Simulations find ghostly whirls of dark matter trailing galaxy arms
Simulations suggest where we might look for the mystery material.
Galaxies are far more than the sum of their stars. Long before stars even formed, dark matter clumped up and drew regular matter together with its gravity, providing the invisible scaffolding upon which stars and galaxies eventually grew.
Today, nearly all galaxies are still embedded in giant “halos” of dark matter that extend far beyond their visible borders and hold them together, anchoring stars that move so quickly they would otherwise break out of their galaxy’s gravitational grip and spend their lives adrift in intergalactic space.
The way dark matter and stars interact influences how galaxies change over time. But until recently, scientists had mainly only examined one side of that relationship, exploring the way dark matter pulls on normal matter.
New research probes the reverse effect: whether and how normal matter might influence its dark counterpart in return. The findings could change our understanding of dark matter’s behavior and help scientists figure out how to finally detect it directly.
Dark matter pinwheels
Galaxies like our Milky Way are famous for their sparkling spiral arms, dotted with diamond-like stars and full of glowing gas and dusty tendrils. They may look like sprays of stars that sweep around the galaxy like a pinwheel, but they’re more like galactic traffic jams—pressure waves that stars and gas pass through as they whirl around the galaxy.
A new study suggests they may have a ghostly shadow—trailing dark matter spirals hovering above and below them. A team of scientists found them in galaxy simulations by looking for traces of a gravitational wake left by the visible spiral arms.
We’ve known what might cause this for nearly a century. In 1943, Subrahmanyan Chandrasekhar—a Nobel Prize-winning theoretical physicist—proposed the existence of something called dynamical friction. This effect happens when a massive object passes through an evenly distributed group of lighter objects.
“Initially, a large particle moving through a uniform field of small particles wouldn’t feel any gravity since it would be pulled on equally from every direction,” says Marcel Bernet, a PhD candidate at The University of Barcelona. “But as it moves, it creates a wake like one made by a boat, and this wake pulls the particle from behind and slows it down, which we’ve seen in satellites around the Milky Way. The dwarf galaxies that orbit our own are slowing down as they spiral closer, instead of speeding up like scientists would have normally expected.”
Bernet wondered whether galaxies’ spiral arms might cause a similar reaction in the dark matter halo. Others had theorized about this possibility but hadn’t examined it very closely.
“For a very long time, astronomers were unaware that baryons (gas and stars) could have such an impact on dark matter, and it led us to conclude that our model for galaxy formation, which included cold dark matter, was wrong,” says Alyson Brooks, an associate professor at Rutgers University who was not involved in the study. Brooks investigates the same kind of phenomenon, but in the cores of dwarf galaxies. “I think we are learning now that if we pay attention to what the baryons are doing, our galaxy formation models are more in agreement with observed galaxies and their mass distribution.”
In search of shadows
Bernet led a team of scientists in the hunt for this dark matter wake they thought could be hidden in galaxy evolution simulations. These programs trace galaxy behavior over immense timescales—plenty long enough for their spiral arms to rotate and potentially impact their surroundings, both seen and unseen.
“Basically what you do is you set up a bunch of particles that represent things like stars, gas, and dark matter, and you let them evolve for millions of years,” Bernet says. “Human lives are much too short to witness this happening in real time. We need simulations to help us see more than the present, which is like a single snapshot of the Universe.”
Several other groups already had galaxy simulations they were using to do other science, so the team asked one to see their data. When they found the dark matter imprint they were looking for, they checked for it in another group’s simulation. They found it again, and then in a third simulation as well.
The dark matter spirals are much less pronounced than their stellar counterparts, but the team noted a distinct imprint on the motions of dark matter particles in the simulations. The dark spiral arms lag behind the stellar arms, forming a sort of unseen shadow.
These findings add a new layer of complexity to our understanding of how galaxies evolve, suggesting that dark matter is more than a passive, invisible scaffolding holding galaxies together. Instead, it appears to react to the gravity from stars in galaxies’ spiral arms in a way that may even influence star formation or galactic rotation over cosmic timescales. It could also explain the relatively newfound excess mass along a nearby spiral arm in the Milky Way.
The fact that they saw the same effect in differently structured simulations suggests that these dark matter spirals may be common in galaxies like the Milky Way. But tracking them down in the real Universe may be tricky.
Bernet says scientists could measure dark matter in the Milky Way’s disk. “We can currently measure the density of dark matter close to us with a huge precision,” he says. “If we can extend these measurements to the entire disk with enough precision, spiral patterns should emerge if they exist.”
“I think these results are very important because it changes our expectations for where to search for dark matter signals in galaxies,” Brooks says. “I could imagine that this result might influence our expectation for how dense dark matter is near the solar neighborhood and could influence expectations for lab experiments that are trying to directly detect dark matter.” That’s a goal scientists have been chasing for nearly 100 years.
Ashley writes about space for a contractor for NASA’s Goddard Space Flight Center by day and freelances in her free time. She holds master’s degrees in space studies from the University of North Dakota and science writing from Johns Hopkins University. She writes most of her articles with a baby on her lap.
