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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