Science News reports that astronomers have found the dark edge of the Milky Way – the unlit mass of dark matter spinning beyond the bright stars – and have determined that our galaxy is 1.9 million light years across:
Astronomers have long known that the brightest part of the Milky Way, the pancake-shaped disk of stars that houses the sun, is some 120,000 light-years across. Beyond this stellar disk is a disk of gas. A vast halo of dark matter, presumably full of invisible particles, engulfs both disks and stretches far beyond them. But because the dark halo emits no light, its diameter is hard to measure.
Now, Alis Deason, an astrophysicist at Durham University in England, and her colleagues have used nearby galaxies to locate the Milky Way’s edge. The precise diameter is 1.9 million light-years, give or take 0.4 million light-years, the team reports February 21 in a paper posted at arXiv.org.
To put that size into perspective, imagine a map in which the distance between the sun and the Earth is just one inch. If the Milky Way’s heart were at the center of the Earth, the galaxy’s edge would be four times farther away than the moon actually is.
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The simulations showed that just beyond the edge of a giant galaxy’s dark halo, the velocities of small nearby galaxies drop sharply.
Using existing telescope observations, Deason and her colleagues found a similar plunge in the speeds of small galaxies near the Milky Way. This occurred at a distance of about 950,000 light-years from the Milky Way’s center, marking the galaxy’s edge, the scientists say. The edge is 35 times farther from the galactic center than the sun is.