Our Milky Way is just one of many billions of galaxies that dot the cosmos—an ordinary spiral in a universe filled with them. The unspecialness of our corner of space, an idea known as the Copernican principle, is a cornerstone of modern cosmology. But it doesn't mean that the Milky Way has to be totally average in every respect.
Among the more than 20 satellite galaxies that hover around the Milky Way in a kind of galactic entourage are two large satellites known as the Large and Small Magellanic Clouds. Stargazers and navigators have known about them since before the age the telescope. Yet today's astrophysicists have had a hard time explaining how they got there. Computer simulations of galaxy formation and evolution tend not to produce bright satellite galaxies akin to the two Magellanic Clouds. So researchers had to ask: Are the simulations flawed—perhaps in the way that they account for the all-important role of the mysterious dark matter—or is the Milky Way just a bit of an oddball?
With help from new supercomputer simulations and from a universe-mapping telescope project called the Sloan Digital Sky Survey, the matter now seems to have been settled.
Sloan survey says? Oddball.
In a state-of-the-art affirmation of the earlier models, the latest round of supercomputer simulations again showed that a Milky Way–size galaxy should rarely gather satellites the size of the Magellanic Clouds. And telescopic observations of thousands of real-life galaxies and their satellites have confirmed that theoretical prediction.
"It's really that the Milky Way is sort of unique," says Michael Busha, an astrophysicist currently based at the University of Zurich in Switzerland. Busha is lead author of a study in the December 20 Astrophysical Journal that compared the results of a new computer simulation, named Bolshoi, to actual galaxy observations from Sloan. In both simulation and observation, the majority of Milky Way–like galaxies have no companions as hefty as the Magellanic Clouds. A handful of galaxies have one such satellite, and very few—roughly 5 to 10 percent—match the Milky Way's count of two large satellite galaxies.
"It's a little odd, but it's not really odd," Busha says of our home galaxy, likening the Magellanic Clouds to an oversize feature on a human face. "One of my colleagues calls them 'the big ears of the Milky Way,'" he says. "You look like a normal person, you don't look strange, you just happen to have large ears."
The discrepancy between how the Milky Way looks and how theory said it ought to look "has been nagging some of us for a number of years," says cosmologist James Bullock of the University of California, Irvine. "I wouldn't go so far as to say I was losing sleep, but maybe tossing and turning some." Bullock and his co-authors recently used a different simulation, called Millennium-II, and a different set of Sloan galaxies to come to a similar conclusion. "Our galaxy is apparently a little unusual—about as unusual as theory predicted," he says. "This is good news. There are some remaining puzzles, but at least this one seems under control."
What may not be under control is a very different disparity between theory and observation of the Milky Way's companion galaxies at the smaller end of the size spectrum. In what is known as the "missing satellite problem," galaxy simulations tend to produce more small, faint satellite galaxies than astronomers actually see near the outskirts of the Milky Way. Many possible explanations have been suggested—perhaps astronomers simply have not yet found all of the Milky Way's satellites, or perhaps small galaxies do not develop as readily as assumed.
Whether the Milky Way's unusually populous coterie of large satellites has anything to do with the missing satellite problem remains to be seen. "What we want to know is: The fact that we have these two bright satellites, what does that say about the rest of the satellite population? What does that say about the missing satellite problem?" Busha says. "We really don't know the answer to that question."
Busha's colleagues are looking into it, however. Louis Strigari and Risa Wechsler of Stanford University have begun to investigate how the Milky Way stacks up against similar galaxies when it comes to somewhat smaller satellites. They dug through Sloan data to identify how often Milky Way analogues host companion galaxies dimmer than the Magellanic Clouds but brighter than the next-brightest Milky Way satellites, the dwarf galaxies Sagittarius and Fornax. Simulations of these intermediate-size satellites predict that the Milky Way ought to have more companions than it does.
"The naive prediction would be that there should be many systems brighter than Fornax and dimmer than the [Small Magellanic Cloud], which we do not see," Wechsler says. "One way out of this problem would be to say that the Milky Way is rare in this respect as well, and that most galaxies do have these extra satellites."
But that explanation does not stand up to Strigari and Wechsler's preliminary analysis. "Here, we found that the Milky Way does seem to be consistent with having a typical number of dimmer satellite galaxies," Wechsler says, although she notes that the error bars are still large. "This is probably telling us that it is the Magellanic Clouds that are atypical, and not the Milky Way system as a whole."
Why supercomputer simulations should generate more midsize satellites than are found near real-life galaxies, including ours, remains an open question. But researchers can at least sleep a bit better knowing that the Milky Way is indeed rather ordinary, big ears notwithstanding.