Galaxies that move together have cosmologists stumped about dark matter

Galaxies that move together have cosmologists stumped about dark matter
The galaxy Centaurus A is viewed by the European Southern Observatory in 2012. Scientists studying the galaxy and several dwarf galaxies surrounding it are stumped by their behavior. (AFP photo / ESO)

Astronomers have discovered that the smaller satellite galaxies around Centaurus A are engaged in a coordinated dance that seems out of sync with our understanding of the large-scale structure of the universe.

The discovery, described in the journal Science, could push physicists to redefine our understanding of dark matter, that mysterious stuff that forms the universe's cosmic web.


Unlike normal matter, dark matter doesn't interact with other matter. It can't be seen or touched. And yet we know it must be there because there's so much of it that its gravitational influence affects the spinning of galaxies. There's more than five times as much dark matter as there is normal matter — normal matter being the stuff that makes up the stars, the galaxies, Earth and every living thing that inhabits it.

There are a lot of theories to explain what dark matter is. Currently, the prevailing idea is that "cold dark matter" forms giant clumps connected by dark matter filaments in a cosmic web.


Large galaxies like the Milky Way are surrounded by large spherical "halos" of dark matter. These galaxies also typically have a sizable coterie of smaller satellite galaxies around them. According to our understanding of dark matter, those satellite galaxies should be distributed all around their galactic host, said study coauthor Marcel Pawlowski, an astrophysicist at UC Irvine.

"They should be rather randomly distributed and move in more or less random directions if we believe our current understanding of cosmology — but they don't really," Pawlowski said.

Take our home galaxy, the Milky Way. Out of 11 satellite galaxies with known velocities, eight seem to orbit in a tight disc that's perpendicular to the plane of the spiral galaxy. (There could be more galaxies; we just can't see them.) The same pattern seems to apply to a number of the satellites around our galactic neighbor, Andromeda: 15 out of 27 surveyed galaxies are arranged in a narrow plane around the host galaxy.

But many scientists figured that the Milky Way (and Andromeda) must be the exception rather than the rule.


"Many astronomers have been concerned about drawing conclusions from the nearest galaxy systems: The census of Milky Way satellite galaxies might be affected by the gas and stars in the Galaxy's disk, and it is not currently possible to measure motions perpendicular to the plane of satellites in Andromeda, meaning its long-term stability remains unknown," Michael Boylan-Kolchin of the University of Texas at Austin, who was not involved in the study, wrote in a commentary.

This animation shows the 3-D spatial distribution of the CenA subgroup of galaxies. (Credit: O. Müller et al., Science, 2018.)

For this paper, an international team of researchers looked outside of our own neighborhood for answers. They focused on the galaxy Centaurus A, which lies about 13 million light years away. Centaurus A is an elliptical galaxy that's also surrounded by an array of satellites. Perhaps studying its companions would shed light on whether the Milky Way was the exception or the rule.

Using archived data, the researchers looked at velocity data for 16 of the known satellite galaxies around Centaurus A. They found that 14 of them appeared to be moving in a common plane around the larger galaxy, not at random. That plane appears to be roughly perpendicular to the dusty disk that surrounds the elliptical galaxy.

Under the current dark matter model, this sort of alignment is supposed to be a one-in-a-thousand sort of event, the scientists said. So what does it mean that the three galaxies that scientists have looked at so far all share the same supposedly rare pattern?

Perhaps these systems were all created by galaxies merging together, which could potentially explain their movement patterns without coming into conflict with our understanding of dark matter, scientists said.

If not, it could mean that our ideas about dark matter need to be tweaked — or perhaps even revised entirely, Pawlowski said. Perhaps dark matter doesn't exist, and there are simply changes to the behavior of gravity in different situations that make it seem like some kind of invisible mass is at work. But modifying models of how gravity works is much easier said than done.

"We kind of know where we have our problems — we just haven't figured out how to solve them," he said. "I think we should be more open-minded and consider alternative approaches."

One of the next steps, he added, would be to continue surveying more large galaxies and their satellites to see which configuration is truly more prevalent than the other.


"We really want to understand it in a global sense," he said.

In any case, any change that moves our understanding forward would be welcomed by the physics community, Boylan-Kolchin said.

"Perhaps most excitingly, any potential resolution of the puzzle of satellite planes is interesting," he wrote. "At worst, we improve our understanding of galaxy formation; at best, we are led to a deeper understanding of the laws of physics."

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