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Dwarf galaxies: breakthrough in bid to find 'fossils' of early universe

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The approach the others used allowed them to claim a detection, but the technique wasn't powerful enough to yield an estimate of the dwarf galaxy's mass or whether the system held more than one dwarf galaxy, Dr. Vegetti says.

Her team's approach allows estimates of both.

Dwarf galaxies don't have the eye-popping appearance of galaxies such as the Pinwheel or Andromeda galaxies.

Over the past several years, astronomers have found 25 newly identified dwarfs orbiting the Milky Way and the Andromeda galaxy. Most are dimmer than any previously known dwarfs. And a few are so dim they could be mistaken for globular clusters, typically collections of 1,000 or more stars whose mutual gravity keeps the group together.

Despite their lack of visual zip, dwarf galaxies have come to be recognized as fossils from the early epochs of galaxy formation.

Over the past decade or so, however, the hunt for these galactic no-seeums has gained momentum, explains Dr. Bullock, who was not part of the research team involved in the observation.

The reason: The currently favored picture of galaxy formation, known as the cold dark matter theory, predicts that far more dwarf galaxies should be present than astronomers have seen as they hunt for starlight from the objects.

Cold dark matter is thought to comprise 90 percent of all the matter in the universe. It emits no light; its presence is inferred from the influence its gravity exerts on galaxies and clusters of galaxies.

Each galaxy is thought to have formed within its own vast cocoon of dark matter, then to have grown as it merged or collided with others.

The theory itself is on firm footing, Bullock explains. But since the theory seems to be pretty robust, it's predictions relating the distribution of numbers and masses of dwarf galaxies should also be robust.

Essentially, the theory allows researchers to derive an estimate of the number of dwarf galaxies that should be present at different masses. Dwarfs with the lowest masses, hence the faintest galaxies, should be the most numerous, Vegetti says.

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