Small galaxies can lose their dark matter in confrontations with Goliath galaxies

New study grew out of two discoveries that threatened to shake the foundations of galaxies as astronomers understand them

The Hubble Space Telescope captures two galaxies in the process of merging. (Nasa)

When small galaxies encounter more massive ones, the former face an ultimatum: Assimilate or be destroyed. But a new study suggests there is a third alternative — confronting the larger galaxy, and paying a cost in dark matter.

A new paper published Monday in the journal Nature Astronomy details a study using high-powered computer simulations of galaxies that showed contrary to the assumptions of astronomers, galaxies can exist without a surrounding halo of dark matter, which scientists also believe is essential for galaxy formation. The trick is that such galaxies must survive close encounters with much more massive galaxies, passing through the larger galaxy and being stripped of their dark matter.

It’s a finding that not only expands scientists’ understanding of galactic evolution and behavior but preserves the important role dark matter plays in the universe and suggests important future experiments for the recently launched James Webb Space Telescope.

The new study grew out of two discoveries that threatened to shake the foundations of galaxies as astronomers understand them.

In 2018, a team of Yale astronomers discovered a dwarf galaxy — NGC 1052-DF2 — that possessed little or no dark matter. Dark matter is a mysterious form of matter believed to make up 85% of the universe. Although scientists cannot see it, they can see dark matter’s gravitational effects, and it’s believed that the enigmatic material helps galaxies to form and hold their shape.

“The reason we believe there is dark matter is by looking at the motions of stars in the galaxy — They’re moving too quickly, and there needs to be something holding those stars together,” said Jorge Moreno, assistant professor of physics and astronomy at Pomona College, California,  and lead author of the new paper. “It’s kind of like the dark matter is the glue that holds everything together.”

A second galaxy, AGC 114905, was also recently found to have little or no dark matter, and together, such observations posed a real problem for astronomers studying galaxies. Here were entities that astrophysics said should not exist.

“When people discovered one and then another galaxy without dark matter, the whole community, we went into panic mode,” Dr. Moreno said. What was thought to be a fundamental part of galaxy behavior had just been shown to be wrong, and some scientists even questioned the reality of dark matter itself. “They said, ‘maybe we don’t need any dark matter, but rather, we need to modify the laws of physics.”

Dr. Moreno is not an observing astronomer, gazing at distant galaxies through a telescope. Instead, he uses supercomputers to model the behavior of galaxies using the data collected from observations and the laws of physics as they are believed to function throughout the cosmos.

It’s a technique that has its advantages since a galaxy’s life cycle spans millions or even billions of years.

“In a simulation, you can actually see the evolution in minutes,” he said. “You can see the galaxy from the moment it’s born, when it’s a child, when it’s an adolescent and when it becomes an adult.”

In December 2020, Dr. Moreno was running a simulation in a new suite of software that allowed him to simulate wide swaths of the universe consisting of thousands of galaxies. It was a simulation that assumed dark matter was essential for galaxy formation and structure, and yet the simulation began cranking out galaxies that lacked a dark matter halo. It also showed just how they got that way — an encounter with a more massive neighbor.

It was assumed that smaller galaxies orbiting larger galaxies would either be destroyed, torn asunder by the gravity of the more massive galaxy, or become assimilated, eaten and absorbed by the larger galactic system.

“What we find is that these galaxies actually don’t choose one or the other, they come up with a third way of surviving,” Dr. Moreno said, which he likens to David confronting the giant Goliath in the Bible. “They go right through the galaxy, and they go through it multiple times, and they survive. For some reason they survive, but they pay a price —  they end up giving up their dark matter.”

This proof of concept has several implications. First it offers an explanation for the observed galaxies and their puzzling lack of dark matter, and one of the astronomers on the team discovered NGC 1052-DF2, Shany Danieli, is the second author of the new paper.

But the finding also strengthens the evidence for the reality of dark matter in the universe.

“We don’t get to get rid of the Dark Matter model. We’ve actually saved the model,” Dr. Moreno said. “People were saying, ‘you know, it could be the case that dark matter is wrong and we need to come up with alternatives.’ My paper alleviates those concerns.”

The third implication of the new findings applies outside the world of science and astrophysics, but within the world of the humans who do the science and astrophysics.

“This discovery adds a new dimension. You have more than two options: you can assimilate, you can be destroyed or you can just confront the Goliath,” Dr. Moreno said. “I’m one of the few astronomers of color doing simulations, and I have indigenous roots. So to me, that resonates a lot with me with the story of my people who, some of them assimilated, some of them were destroyed.”

Dr. Moreno himself took a third path, that of choosing to make a place for himself in the astronomy community so that people that look like him can be included. And although he himself does not have Cherokee heritage, he named the galaxies created in his simulation after the seven clans of that native American tribe: Bird, Blue, Deer, Long Hair, Paint, Wild Potato, and Wolf. Dr. Moreno is not himself Cherokee, but a colleague and friend of his is Cherokee.

“That’s how I learn about those names,” Dr. Moreno said. “I asked him and he consulted with the Cherokee elders, basically, asking permission for me to use the names and they said yes.”

But the new study doesn’t merely shore up dark matter theories or explain recent observations. It makes specific predictions about what observing astronomers should see as they continue watching the skies.

“We make a prediction that 30% of massive galaxies should have at least one satellite without dark matter,” Dr. Moreno said. “A few weeks ago, we launched the JWST, the James Webb Space Telescope, and that’s an ideal instrument for people to go and look around other massive galaxies and see if they can find satellites like this.”