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Brightest galaxies grew early after Big Bang

(Download Image) Photo courtesy of Subaru Telescope/National Observatory of Japan An infrared image of a galaxy cluster seen at a distance corresponding to 65 percent back to the Big Bang.

Contrary to popular belief, the stellar mass of the largest galaxies, called bright cluster galaxies, experienced an early period of rapid growth 9 billion years ago, some 4 billion years after the Big Bang.

In fact, the galaxies achieved more than 90 percent of their final stellar mass at the time they were observed, and are not significantly different from their stellar mass today.

The scientific consensus is that galaxies began as small density fluctuations in the early universe and gained most of their weight more recently by swallowing up other galaxies that came too close.

Stars begin to form relatively quickly within sub-galactic sized building blocks, which are subsequently assembled into galaxies. But exactly when this assemblage takes place has stumped the scientific community.

But new research by an international team of astronomers, including Adam Stanford of the Laboratory, shows a new picture of galaxy assembly in which Bright Cluster Galaxies (BCG) experience an early period of rapid growth rather than a prolonged hierarchical assembly.

The group first studied the ages of the stars within these galaxies by looking at the infrared wavebands. The wavebands are less sensitive than optical light to the presence of young stars and are a more accurate tracer of the underlying old stellar population and, in turn, of the stellar mass of the systems.

The team effectively weighed the galaxies and found that despite feeding on a constant diet of small galaxies, the heaviest galaxies have not increased their weight over the last 9 billion years. In a universe with an age of 13.7 billion years old, these results spark a debate as to how these galaxies put on so much weight in the first few billion years after the Big Bang.

The research appears in the April 2 edition of the journal, Nature.

April 10, 2009

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Anne M. Stark
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