科学美国人60秒:Milky Way Makes Small Massive Gain

Weighing the Milky Way is no easy task. It’s not like you can just step outside the galaxy and place it on a humongous scale.

Plus, most of the galaxy’s heft is in invisible dark matter, which betrays its presence only through its gravitational pull. So astronomers have devised clever ways of inferring the mass of our galaxy by measuring how it pulls on smaller objects moving around it.

A new estimate, based on its effect on the motion of a nearby dwarf galaxy, has the Milky Way a bit beefier than some past measurements that relied on the motions of massive stars. The study is in the Astrophysical Journal. [Michael Boylan-Kolchin et al., The Space Motion of Leo I: The Mass of the Milky Way's Dark Matter Halo] (free preprint)

The researchers first used the Hubble Space Telescope to track the dwarf galaxy Leo I, which at 850,000 light-years away is one of the most distant of the satellite galaxies swarming around the Milky Way. Then they simulated galaxies able to host such an object. Turns out that anything less massive than one trillion suns is very unlikely. In fact, the researchers estimate that the Milky Way’s dark matter has as much mass as 1.6 trillion suns. That’s some pretty heavy-duty astrophysics.

—John Matson