Dark Matter Helped Early Galaxies Survive "Massacre"

By Ker Than
for National Geographic News

The "ignition" of the first stars half a billion years after the big bang led to a cosmic massacre that spared just one out of every thousand galaxies. Survival depended on having large clouds of the mysterious substance known as dark matter, a new supercomputer model suggests.

Within dark matter clouds, normal matter was in the process of coalescing into young stars.

These stars, however, would have been sending out damaging radiation.

Larger dark matter clouds would have attracted more normal, or visible, matter, which means that larger galaxies would have had enough material to survive even after being blasted by radiation from their neighbors.

Smaller galaxies, meanwhile, would have had all their stars and star-forming material vaporized, leaving behind barren dark matter clumps.

"This is a case where the bullies really win out," said study team member Carlos Frenk, an astrophysicist at Durham University in the U.K.

"The galaxies that managed to make the stars that fried the early universe were the ones that managed to accumulate dark matter the fastest."

"Missing" Satellites

For a long time after the galactic massacre, no new galaxies were able to form, according to the new simulations by Frenk and Takashi Okamoto of the University of Tsukuba in Japan.

Dark matter, meanwhile, continued to merge and grow into ever larger structures.

Then, around 10 to 12 billion years ago, some of the dark matter clumps grew massive enough to counteract the radiation from the survivor galaxies.

At this point the dark matter could once again "protect" normal matter, and larger galaxies were finally able to take shape.

This model, recently presented at the Royal Society 2009 Summer Science Exhibition in London, could explain the Milky Way's "missing satellite" problem, said astrophysicist Andrew Benson of the California Institute of Technology.

So far, astronomers know of only about 20 satellite galaxies of the Milky Way, but according to a key theory of galaxy formation, there should be thousands.

That's because big galaxies like the Milky Way are thought to have formed through the violent mergers of many smaller galaxies.

Any discarded remnants that didn't make it into the larger structure would have become satellite galaxies.

But if the new model is correct, then the Milky Way's "missing" satellite galaxies never formed in the first place, said Benson, who was not involved in the new study.

"Professor Frenk has shown that if you can prevent the formation of galaxies very early in the universe, you can reduce the number of galaxies that you would expect to see around the Milky Way down to a level that is more compatible with what we actually observe."