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Some of the Universe's First Galaxies Discovered

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Author Topic: Some of the Universe's First Galaxies Discovered  (Read 53 times)
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« on: November 07, 2009, 07:05:30 pm »

Some of the Universe's First Galaxies Discovered

A new survey has found 22 of the earliest galaxies to form in the universe, confirming the age of one at just 787 million years after the theoretical Big Bang.

These and other galaxies from the universe's childhood could help shed light on the conditions that governed the early universe.

With recent technological advances, astronomers have been able to observe more of the so-called reionization era, the farthest back in time that astronomers can observe.

Universe's first light

For the first few hundred thousand years after the Big Bang (which took place about 13.7 billion years ago), the universe was a hot, murky mess, with no light radiating out. Because there is no residual light from that early epoch, scientists can't observe any traces of it.

But about 400,000 years after the Big Bang, temperatures in the universe cooled, electrons and protons joined to form neutral hydrogen (meaning it had no charge), and the murk cleared. Some time before 1 billion years after the Big Bang, neutral hydrogen began to form stars in the first galaxies, which radiated energy and changed the hydrogen back to being ionized, or charged. This was what astronomers call the reionization period.

But while astronomers know that this period was over by about the time the universe was 1 billion years old, they don't know exactly when it began when the first stars and galaxies began to illuminate the universe. They also don't know whether reionization started gradually or instantly.

To help answer this question a team of astronomers led by Masami Ouchi of the Carnegie Observatories used a technique for finding some of the early, extremely distant galaxies.

"We look for 'dropout' galaxies," Ouchi said. "We use progressively redder filters that reveal increasing wavelengths of light and watch which galaxies disappear from or 'dropout' of images made using those filters."

Dropout galaxies

The specific wavelengths of light at which the 'dropout' galaxies appear can tell astronomer's their distance and age.

Ouchi and his colleagues studied an area over 100 times larger than any previous such study and so had a larger sample of galaxies.

"Plus, we were able to confirm one galaxy's age," Ouchi said. "Since all the galaxies were found using the same dropout technique, they are likely to be the same age."

The team's observations were made from 2006 to 2009 with the wide-field camera of the 8.3-meter Subaru Telescope in Hawaii.

Ouchi and his team compared their observations with those from other studies looked at the rates of star formation, which can be gleaned from data on the density and brightness of galaxies, and found that they were dramatically lower from 800 millions years to about one billion years after the Big Bang, than thereafter.

Accordingly, they calculated that the rate of ionization would be very slow during this early time, because of this low star-formation rate.

"We were really surprised that the rate of ionization seems so low, which would constitute a contradiction with the claim of NASA's WMAP satellite. It concluded that reionization started no later than 600 million years after the Big Bang," Ouchi said.

"We think this riddle might be explained by more efficient ionizing photon production rates in early galaxies," he added. "The formation of massive stars may have been much more vigorous than in today's galaxies. Fewer, massive stars produce more ionizing photons than many smaller stars."

Ouchi's findings will be detailed in a December issue of the Astrophysical Journal.

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