Some nearby dwarf galaxies are causing headaches for astrophysicists who say they don't fit the currently accepted standard model of cosmology.
Specifically, they don't match the accepted model of how galaxies form, say researchers at Western Case Reserve University in Ohio and the Rochester Institute of Technology in New York.
"The model predicts that dwarf galaxies should form inside small clumps of dark matter and that these clumps should be distributed randomly about their parent galaxy," study co-author David Merritt in Rochester says. "But what is observed is very different. The dwarf galaxies belonging to the Milky Way and Andromeda are seen to be orbiting in huge, thin disk-like structures."
This is another departure from the model, the researchers report in the publication Monthly Notices of the Royal Astronomical Society, which holds that the galaxies should independently move in random directions.
Simulations based on the standard model did not match the disk-like structures that have been observed, they said.
"We see the satellite galaxies are in a huge disk and moving in the same direction within this disk, like the planets in our solar system moving in a thin plane in one direction around the aun," Case Western scientist Marcel Pawlowski says. "That's unexpected and could be a real problem."
Several components of the current standard model, including dark energy and dark matter, have been included because the model was proving to be inconsistent with what was being observed, the researchers said.
Some astrophysicists, faced with the failure of the standard model to replicate what is being observed, have been considering alternate theories.
There may be a different -- and older -- explanation behind the dwarf galaxies' formation that may have to be revisited, they say.
That explanation holds the dwarf galaxies are the result of a collision in space between two giant galaxies that ripped material off of both and flung the debris out to great distances, where it slowly coalesced into the dwarf galaxies seen today.
"This 'tidal' model can naturally explain why the observed satellites are orbiting in thin disks," Merrit says.
The new study involved institutions in six countries and more than a dozen scientists, who concentrated on the dwarf galaxies surrounding our own Milky Way.
"But we also have Andromeda," Pawlowski says. "The chance to have two galaxies with such huge disks of satellite galaxies is less than one in 100,000."
The standard model on how galaxies formed is a long-held one, the researchers acknowledge, but no theory can ignore new evidence, they say.
"When you have a clear contradiction like this, you ought to focus on it," Merritt says. "This is how progress in science is made."