Galaxy rotations raise doubts on dark matter

An odd distribution in the orbits of small “satellite” galaxies circling a larger galaxy called Centaurus A is making scientists wonder how well they understand the role of dark matter in shaping the universe we know today—or even whether it exists at all.

Satellite galaxies are common – our own Milky Way galaxy has about 30 – and come in many sizes, ranging from about 1% the size of the primary galaxy to as small as only a few thousand stars. Astrophysicists believe these galaxies were formed separately from their primaries and were then gravitationally captured. 

Where it gets complicates is that standard cosmological theories predict that dark matter played a role in these captures.

Dark matter is an as-yet unknown material believed to comprise the bulk of the universe’s matter. Not only does it pervade today’s galaxies, but theory says that early in the universe’s history it would have condensed under the force of gravity into cobweb-like filaments extending outward from large galaxies such as Centaurus A. 

This means that as each satellite galaxy was captured, it would have interacted gravitationally with those filaments — differently for each one, depending on which filaments it most strongly interacted with. The result, says Oliver Müller of the University of Basel, Switzerland, is that these satellite galaxies should now be circling their primary on random orbits, “like bees around a beehive”.

But that’s not what Müller team found when they looked at Centaurus A. In a study to be published in the journal Science, they found that for the 16 of these galaxies whose motions they could measure, 14 appeared to be orbiting in a single plane, rotating in the same direction, like the planets in our solar system. The likelihood of that happening by chance, he says, is less than 0.5%.

Previous research had revealed that the same applies to the Milky Way galaxy’s satellite galaxies, as well as those circling the neighbouring Andromeda Galaxy. But scientists had simply assumed that something — perhaps a unique arrangement of dark matter filaments in our own galactic neighbourhood, had somehow channeled satellite galaxies into this pattern. 

The fact the same pattern held at the Andromeda Galaxy didn’t do much to dispel this notion because the two galaxies are close enough together that the same oddity could have applied to both. “[The thinking was that] we are just in a special system … an outlier,” says Müller.

But Centaurus A is 13 million light-years away — several times farther than the Andromeda Galaxy. The fact its satellites orbit in a similar manner shows that planes of satellites may be more common than we thought, he adds. 

The implication is that we may not understand the process of satellite galaxy capture as well as we thought. But the find could have broader ramifications. “It could be that we have a bad understanding of the formation of ‘structure’ in the universe,” Müller says. 

It is even possible there is something wrong with the entire theory of dark matter, all the way down to the question of whether it exists. “I don’t say it refutes dark matter, but it challenges it,” he says.

Michael Boylan-Kolchin, an astrophysicist at the University of Texas in Austin, Texas, and author of a second paper in Science, agrees. 

Although the existence of dark matter is accepted by the vast majority of astrophysicists, he says, there is a minority who think it doesn’t exist, and that its apparent effects actually reveal an error in our theory of gravity. 

The new finding does indeed add support to that view, but he thinks it’s too soon to start rejecting the entire concept of dark matter. That’s because dark matter explains aspects of the universe other than the motions of satellite galaxies. 

“Dark matter explains so many different observations at so many different cosmic times, with so many different scales, that it’s hard to get away from,” Boylan-Kolchin says.{%recommended 6050%}

That said, he thinks the new find is exciting. “It’s important to try to poke at the standard cosmological model and see where its shortcomings might be,” he says, “because there is where you start to reveal any issues that will either lead you to a better understanding of the model or having to reject the model and come up with a better one.” 

The next step, he says, will be for astronomers to study the motions of these galaxies in more detail. That’s because Müller’s team only measured their direction in our line of sight, showing that those on one side of Centaurus A were receding, while those on the other were approaching. 

(This is done by looking at Doppler shifts in the frequencies of light in their spectra.) 

That is a good start, but these galaxies may also be moving in other directions — the equivalent of up-and-down or side-to-side. If so, they might look like they are rotating in tidy formation when in fact they are flying off sideways, every which-way.

To measure this, astronomers will have to watch these galaxies over time, precisely enough to see them move. “It’s akin to measuring the growth of human hair if the human is standing on the Moon,” says Boylan-Kolchin. “That’s very difficult, but within our capabilities — perhaps not for Centaurus A in the next few years, but for Andromeda, I think we should be able to tell.”