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.

7 Comments

1. 1. Rowan Grigg 09:45 AM 12/30/11

   I live far enough from the city glow (and in the southern hemisphere) to see the Magellanic clouds in all their glory each night. But tell me, are they *real* galaxies with potentially viable stars and planets and et and such?

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2. 2. tharter in reply to Rowan Grigg 11:07 AM 12/30/11

   Sure they are. They are MUCH smaller than the Milky Way, and have a different star-forming history but they're just galaxies. There must presumably be planets and such.

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3. 3. MadScientist72 11:32 AM 12/30/11

   A couple things came to mind when I read this:
   "Milky Way, what big ears you have!" and "Oh great! We're living in the Prince Charles/Ross Perot of galaxies!"
   
   But, seriously, is it possible that the Magellanic Clouds didn't form as companions to the Milky Way, but were picked up by its gravity sometime later? Maybe some of our natural companions got flung off into space in the disruption caused by picking up a couple of galactic hitchhikers?

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4. 4. jtdwyer 10:46 PM 12/30/11

   Without being able to read the referenced research report it's difficult to assess, but it's abstract states:
   "The statistics of the number of MCs in the lambda cold dark matter model are in good agreement with observations of a large sample of Sloan Digital Sky Survey (SDSS) galaxies. Under the subhalo abundance matching assumption of a relationship with small scatter between galaxy r-band luminosities and halo internal velocities v[max], we make detailed comparisons to similar measurements using SDSS Data Release 7 data by Liu et al. Models and observational data give very similar probabilities for having zero, one, and two MC-like satellites."
   
   There are several additional factors that may have affected these results.
   
   The researchers selected galaxies from the survey that had similar apparent luminosities to the Milky Way. Unfortunately, the apparent luminosity of spiral galaxies depends in large part to factors such as the observational aspect angle to the disk plane and the degree of stellar obscuration produced by gas clouds. When viewed edge-on, spiral galaxies' apparent luminosity is reduced since the entire disk appears to be non-luminous. The galaxies selected for comparison with the Milky Way may have been less similar than presumed.
   
   When viewed full on (from 'above' or 'below'), it can be very difficult to distinguish objects within a galaxy's halo region from those in the galactic disk. Those on the opposite side of the disk may be completely obscured. A significant number of companion galaxies may have avoided identification.
   
   Then there's the dark matter halo assumption. While spiral galaxy disk objects' velocities do not diminish with increasing radii, as predicted by Kepler's laws of planetary motion, observable halo objects (presumed here to be enveloped by dark matter) DO independently orbit the remote central galactic mass 'just like planets in the Solar system'. This observation argues against the presence of a massive dark matter halo in the Milky Way galaxy. Is the Milky Way not only one of the few similar galaxies that have Magellanic Cloud-like companions but one of a few galaxies without dark matter? Not likely. Please see: Bratek et al, (2011), "Keplerian Ensemble Approximation. The issue of motions of Galactic halo compact objects", http://arxiv.org/abs/1108.1629v1
   
   For additional comments and references, please see" Dwyer, (2011),
   "On not being the first to discover no galactic dark matter", http://www.sciencewithoutfiction.com/uploads/JDwyer.PDF

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5. 5. David Russell 09:19 PM 1/2/12

   From what I have seen the reach or affect of the Milky Way is about 1,000,000 light years and considering Andromeda is at least as big as the Milky Way I suspect it affects about 1,000,000 light years of space in its area. It will be getting more interesting over the next 2 billion years as they meet.

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6. 6. jack.123 06:16 AM 1/6/12

   If Andromeda comes in edge first it will look like our own Milky Way but if it comes in face first it will be spectacular turning night into day for half of the year.For hundreds of millions of years it will fill the sky that is of course if Earth is on the right side of the galaxy when it comes,and if there is anybody still here to see it.If there is any reason to travel in too the future it would be to see this event.

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7. 7. verdai 05:52 PM 1/10/12

   just so we have our halo.

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