A Hubble image of the dark-matter-free galaxy DF2.

Science

These galaxies have no dark matter — and astronomers may finally know why

A galaxy 65 million light years away holds secrets to an ancient traffic jam, which may have kicked out dark matter in the process.

Zili Shen, Pieter van Dokkum, NASA

Dark matter is an invisible, intangible substance thought to make up roughly five-sixths of all matter in the universe. But scientists turned to an unusual inspiration to study dark matter: the TV show House.

Like Hugh Laurie’s titular character in the show, the authors whittled down “symptoms” to find a new way for giant collisions between galaxies to expel dark matter, which could yield critical insights on the elusive phenomenon’s properties. The results were published Wednesday in Nature.

HERE’S THE BACKGROUND — Astronomers have suspected the existence of dark matter for roughly a century after detecting signs that galaxies held more mass than they could account for. When researchers analyzed the speed of stars circling around the center of the Milky Way and of galaxies moving in distant clusters, they all seemed to be going so fast that they should have overcome the force of gravity drawing them inward and flown out into the void beyond.

Scientists think that the gravitational pull from an invisible “dark matter” keeps these galaxies and clusters from tearing themselves apart. Dark matter doesn’t seem to be made out of visible “baryonic” matter like electrons, neutrons, and protons, so many physicists have turned instead to a new kind of exotic particle that interacts very weakly with other matter and the other known forces. But like baryonic matter, it exerts gravity.

Previous research suggested that ordinary matter lacks enough gravity to have pulled itself into galaxies within the universe's current age. As such, researchers suspected galaxies might only form if dark matter was involved.

However, in 2018, astronomers discovered one large galaxy, NGC 1052-DF2, mysteriously possessed several hundred times less dark matter than expected from the conventional theory of galaxy formation. Then, in 2019, they discovered another large galaxy, NGC 1052-DF4, that was similarly devoid of dark matter. How might these galaxies have formed?

NGC 1052-DF2, discovered in 2018, seems to have almost no matter — making it a faint, ghostly cloud of stars. NASA, ESA, and P. van Dokkum (Yale University)

Digging into the details — In the new study, the scientists found inspiration from the TV series House. In the series, a cantankerous doctor seeks solutions for mysterious medical ailments patients suffer from.

“They do ‘differential diagnosis’ in the series, basically writing down all the symptoms and possible causes, and trying to find which disease explains all the symptoms,” study lead author Pieter van Dokkum, an astronomer at Yale University, tells Inverse. “That inspired me to do the same with DF2 and DF4 — write down all we know, and see if there is a simple explanation for it all.”

DF2 and DF4 might have formed from a giant collision between galaxies, astrophysicist Ji-hoon Kim at Seoul National University, who did not participate in this research, tells Inverse. The ordinary matter in these galaxies would have stopped with the impact, whereas their dark matter, being virtually intangible, would have passed through.

A similar effect separating ordinary matter from dark matter may have occurred on a much larger scale with the Bullet Cluster about 3.8 billion light-years away. It formed after two large clusters of galaxies collided at roughly 10 million miles per hour, the most energetic event known in the universe since the Big Bang.

DF2 and DF4 orbit a larger galaxy called NGC 1052, located roughly 65 million light years from Earth. The light the researchers analyzed from these bodies “left the galaxies when the asteroid that killed the dinosaurs hit the Earth,” van Dokkum notes.

The astronomers noted that DF2 and DF4 are roughly 6.8 million light years apart and moving away from each other at about 800,000 miles per hour. Prior work suggested that a collision between galaxies could result in dark matter-free galaxies hurtling away from each other at such speeds. Such an impact may have generated shock waves in the gas of DF2 and DF4 that may help explain the many bright star clusters seen in those galaxies.

By following the motions of DF2 and DF4 back in time, the scientists suggested they originated from a collision roughly 8 billion years ago. When they used advanced image-analysis techniques compiled last year on galaxies surrounding NGC 1052, they discovered that seven to 11 galaxies and other cosmic structures near NGC 1052, including DF2 and DF4, form a long trail.

“We were really surprised to find a galaxy without dark matter in 2018. Finding a second one in 2019 was super odd, and we had no explanation,” van Dokkum says. “Now we think we are seeing about 10, and that’s just amazing.”

The researchers suggest that all the structures discovered may be debris from this cosmic impact. “The key moment where everything clicked was discovering the other galaxies along the DF2-DF4 axis — the trail was expected in this model, and seeing it was a really big moment,” van Dokkum says. “We now finally have an explanation for all these weird findings over the past few years.”

Inverted view of region to highlight the regions around the galaxies near NGC 1052-DF4. M. Montes et al.

WHAT’S NEXT? — To fully verify that the trail the researchers detected indeed came from a single collision, their 3-D positions and 3-D velocities, their level of dark matter, and the ages of their stars “should be measured and match the simulations’ prediction,” astrophysicist Eun-jin Shin at Seoul National University, who did not participate in this study, tells Inverse.

Van Dokkum notes they can collect such details in the next few months and years. If these findings confirm the so-called “bullet dwarfs” scenario, “it can be more evidence for the existence of dark matter, and will shed light on its nature,” Kim says.

For example, although dark matter apparently barely interacts with ordinary matter, it remains an open question as to whether it interacts with itself. Analyzing the aftermath of the ancient collision that gave rise to DF2 and DF4 could shed light on how dark matter behaved before, during, and after the event.

“The hope is that we can use this system, and others like it if we find them, to test the nature of dark matter,” van Dokkum says. “That may require the James Webb Space Telescope, or even 30-meter-class telescopes on the ground.”

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