Hubble Astronomers Uncover An Overheated Early Universe

WASHINGTON -- During a period of universal warming 11 billion years
ago, quasars -- the brilliant core of active galaxies -- produced
fierce radiation blasts that stunted the growth of some dwarf
galaxies for approximately 500 million years.

This important conclusion comes from a team of astronomers that used
the new capabilities of NASA's Hubble Space Telescope to probe the
invisible, remote universe. The team's results will be published in
the October 10 issue of The Astrophysical Journal.

Using Hubble's Cosmic Origins Spectrograph (COS), the astronomers
identified this era, from 11.7 to 11.3 billion years ago, when the
ultraviolet light emitted by active galaxies stripped electrons off
helium atoms. The process, known as ionization, heated the
intergalactic helium from 18,000 degrees Fahrenheit to nearly 40,000
degrees. This inhibited the gas from gravitationally collapsing to
form new generations of stars in some small galaxies.

Because of its greatly improved sensitivity and lower background
"noise" compared to previous spectrographs in space, the COS
observations were ground-breaking. The observations allowed
scientists to produce more detailed measurements of the intergalactic
helium than previously possible.

"These COS results yield new insight into an important phase in the
history of our universe," said Hubble Program Scientist Eric Smith at
NASA Headquarters in Washington.

Michael Shull of the University of Colorado in Boulder and his team
studied the spectrum of ultraviolet light produced by a quasar and
found signs of ionized helium. This beacon, like a headlight shining
through fog, travels through interspersed clouds of otherwise
invisible gas and allows for a core sample of the gas clouds.

The universe went through an initial heat wave more than 13 billion
years ago when energy from early massive stars ionized cold
interstellar hydrogen from the big bang. This epoch is called
reionization, because the hydrogen nuclei originally were in an
ionized state shortly after the big bang.

The Hubble team found it would take another two billion years before
the universe produced sources of ultraviolet radiation with enough
energy to reionize the primordial helium that also was cooked up in
the big bang. This radiation didn't come from stars, but rather from
super massive black holes. The black holes furiously converted some
of the gravitational energy of this mass to powerful ultraviolet
radiation that blazed out of these active galaxies.

The helium's reionization occurred at a transitional time in the
universe's history when galaxies collided to ignite quasars. After
the helium was reionized, intergalactic gas again cooled down and
dwarf galaxies could resume normal assembly.

"I imagine quite a few more dwarf galaxies may have formed if helium
reionization had not taken place," Shull said.

So far, Shull and his team only have one perspective to measure the
helium transition to its ionized state. However, the COS science team
plans to use Hubble to look in other directions to determine if
helium reionization uniformly took place across the universe.

For illustrations and more information about these results, visit:

http://hubblesite.org/news/2010/31

and

http://www.nasa.gov/hubble

Source: NASA

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