An unusual galaxy far, far away is stumping astronomers not because of what’s there, but because of what’s missing. About 65 million light-years away, the galaxy called NGC1052-DF2 is dim and diffuse, coming in at about one two-hundredths the mass of our Milky Way. Normally, not all of a galaxy’s mass is visible. In addition to a mix of ordinary matter—like stars and planets and manatees—galaxies are expected to contain dark matter, an invisible substance that makes up most of the mass in the universe. Although we can’t directly observe it, we know dark matter is there because we can see how its gravity affects ordinary matter. Based on the ratio in other galaxies, an isolated galaxy like NGC1052-DF2 should have about a hundred times more dark matter than ordinary matter. But this one appears to have … almost none, scientists report today in Nature.
Using a cluster of lenses called the Dragonfly Telephoto Array, a team led by Yale University’s Pieter van Dokkum took a close look at NGC1052-DF2. By tracking the motion of 10 embedded star clusters, the team could determine how much mass is tucked into the galaxy. And surprisingly, it’s about the same amount of mass they’d expect to see from the galaxy’s stars alone. “We thought the dark matter was not just an optional component of galaxies,” van Dokkum says, noting that the team has found several other similarly perplexing galaxies to scrutinise.
One strange observation doesn’t necessarily break a theory. But finding a galaxy that’s more or less devoid of dark matter certainly suggests a few tantalising things. First, it challenges ideas about how galaxies form. “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 number of stars that formed and the dark matter there, at least when the galaxy formed.” In other words, no dark matter, no galaxy.
Van Dokkum and his colleagues outline several scenarios that could explain how this dark matter-free galaxy formed and evolved, but he doesn’t like any of them. Wechsler is similarly perplexed and says that if it turns out that galaxies can form in the absence of dark matter, it would be stunning. “I would need to rethink what a galaxy is,” she says. Paradoxically, the absence of dark matter in this particular stellar conglomerate also suggests that dark matter does, in fact, exist.
In theories proposing alternatives to dark matter, such as modifications to our understanding of gravity, whatever is mimicking the dark matter signature is not something that can be turned on or off—it should always be there. So, van Dokkum says, “by not detecting the dark matter, we prove it’s real.”
Credit: Nadia Drake for The National Geographic, 28 March 2018.