Galaxies are thought to be composed of visible matter—like stars, planets, and dust—and dark matter, an invisible substance that is believed to make up most of the mass in the universe. But a small, distant galaxy is challenging everything we thought we knew about galaxy formation.
The galaxy, charmingly named NGC1052-DF2, is about 65 million light-years away and is about one two-hundredths the mass of our own Milky Way galaxy. Based on what we know of other galaxies of similar mass, it should contain almost one hundred times more dark matter than visible matter. Yet according to Pieter van Dokkum from Yale University, this galaxy has almost none.
Van Dokkum and his colleagues used the Dragonfly Telephoto Array to track the motion of 10 known star clusters to determine how massive the galaxy is. Based on their observations, they found the mass to be what you’d expect to see from the stars alone, suggesting that the galaxy has no dark matter.
If galaxies can form in the absence of dark matter, it really challenges our understanding of galaxy formation. Here’s Nadia Drake reporting for National Geographic:
“In modern galaxy formation theory, our understanding is that galaxies form in a dark matter halo,” says Stanford University’s Risa Wechsler. “There’s a pretty tight relationship between the amount of stars that formed and the dark matter there, at least when the galaxy formed.”
Scientists have various theories on how this galaxy came to be. One idea suggests it’s a result of a galaxy merger, and that the gas concentrated after being ejected. Another idea is that it formed from matter spewed out by quasars. Van Dokkum’s group suggests it may have formed as a large, nearby galaxy concentrated a huge amount of gas, creating a “mini-whirlpool” from where the stars formed. Each of these theories needs to be examined to determine if it could have formed NGC1052-DF2.
Paradoxically, by finding a galaxy with little-to-no dark matter, it further suggests that dark matter does in fact exist. There’s a new idea called emergent gravity that proposes there is no dark matter. This theory postulates that gravity works differently on a galactic scale in a way that mimics the presence of dark matter. If that’s true, then every galaxy would follow these different laws of gravity and look like they have dark matter—without exception. NGC1052-DF2 disproves that theory, providing further evidence to the actual existence of dark matter.
Photo credit: Gemini Observatory/NSF/AURA/Keck/Jen Miller