Do You Want To Weigh A Galaxy? Now There’s A New Way

The Milky Way from the Australian outback. Structuresxx

Weighing a galaxy is not a problem you might encounter every day, but it is often quite the problem for astronomers. The mass of a galaxy is important for deriving certain properties, and current methods have a lot of uncertainties.

A new method is now being proposed to measure the mass of a galaxy, in particular our own. Researchers from the University of Arizona and the University of Cambridge have tried to solve the problem by using the smaller galaxies surrounding the Milky Way as a way to estimate its mass. Their efforts are reported in The Astrophysical Journal.

These satellite galaxies orbit the Milky Way, so the mass of our galaxy strongly affects their motion. Usually, researchers use the satellite galaxies' position and velocity to make the mass estimate, but this research team used a related but different quantity: angular momentum. Since the galaxies follow an elliptical orbit, their velocity will be slightly different depending on if they are nearer or further away. Angular momentum is unaffected.

"Think of a figure skater doing a pirouette," lead author Ekta Patel, from the University of Arizona, said in a statement. "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."

The team calculated the full three-dimensional motion of 50 satellites of the Milky Way and combined that information with statistical models and simulations. They used virtual universes with 20,000 “host galaxies” as proxies for the Milky Way. These were surrounded by 90,000 satellite galaxies. This approach helped them estimate the mass of the Milky Way to be 960 billion times the mass of the Sun. The team claim that this method has less uncertainty compared to previous estimates of the Milky Way’s mass.

As new observatories come online, more and more ultra-faint galaxies that surround the Milky Way become known to astronomers. The team hope to use this data to make more robust measurements of the mass of the Milky Way and to study properties like the dark matter content of our galaxy.

"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," co-author Gurtina Besla added. "It is perfectly suited to take advantage of the current rapid growth in both observational datasets and numerical capabilities."

This research was presented at the 232nd meeting of the American Astronomical Society in Denver.

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