Powerful, influential figures exert a irresistible pull, gathering an entourage around them. It's a pattern that repeats on celestial levels. Our planet has the moon, but also a host of other artificial satellites that we've used to boost Earth's follower count. The sun has planets, moons, asteroids, and comets. But neither can compare to galaxies like our Milky Way, which not only hosts hundreds of billions of stars but also has additional satellites, entire dwarf galaxies that chill in our galaxy's neighborhood.
Astrophysicists used to think that the Milky Way's followers were a little strange, because many of them acted a lot like planets around a sun—orbiting our galaxy in a plane, perpendicular to the distinctive spiral.
If the dwarf galaxies were following the standard model of cosmology—a theory which underpins most of our understandings of how the universe was formed—they should have been all over the place, scattered around our galaxy's perimeter like autograph seekers mobbing a TV star, not arrayed in an orderly line, like a string of bodyguards.
Then researchers noticed that Andromeda also had dwarf galaxies arrayed in a plane around its center. Still weird, they thought, but maybe these two were just outliers.
But now, a study published in Science suggests that these patterns might not be as odd as they first appeared. A similar row of dwarf galaxies was discovered in 2015 around Centaurus A, a galaxy 13 million light years away.
With three galaxies all showing signs of a similar pattern, cosmologists are trying to figure out how to reconcile new observations with established theories.
“We showed that these structures are more common that we thought. Until now it was thought that, okay, this is just a peculiar thing around the Milky Way and Andromeda. But now we show that these things show up in other isolated systems. So now we need an explanation for this kind of structure,” says Olivier Müller, the lead author of the paper. “I'm excited to see what people will come up with now to explain it.”
There are a few ideas being tossed around trying to understand what might be happening around these larger galaxies. Some of the theories focus on the idea of tidal forces in the formless void of space.
“If we have interaction between two bigger galaxies, then in this interaction there will be some bridges or tidal bridges [between the galaxies]," Müller says. "The gravity of the larger galaxy will just rip out stars and gas similar to the gravity of the moon, which makes the tides on the Earth.” The resulting dwarf galaxies would orbit the larger ones in a plane and along the same direction, similar to what Müller and colleagues observed.
But researchers have no idea how long these tidal dwarf galaxies would last after a collision, and that does have implications for cosmology. “The thing about tidal dwarf galaxies is that we don't know how long they live. Can they be stable? This is in contrast to the standard model, where we think that the dwarf galaxies are the building blocks of the universe—they are the first galaxies that are created and they merge [to form larger galaxies], so they are the oldest objects. But with tidal dwarf galaxies, they would be the youngest objects,” Müller says.
Going from building block to afterthought is a lot to reconcile.
“People think we have detected dark matter, but dark matter is only a hypothesis,” Müller says. “We are still looking for it.”
Dark matter is a critical component of the standard model, helping to explain the gravitational pull between objects in the universe, which can't be explained by the visible matter in the universe.
“I think obviously not having detected dark matter is a problem for that model. But people have a very strong belief in the model for good reason. It explains so much, but this is one area where maybe it's not able to explain things so well, at least at present,” says Michael Boylan-Kolchin, a theoretical astrophysicist who wrote an accompanying perspective on the study.
“I think people are excited because... science can progress by looking for areas where the model doesn't work, and seeing whether that's something that needs to be updated in the model, if it needs to be studied a bit better, or whether there's a more fundamental issue," Boylan-Kolchin says.
This study isn't a death knell for the standard cosmological model, which can explain what happened in the universe moments after the Big Bang, calculate the number of atoms that were present in the universe minutes after it started, explain the presence of cosmic background radiation, and explain how matter is distributed in the universe today. It's a greatest hits list that's hard to beat.
“It's not just explaining observations today, but also cross a wide range of time,” Boylan-Kolchin says. “There's no other model that can come come close to doing that.“
How to reconcile the observations with the theory? More calculations, more thinking, and more data.
For this study, Muller and colleagues looked at the velocities of the satellite galaxies in relation to our perspective here on Earth to extrapolate more detail about their movement around Centaurus A.
“We're able to measure the velocities of these galaxies along our lines of sight, but we're not able to measure the velocity of the galaxy perpendicular to our line of sight,” Boylan-Kolchin says. “So we don't know if they're rotating in planes or if they look like they're rotating in planes.”
There are ways to answer that question. The upcoming James Webb Space Telescope, in addition to the stalwart Hubble, could help astrophysicists study more galaxies and their satellites. By comparing images of the same galaxy taken months or years apart, Boylan-Kolchin says, researchers might be able to get a better sense of how dwarf galaxies are orbiting their hosts, especially if researchers want to take a look at the nearby Andromeda. But it won't be easy.
“It's like trying to measure hair growth on the moon—from Earth. It's very slow, very fine measurements, but it should be possible,” Boylan-Kolchin says.
At the very least, these new observations will let people take a closer look at how our galaxies formed.
“I think now people have to take it seriously, more seriously than before.” Müller says. “Before, there was always this suspicion that we are atypical, we are just a special case in the universe, that normally it works. But now we have shown that another galaxy group nearby also has this feature, so the community has to figure out how we can make such structures more frequently.”