Many large galaxies—our Milky Way among them—are orbited by a retinue of smaller, fainter companions: dwarf galaxies. Some are probably nearly as old as the universe itself, isolated islands of ancient stars that never managed to glom onto a larger galaxy; others are younger, born from the shredded remains of bigger galaxies that collided and ripped one another apart. Regardless of whether any given dwarf galaxy is a primordial building block or a late-stage leftover from a galactic merger, studying these diminutive objects is arguably one of the best ways to learn about how galaxies and other large cosmic structures emerge, interact and grow. And studying the large-scale structures in turn is one of the best ways we have of understanding the fundamental rules of the universe we live in.
In some respects, watching dwarf galaxies orbiting their larger hosts is rather like watching bees humming around a hive. Dwarfs, like bees, appear aimless in their flights—up and down, left and right, with no obvious rhyme or reason. But both have a hidden order. Bees surf around their hives on wind currents carrying the scents of flowers and pheromones, and the orbits of dwarf galaxies are dictated in part by something even more mysterious: the gravitational pull of dark matter. This theoretical, invisible substance is more felt than seen, for it emits no light; the only way we infer its existence is by how it warps the space around it. Crude maps of its distribution indicate dark matter forms a sort of cosmic web, where galaxies large and small are gravitationally glued to filaments and sheets of dark matter. “We have this web of dark matter that feeds the host galaxy from all angles and directions, and that’s why we find so many different orbits and motions of these dwarf galaxies,” says Oliver Müller, a PhD candidate at the University of Basel.
In a new study published Thursday in Science, Müller and colleagues charted how 16 dwarfs moved around the “beehive” of Centaurus A, a large galaxy upward of 10 million light-years distant from the Milky Way. They found that rather than following random orbits, 14 of the dwarfs of Centaurus A are surprisingly aligned and coplanar, that is, they share the same orbital plane—a bit like bees flying in a well-ordered ring around a hive. The puzzling configuration—which Müller’s simulations suggest only have a 0.5 percent probability of occurring by happenstance—has some scientists questioning just how much they understand the behavior and environment around these satellites. If seen around many other galaxies across the universe, Müller says, such bizarrely coplanar dwarfs could even challenge cosmologists’ “Lambda cold dark matter” (LCDM) conception of the universe—the standard model used to explain how galaxies and galaxy clusters emerge and evolve.
But although the new study emphasizes the rarity of aligned coplanar dwarf galaxies, such configurations have been observed before. In fact, prevailing theories suggest that one out of every 10 dwarf galaxies should possess some kind of alignment; at least that’s what computer simulations suggest.
What is unique about this particular alignment is “this is the first example of this kind of configuration seen outside of our local (galactic) group, so that’s quite interesting,” says Carlos Frenk, a cosmologist at Durham University in England. Coplanar dwarf galaxies have been found for the Milky Way and Andromeda, Frenk notes, but the 14 around Centaurus A are the first observed circling a more distant galaxy.
In their study, Müller and his team argue that if coplanar alignments of dwarf galaxies are widespread, this would pose a worthy challenge to the LCDM model—which predicts a random distribution of dwarfs. Finding many coplanar arrangements would suggest, in short, our already limited understanding of dark matter is even more incomplete than previously appreciated. Frenk, for one, is not entirely convinced, pointing out the LCDM model remains in excellent agreement with the vast majority of decades’ worth of observations. “The standard model works quite well,” Frenk says. “Why would it just stop in an instant? It’s interesting, but not a challenge that threatens the cosmological paradigm at this point.”
Unfortunately, taking a more complete census of dwarf galaxies would take decades. Observing the movements of dwarf galaxies is extremely nuanced, in large part because we can more easily measure the speeds of far-distant objects that move either away from or toward us in the sky, rather than those that move across the celestial sphere. And as their name implies, dwarf galaxies are extremely small and dim, making their discoveries even more of a challenge the farther out one looks from the Milky Way. All these effects act to bias the observational data, muddying the waters so that a clear trend either of coplanarity or of disorder becomes hard to see. The only solution is to find and study dwarf galaxies, near and far, using multiple overlapping and time-consuming methods.
Meanwhile, intriguing observations like these might do more to actually confirm the success of the LCDM model than to replace it. Michael Boylan-Kolchin, an astrophysicist at The University of Texas at Austin and author of an accompanying commentary in Science, says the standard model has overcome similar challenges before. “The LCDM has faced many tests and is still the standard cosmological model; I expect the same will be true in the case of satellite planes,” he says. “However, I also think it is essential to fully investigate all potential inconsistencies. It is the only way we increase our confidence in theories or, in rare cases, find the flaws that demand fundamental revisions.”
As a part of his dissertation work, Müller plans to spend time in the upcoming months using the Dark Energy Camera in Chile to study the Centaurus A system with the hopes of finding more dwarf satellites. Another dozen or so in nonplanar orbits could make the reported trend disappear. For now though, he is excited about this oddity, because it may actually be the norm. “These systems aren’t just an outlier—they look like they’re more common. I think we have to take this very seriously…. I want us to do further studies of this structure. I want to show that this isn’t just a coincidence.”