Our Milky Way galaxy is surrounded by a dozen smaller orbiting galaxies. The size of this cosmic neighborhood has perplexed astrophysicists for some time because the currently favored theory of galaxy formation predicts 10 times as many satellites. But new computer simulations run by Andrey Kravtsov of the University of Chicago and his colleagues have shown that the relative paucity of Milky Way companions may not be such a concern after all.

Standard cosmology says that most of the matter in the universe is an unknown substance that moves slowly and does not emit light. Over time, small clumps of this cold dark matter have merged to form increasingly large clumps. Above a certain mass threshold, normal matter inside these dark matter objects, called halos, forms stars, and eventually galaxies. Computer models incorporating this hierarchical progression have successfully reproduced the cosmic web that astronomers can see in their largest maps of the heavens. But it has been hard to accurately simulate individual galaxies and their vicinities because there are more details to worry about at these smaller scales.

The new simulations, detailed in the July 10 issue of The Astrophysical Journal, probed the formation history of a Milky Way-type environment (see image) using relatively small time increments, which allowed the researchers to follow the ebb and flow of the dark matter. In so doing, they discovered that as gravity pulled everything in, tidal forces from larger halos stripped mass off of smaller halos. Only the heftiest of the sub-halos formed dwarf galaxies before the loss of mass put an end to star formation. In the end, the predicted number of luminous galactic satellites matched observations.

Some theorists had tried to solve the so-called satellite problem by introducing new physics, such as warm dark matter that mixed with the cold. The current results show "that the problem can be solved without using anything exotic," Kravtsov said. "There is no question about the physical ingredients of our model."