Puzzling Dwarf Galaxies Finally Make Sense

It has long been a mystery why small galaxies don?t have as many stars and matter in their centers as predicted. Now scientists have found the answer with a new simulation of galaxy and star formation.

All galaxies are thought to form inside large cradles of invisible stuff called dark matter. Scientists can't measure dark matter directly, but they suspect it's there because of its telltale gravitational tug on regular matter. And miniature galaxies called dwarf galaxies seem to have even higher proportions of dark matter than regular galaxies.

The so-called cold dark matter theory does very well at explaining how most large structures in our universe form. But it cannot describe dwarf galaxies that well: It predicts that they should have more mass, including both normal and dark matter, in their cores than they do.

But the new supercomputer model includes both cold dark matter theory, as well as detailed theories of star formation, scientists say.

"Most previous work included only a simple description of how and where stars formed within galaxies, or neglected star formation altogether," said Fabio Governato, an astronomer at the University of Washington. "Instead we performed new computer simulations, run over several national supercomputing facilities, and included a better description of where and how star formation happens in galaxies."

The model accurately predicts some of the types of dwarf galaxies that we see around us in the universe. Governato and colleagues report their results in the Jan. 14 issue of the journal Nature.

A key to the new model is that it includes a phenomenon known as stellar wind. When very large stars die, they explode in big blasts called supernova that create huge winds that sweep gas away. When enough gas is pushed out of the center of galaxies, the gravitational pull there is diminished and dark matter, as well as normal matter, just drifts away.

Since dwarf galaxies are so small to begin with ? they typically have around one-tenth, or less, of the mass of the Milky Way ? they have a harder time holding on to their mass than larger galaxies.

When this piece of the puzzle was included in the simulations, the empty-centered galaxies finally made sense.

"The cold dark matter theory works amazingly well at telling where, when and how many galaxies should form," Governato said. "What we did was find a better description of processes that we know happen in the real universe, resulting in more accurate simulations."

The researchers used their model to simulate the formation of two dwarf galaxies. While these results are impressive, they must be shown to hold true for a wider range of galaxy masses and environments, said Marla Geha, an astronomer at Yale University who did not work on the project.

Nonetheless, "a strong hint that this team is heading in the right direction is visually evident ? images

of their simulated galaxies are nearly indistinguishable from the real thing," she wrote in an accompanying essay in the same issue of the journal Nature.

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