Three years ago, Filippo Fraternali and his colleagues spotted a half dozen mysteriously diffuse galaxies, which looked like sprawling cities of stars and gas. But unlike almost every other galaxy ever seen—including our own Milky Way—they didn’t seem to be enshrouded in huge masses of dark matter, which would normally hold those stellar metropolises together with their gravity. The scientists picked one to zoom in on, a modest-sized galaxy about 250,000,000 light-years away, and they pointed the 27 radio telescope antennas of the Very Large Array in New Mexico at it.
After gathering 40 hours’ worth of data, they mapped out the stars and gas and confirmed what the earlier snapshots had hinted at: “The dark matter content that we infer in this galaxy is much, much smaller than what you would expect,” says Fraternali, an astronomer at Kapteyn Astronomical Institute of the University of Groningen in the Netherlands. If the team or their competitors find other such galaxies, it could pose a challenge for scientists’ view of dark matter, the dominant perspective in the field for at least 20 years. Fraternali and his team published their findings in December in the Monthly Notices of the Royal Astronomical Society.
Based on decades of telescope observations and computer simulations, scientists have come to think of dark matter as the hidden skeleton of the cosmos; its “joints” are massive clumps of invisible particles which host galaxies large and small. But Fraternali isn’t the first to glimpse an exception to that rule. A few years ago, Pieter van Dokkum, an astronomer at Yale, and his colleagues discovered similar galaxies with the Hubble telescope that also seemed to lack dark matter. “These galaxies that we found in 2018, they created a lot of controversy and discussion and follow-up work because they were unexpected and difficult to explain,” van Dokkum says.
Those other galaxies lived in a crowded environment, where bigger, neighboring galaxies frequently fly by, possibly pulling away dark matter with them. In contrast, Fraternali’s galaxy is pretty isolated, with no such bothersome neighbors, so its dearth of dark matter can’t be explained that way. “It could be very significant,” van Dokkum says. “How do you get stars and gas in that location together without the help of dark matter?”
These strange objects have come to be called “ultra-diffuse galaxies.” They’re extreme outliers: In terms of their mass, they’re minuscule, but they’re spread out over vast distances. Some are as large as the Milky Way, but with only a hundredth as many stars—or even fewer. They’re so close to being transparent that they’re tough to spy in the night sky. “They’re slightly fainter in the center, so they’re difficult to detect. Now, with better telescopes and deeper observations, they have become more well known,” says Mireia Montes, an astronomer at the Space Telescope Science Institute in Baltimore and an expert on such galaxies.
Starting in the 1960s, American astronomer Vera Rubin and others first revealed the likely existence of unseen, or “dark,” matter while measuring how fast stars in galaxies whirl around the center, showing that inner stars orbit at different speeds than outer ones. Based on those stars’ rotation, scientists calculated how much mass the galaxy must have to keep them constantly orbiting, rather than being flung into space. For many galaxies, that mass was many times larger than that of all the stars added up. Scientists resolved the problem by inferring the presence of some kind of dark matter, which doesn’t emit or reflect light, and which must be making up the rest of the mass that’s holding the galaxy together.
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But the measurements by Fraternali and his team show that, for their extremely diffuse galaxy, there’s no need to invoke dark matter. The rotation speeds they measure totally match up with the mass of the stars and gas clouds they observed, without requiring any extra mass that can’t be seen. Montes and her research group aim to study these galaxies, including in their outskirts, in more detail in case there’s missing matter that Fraternali hasn’t detected. But at least for now, this ghostly galaxy remains a conundrum.
Fraternali points out that their galaxy, known as AGC 114905, has a big X factor: It’s tilted. Some galaxies are shaped like flying saucers, and if telescopes on Earth can see them edge-on, that makes observing them easy for astronomers. They can see the orbiting stars on one side of the galactic disc moving toward us, and those on the other side orbiting away from us. If they can measure those stars’ speeds, they can estimate the galaxy’s mass—and figure out if some of the total must be composed of dark matter. But Fraternali calculates that AGC 114905 is inclined by a little more than 30 degrees, so astronomers have to correct their mass measurements to account for that tilt. If they’re wrong about that degree of tilt, their measurements actually could leave plenty of room for dark matter.
But assuming the team is right, it’s not yet clear exactly what kind of exception their galaxy may be. Is it a really weird cosmic object that nobody understands? Or is it a harbinger of bigger problems for dark matter theories?
So far, it doesn’t fit any of the proposed explanations for the origins of ultra-diffuse galaxies. Some astronomers speculate that such a low-dark-matter galaxy could be the remnant of a pair of bigger galaxies tugging at each other with their gravitational pull during a close flyby, leaving a puffy blob of stars and gas in their wake. But there’s no giant galaxy next door, so that doesn’t explain it, van Dokkum says.
Another theory is that it could be the remnant of past stellar explosions. All stars eventually die, and some go out with a bang, going supernova. Over time, supernovas might spread out parts of a galaxy, expelling matter, including gas clouds. But that’s not the case with AGC 114905, Fraternali says, since it’s still filled with lots of gas, which serves as the building material and fuel for new stars. And if the galaxy used to be much more concentrated eons ago, one would expect lots of compact clusters of stars left over today, indicators of a denser past. But the galaxy lacks many clusters like that, van Dokkum says.
In fact, AGC 114905 doesn’t seem to fit any model that includes dark matter. For decades, scientists like Laura Sales, an astrophysicist at UC Riverside, have simulated the cosmos on powerful computers, trying to show how dark matter models can reproduce the myriad galaxies astronomers spot with their telescopes. “We quickly looked into our simulations, and we don’t have something like this galaxy,” she says.
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Instead, Fraternali’s galaxy and others like it could point to a need for alternatives to dark matter. When scientists infer the presence of hidden matter lurking about, they’re making assumptions about how gravity works. But what if gravity operates a bit differently than they thought?
In their work, Fraternali and his colleagues tested a leading contender among dark matter alternatives, called MOND, for Modified Newtonian Dynamics, which involves tweaking Isaac Newton’s law of gravity. First proposed by Israeli physicist Mordehai Milgrom in the 1980s, MOND hypothesizes that standard gravitational physics, which accurately explains the motions of objects with high gravitational accelerations, like planets in our solar system, might not apply the same way to slowly orbiting stars at the edge of a galaxy’s disc. So the discrepancy between the expected velocities of the stars in galaxies and how fast they appear to be moving may not indicate missing mass, but rather a math error, if the MOND gravitational law is right. But while the MOND model fares well with more normal galaxies, it too couldn’t explain the rotation of Fraternali’s team’s fluffy galaxy. It fared just as poorly as dark matter models do.
It’s too early to tell whether AGC 114905 indicates a problem with theories of dark matter, Sales says. For now, Fraternali and others will continue to examine these enigmatic and previously overlooked galaxies, including with the newly launched James Webb Space Telescope, in the hopes of resolving this riddle. “It’s not like we’re probing the edge of the universe, or trying to see a tiny planet next to a star. It’s actually doable with the tools that we have,” van Dokkum says. “To me, that makes it exciting.”
Updated 01/10/2022 1:00 PM ET: This story has been updated with the galaxy's correct distance from Earth in light-years.
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