these measurements right now is largely due to our ability to detect (or rather not detect) these very faint dwarf galaxies that orbit our galaxy. Knowing the precise mass estimate for the Milky Way can help us determine how well this theory actually applies,” explains Patel.

The study makes use of the Illustris-Dark simulation, a branch of the Illustris Project that encompasses the evolution of dark matter particles for 13.8 billion simulated years. Using simulated analogs from Illustris-Dark, the study dives into exploring the relationship between the mass of Milky Way’s halo — which is made up of dark matter and theorized to dominate our galaxy’s mass — and the angular momenta of satellite galaxies. Though the method has an error bar of 30 percent, the strong relationship that is discovered between host halo mass and angular momenta of satellite galaxies gives this method the potential to be extended to larger datasets.

“Our method does not necessarily provide the most accurate or precise Milky Way mass estimate to date, but the novelty is the method, which we hope will be extremely useful in upcoming years as the observational data sets and the simulations we work with continue to grow and improve,” says Patel.
This stunning panorama taken in the Chilean Atacama Desert shows the Milky Way hovering just above the horizon, while the Large and Small Magellanic Clouds (satellite galaxies of the Milky Way) glow brightly to the left.
ESO/Y. Beletsky
The study uses only nine of the brightest 50 known galaxies, but the unique approach does set the stage for more accurate measurements. Though the results from this study must be taken with a grain of salt, as there is a rather large uncertainty, the precision obtained using this method can only continue to improve as information about satellite galaxies is added in greater detail.

“This data is being measured with space missions like the Gaia spacecraft, so I think we're not that far from the most precise Milky Way mass estimate to date and our method provides one way to obtain that,” explains Patel.

Moreover, when missions like the Large Synoptic Survey Telescope detect fainter and farther galaxies in the future, studies like these will provide a testing ground for our cosmological models — the cold dark matter theory, helping us unlock one of the many cosmic wonders.


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