Thirteen million light years away, a handful of relatively small galaxies revolve in a disk around the larger elliptical galaxy Centaurus A. There's nothing unusual about a collection of small galaxies orbiting a large one. But the behavior of these particular galaxies doesn’t fit our existing models of how the Universe works. Some astronomers say that new observations could prompt a reconsideration of the standard cosmological model.
In the standard cosmological model, or λCDM, the unseen gravitational influence of dark matter provides an invisible framework for the regular matter that we can actually detect, which only accounts for 4.6 percent of the Universe's contents. The model does a good job of explaining nearly everything about the large-scale structure of our Universe, from its earliest moments to the present day, and there’s a lot of very strong evidence to support it.
But the model doesn’t seem to explain why a group of small galaxies, called dwarf or satellite galaxies, seem to orbit Centaurus A in a neat orbital plane like planets orbiting a star.
Disk or cloud?
According to the λCDM model of the Universe, satellite galaxies should form a more or less spherical cloud around the central galaxy, orbiting in random directions and at random angles. If you drew a map of those orbits, it should look more like electrons spinning around the nucleus of an atom, rather than like planets orbiting a star in neat concentric ellipses. In fact, computer simulations produce galaxies orbiting another galaxy on a flat plane, like planets around a star, only about half a percent of the time. In the real Universe, however, satellite galaxies appear to align their orbits in a shared plane much more often.
Our own Milky Way galaxy has several satellite galaxies, and most of them appear to orbit in the same plane, although it’s hard to be sure since we’re observing them through the gas and stars of our own galactic disk. Elsewhere, 13 of the Andromeda Galaxy’s 15 satellites seem to orbit in the same plane, not the random cloud of orbits predicted by the standard model.
Astrophysicists mostly dismissed the Milky Way and Andromeda as statistical flukes, but new observations of Centaurus A and its collection of satellites appear to show the same thing: 14 out of the 16 satellite galaxies for which we have good velocity data seem to be orbiting their larger host galaxy together in the same plane.
A galactic puzzle
So why are these galaxies whirling merrily through space in seeming defiance of the λCDM model?
“There is no obvious answer, unfortunately, or maybe expectedly—otherwise planes of satellite galaxies would not be seen as a problem and we wouldn’t talk about them as much,” said astrophysicist Marcel Pawlowski of the University of California, Irvine, who coauthored the study with astrophysicist Oliver Müller of the University of Basel and colleagues. The data raises important questions, however. If Pawlowski and his colleagues are correct, the dwarf galaxies’ strangely orderly behavior could point to a major flaw in the standard cosmological model.
That would be a major problem for physicists to grapple with: they would need to either revise the λCDM model or find an entirely new way to explain how the Universe works. Most astrophysicists are far from sold on the idea of replacing the standard model, though.
“It would take very strong evidence of an insurmountable problem to give up on λCDM, or at least a model that looks very much like it,” said astrophysicist Mike Boylan-Kolchin of the University of Texas, who commented on the study.
Still room for debate
For the moment, it’s not clear that the Centaurus A galaxies actually pose such an insurmountable problem. For that to be the case, astrophysicists would need to be very certain that the dwarf galaxies really are moving in a stable orbital plane, and Boylan-Kolchin says there’s still reason to question that.
For one thing, the smaller galaxies that seem to orbit in planes around both Andromeda and the Milky Way also have a wide range of ages, masses, and ratios of chemical elements. Boylan-Kolchin says that if the galaxies all formed as the result of the same collision, they should be the same age and have similar chemical compositions. The λCDM model explains that variation through a series of collisions, but alternatives don't.
And to challenge the λCDM model, scientists would need to rule out other explanations for the dwarf galaxies’ orbits. For instance, dark matter tends to form filaments, and the filaments' gravitational effects work like invisible highways in space, channeling matter in a particular direction. Those filaments could have shaped the direction in which matter accreted onto these galaxies’ haloes, some scientists say. But Pawlowski says that simulations already account for those structures and still only rarely produce co-rotating galaxy systems.
Another possibility is that in the λCDM model, the dark matter structures that shape the arrangement of galaxies are on a scale of about 10 to 20 times the distance from Earth to Centaurus A. That larger-scale distribution of dark matter could cause planar structures like the ones astronomers are seeing. After all, Andromeda and Centaurus A’s satellite galaxies seem to orbit on the same plane. But because it’s a quirk of the local (relatively speaking) dark matter structure, it wouldn’t typically show up in simulated model universes.
Raising the stakes
To resolve the question, astronomers need more data. “This process is a crucial component of science: continually testing models, identifying weaknesses, and understanding whether those weaknesses indicate an incomplete understanding of the model or a need for a new paradigm,” said Boylan-Kolchin. “My money is on λCDM, but it would be thrilling to find out that it is wrong.”
The first step is simply to observe more galaxies, according to Pawlowski, especially ones farther from our cosmic neighborhood. That will give astronomers a better idea of how common it actually is for satellite galaxies to orbit on a common plane instead of in a cloud, and it could rule out any local quirks.
“That could reveal correlations with the host galaxy, its history, environment, or other properties that could point at a solution. I think everybody agrees that obtaining more observational data is the best way forward,” Pawlowski said, but he added that simulations still have a lot to reveal about satellite galaxies’ behavior, their formation, and the implications for the standard model.
Meanwhile, Boylan-Kolchin proposes looking closely at the satellite galaxies and measuring their velocity perpendicular to the orbital plane. If they’re really orbiting together in the same plane, there shouldn’t be much “sideways” movement at all. Otherwise, these orbital planes are probably just coincidence—a temporary alignment of galactic orbits that we just happen to be around at the right time to observe. Centaurus A is too distant for that kind of observation, but in the next few years, the James Webb Space Telescope may make it possible for Andromeda and the Milky Way’s satellite galaxies.
“At worst, we improve our understanding of galaxy formation; at best, we are led to a deeper understanding of the laws of physics,” wrote Boylan-Kolchin. “The distribution of satellites around nearby galaxies does not yet rise to the level of a crisis driving us to a new understanding, but the results of Müller et al. raise the stakes.”
Science, 2017. DOI: 10.1126/science.aao1858 (About DOIs).
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