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No difference. Three of the Milky Way's dwarf satellite galaxies, W1, Leo II, and Fornax (left to right), have the same mass despite vastly different numbers of stars.

J. Bullock, M. Geha, and R. Powell

No Slimming Down for Dwarf Galaxies

Imagine putting 100 people of various sizes on a scale and finding they all weigh the same. Astronomers have discovered something equally puzzling while surveying a group of tiny galaxies that neighbor the Milky Way: No matter how many stars they contain, every galaxy has the same mass. The results may reveal something profound about how galaxies form--or even about the very nature of the mysterious substance known as dark matter.

The Milky Way is surrounded by 23 so-called dwarf galaxies that are the least luminous galaxies known. Hoping to learn more about these dim collections of stars, cosmologist Manoj Kaplinghat of the University of California, Irvine, and colleagues measured the mass of 18 of them--the most complete accounting yet. The team estimated the mass by observing the speed of a representative sample of stars in each galaxy. A bigger spread in velocity implies a higher mass.

In spite of the fact that the dwarfs contain very different amounts of stars--ranging from a few thousand to more than 10 million--the total masses in the central regions were all equivalent to about 10 million suns, the team reports tomorrow in Nature. This implies that the seemingly smallest galaxies have a lot more invisible dark matter than do their more luminous neighbors.

Why such a disparity exists is hard to explain, Kaplinghat says. Also strange is the lack of galaxies below this mass. "It seems to be telling us there is a threshold when galaxies form," he says. This could be because smaller clumps of matter have too little gravity to form stars and therefore never become visible to us.

A more speculative possibility is that small clumps of matter never form in the first place. This would impact the current favorite model of dark matter, which assumes that massive particles condense into clumps that end up being the gravitational seeds for galaxies. Because these heavy particles are "cold"--meaning they move slowly--they can condense to form very small clumps, as well as very big ones. However, a lighter, "warm" dark-matter particle would make only big clumps and therefore might explain the lack of galaxies at the low-mass end. If the dark matter were warm, it might require different search methods than those currently being applied in underground lab experiments and at the Large Hadron Collider in Geneva, Switzerland, Kaplinghat says.

"This is an intriguing result," says cosmologist Piero Madau of the University of California, Santa Cruz. As to what this might mean, he says, "I have a slight preference for not modifying the properties of the dark-matter particle."