Weighing the Milky Way

The vast majority of a galaxy’s mass (mostly dark matter) is located in its halo, a vast, surrounding region containing few, if any, stars and whose shape is largely unknown.

In a widely accepted cosmological model, dark-matter filaments span the entire universe, drawing luminous (“regular”) matter with them. Where they intersect, gas and dust accumulate and coalesce into galaxies.

Over billions of years, small galaxies merge to form into larger ones, and as those grow in size and their gravitational pull reaches farther and farther into space, they attract a zoo of other small galaxies, which then become satellite galaxies. Their host galaxy determines their orbits, much like the sun’s gravitational pull directs the movement of planets and bodies in the solar system.

“We now know that the universe is expanding,” says Ekta Patel, a fourth-year graduate student in the astronomy department and Steward Observatory at the University of Arizona. “But when two galaxies come close enough, their mutual attraction is greater than the influence of the expanding universe, so they begin to orbit each other around a common center, like our Milky Way and our closest neighbor, the Andromeda Galaxy.”

Although Andromeda is approaching the Milky Way at 110 kilometers per second, the two won’t merge until about 4.5 billion years from now. According to Patel, tracking Andromeda’s motion is “equivalent to watching a human hair grow at the distance of the moon.”

Because it’s impossible to “weigh” a galaxy simply by looking at it—much less when the observer happens to be inside of it, as is the case with our Milky Way—researchers deduce a galaxy’s mass by studying the motions of celestial objects as they dance around the host galaxy, led by its gravitational pull.

Such objects—also called tracers, because they trace the mass of their host galaxy—can be satellite galaxies or streams of stars created from the scattering of former galaxies that came too close to remain intact.

Unlike previous methods commonly used to estimate a galaxy’s mass, such as measuring its tracers’ velocities and positions, the approach Patel and her coauthors developed uses their angular momentum, which yields more reliable results because it doesn’t change over time.

The angular momentum of a body in space depends on both its distance and speed. Since satellite galaxies tend to move around the Milky Way in elliptical orbits, their speeds increase as they get closer to our galaxy and decrease as they get farther away. Because the angular momentum is the product of both position and speed, there is no net change regardless of whether the tracer is at its closest or farthest position in its orbit.