Astronomers Discover a Strange Galaxy Without Dark Matter

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three years ago, Filippo Fraternali and his colleagues observed half a dozen mysteriously dispersed galaxies that looked like stars and gas giant cities. But unlike nearly every other galaxy—including our own Milky Way—they didn’t seem to be enveloped in the vast masses of dark matter that would normally hold those stellar metropolises together with their gravity. Scientists chose one to zoom in on, a modestly sized galaxy about 250,000 light-years away, and pointed the 27 radio telescope antennas of the Very Large Array in New Mexico.

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After gathering 40 hours of data, they mapped out the stars and gas and confirmed what the earlier snapshots indicated: “The dark matter content we estimate in this galaxy is much less than you might expect.” It is,” says Fraternali, an astronomer at the Kapten Astronomical Institute at the University of Groningen in the Netherlands. If the team or their competitors find other such galaxies, it could pose a challenge to scientists’ view of dark matter, the dominant perspective in the region for at least 20 years. fraternity and its team published its findings in december Monthly Notices of the Royal Astronomical Society,

Based on decades of telescope observations and computer simulations, scientists have come to regard dark matter as the hidden skeleton of the universe; Its “couples” are massive clumps of invisible particles that host galaxies large and small. But Fraternali isn’t the first to glimpse an exception to that rule. A few years ago, Peter van Dokkum, an astronomer at Yale, and his colleagues with the Hubble Telescope discovered similar galaxies that also lacked dark matter. “These galaxies we found in 2018 caused a lot of controversy and discussion and follow-up because they were unpredictable and difficult to explain,” says Van Dokkum.


Those other galaxies lived in crowded environments, where larger, neighboring galaxies frequently flew by, possibly dragging dark matter along with them. In contrast, Fraternally’s galaxy is very isolated, with no such disturbed neighbors, so its lack of dark matter cannot be explained as such. “It can be very important,” Van Dokkum says. “How can you get stars and gas together at that location without the help of dark matter?”

These strange objects have come to be called “ultra-diffuse galaxies”. They are highly external: in terms of their mass, they are small, but they are spread over vast distances. Some are as big as the Milky Way, but only one-hundredth as many stars—or even less. They are so close to being transparent that it is difficult to spy on the night sky. “They are slightly fainter in the center, so they are difficult to detect. Now, with better telescopes and deeper observations, they have become more famous,” says Miria Montes, an astronomer at the Space Telescope Science Institute in Baltimore and an expert on such galaxies. Huh.

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In the early 1960s, American astronomer Vera Rubin and others uncovered for the first time the possibility of the existence of the unseen, or “dark”, by measuring how fast the stars in galaxies revolve around the center, showing That the inner stars orbit at a different speed than the outer ones. Based on the rotation of those stars, the scientists calculated how much mass the Milky Way would need to have for them to keep orbiting continuously, instead of drifting through space. For many galaxies, that mass was many times greater than the sum of all the stars. Scientists solved the problem by inferring the presence of some kind of dark matter, which does not emit or reflect light, and which would be making up the rest of the mass that holds the galaxy together.

But measurements by Fraternali and his team show that, for their extremely diffuse galaxy, there is no need to imply dark matter. The rotational speed they measure coincides perfectly with the mass of the stars and gas clouds they observe, without the need for any additional mass that cannot be observed. Montes and his research group aim to study these galaxies, including their outskirts, if there is any missing matter that is not detected fraternally. But at least for now, this ghostly galaxy remains an enigma.

Fraternali explains that his galaxy, known as AGC 114905, has a large X factor: it is tilted. Some galaxies are the size of a flying saucer, and if telescopes on Earth can see them from the edge, it would make it easier for astronomers to observe them. They can see stars orbiting on one side of the galactic disk moving toward us, and those orbiting away from us on the other. If they can measure the motion of those stars, they can estimate the mass of the galaxy – and figure out whether some of the total must be made of dark matter. But Fraternali calculated that AGC 114905 has an inclination of a little over 30 degrees, so astronomers will have to correct their mass measurements to account for that inclination. If they’re wrong about that degree of inclination, their measurement could actually leave a lot of room for dark matter.

But assuming the team is right, it’s not yet clear what kind of exception their galaxy might be. Is this really a strange cosmic object that no one understands? Or is it a harbinger of bigger problems for dark matter theory?

So far, this does not fit into any proposed explanation for the origin of ultra-diffuse galaxies. Some astronomers speculate that such a low-dark-matter galaxy may be the remnant of a pair of larger galaxies that, during a close flyby, pulled each other along with their gravitational pull, causing the stars in their wake. And a bloated drop of gas goes out. But there’s no giant galaxy next door, so that doesn’t explain it, Van Dokkum says.

Another theory is that it may be the remains of past stellar explosions. All stars eventually die, and some go out with a bang as they go supernova. Over time, supernovae can stretch out parts of a galaxy, ejecting matter, including clouds of gas. But that’s not the case with AGC 114905, Fraternali says, because it’s still filled with a lot of gas, which serves as the building material and fuel for new stars. And if the Milky Way used to be much more concentrated, one would expect that today there are many compact clusters of stars left, indicative of a denser past. But the Milky Way lacks many such clusters, Van Dokkum says.

In fact, AGC 114905 doesn’t seem to fit Any Model that includes dark matter. For decades, scientists like Laura Sells, an astrophysicist at UC Riverside, have simulated the universe on powerful computers, trying to show how dark matter models can reproduce the myriad galaxies astronomers have with their telescopes. Huh. “We quickly saw that in our simulations, and we have nothing like this galaxy,” she says.

Instead, Fraternally’s galaxy and others like it may point to the need for an alternative to dark matter. When scientists infer the presence of hidden matter, they infer how gravity works. But what if gravity worked a little differently than they thought?

In their work, Fraternali and his colleagues test a leading contender among dark matter alternatives called MOND for modified Newtonian dynamics, which involves changing Isaac Newton’s law of gravitation. First proposed by Israeli physicist Mordehai Milgrom in the 1980s, MOND envisions that standard gravitational physics, which accurately explains the motions of objects with high gravitational acceleration, like the planets in our solar system, may But the same way does not apply to orbiting stars. of the disk of a galaxy. So the discrepancy between the expected velocities of stars in galaxies and how fast they appear to be moving may not indicate a missing mass, but rather an error of mathematics, if the MOND law of gravity is correct. But while the MOND model performed well with more normal galaxies, it also could not explain the rotation of the forgetful galaxy Fraternali’s team. It performed just as poorly as dark matter models do.

Sales says it’s too early to tell whether AGC 114905 indicates a problem with dark matter theories. For now, in hopes of solving this puzzle, Fraternali and others will continue to investigate these enigmatic and previously unseen galaxies, including the newly launched James Webb Space Telescope. “It’s not like we’re probing the edge of the universe, or trying to see a minor planet next to a star. It’s really doable with the instruments we have,” Van Dokkum says. “For me, that makes it exciting.”

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