Missing dwarves. According to theory, large galaxies should be surrounded by hundreds of small satellites, like the ones shown in this supercomputer simulation.

The Case of the Missing Dwarves

Some of the dwarf galaxies swarming around our Milky Way were much bigger and more massive in the distant past, according to a new computer simulation. If correct, the study would solve a nagging problem for cosmologists who have failed to detect the many dwarf galaxies predicted by the favored version of the big bang scenario.

Computer simulations of the evolution of the universe predict that major galaxies like our own should be surrounded by hundreds of small clumps of dark matter that might evolve into dwarf galaxies. Embarrassingly for the theorists, however, only a handful of dwarf galaxies have been found around the Milky Way and other large galaxies. To explain the discrepancy, astronomers have suggested that star formation only occurs in the most massive of dark matter clumps--clumps far bigger than most dwarf galaxies--leaving the smaller ones invisible. But if that's right, it's hard to explain why stars are seen even in some small dwarves. Apparently, a major part of the puzzle is still missing.

The missing piece may now have been found. In a paper to be published in the 10 July issue of The Astrophysical Journal, theorists led by Andrey Kravtsov of the University of Chicago claim that the visible dwarfs may be leftovers of larger galaxies that formed stars when they were much more massive. In the team's detailed supercomputer simulations, one in every 10 dwarves lost a large part of its mass--up to 99%--due to the gravitational pull of its parent or neighboring galaxies. These once-massive galaxies are now the puzzlingly luminous small dwarves.

The team's paper is "a nice piece of work ... that is probably correct," says Jeremiah Ostriker of Princeton University. But Simon White of the Max Planck Institute for Astrophysics in Garching, Germany, isn't convinced that the new study solves the puzzle completely. He says more compelling support for the current dark matter theory could come from studies seeking to detect the predicted invisible clumps of dark matter as they bend the light of distant quasars.

Related sites
Abstract of paper with link to full text
National Center for Supercomputing Applications
Background information on cold dark matter
Simulation of structure formation in a cold dark matter universe