When Dwarfs Gave Way to Giants (Forwarded)

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For Release: EMBARGOED until 2:00 p.m. EDT, Wednesday, May 17, 2006

Release No.: 06-15

When Dwarfs Gave Way to Giants

Cambridge, MA -- The first galaxies were small -- about 10,000 times
less massive than the Milky Way. Billions of years ago, those
mini-furnaces forged a multitude of hot, massive stars. In the process,
they sowed the seeds for their own destruction by bathing the universe
in ultraviolet radiation. According to theory, that radiation shut off
further dwarf galaxy formation by both ionizing and heating surrounding
hydrogen gas. Now, astronomers Stuart Wyithe (University of Melbourne)
and Avi Loeb (Harvard-Smithsonian Center for Astrophysics) are
presenting direct evidence in support of this theory.

Wyithe and Loeb showed that fewer, larger galaxies, rather than more
numerous, smaller galaxies, dominated the billion-year-old universe.
Dwarf galaxy formation essentially shut off only a few hundred million
years after the Big Bang.

"The first dwarf galaxies sabotaged their own growth and that of their
siblings," says Loeb. "This was theoretically expected, but we
identified the first observational evidence for the self-destructive
behavior of early galaxies."

Their research is being reported in the May 18, 2006 issue of Nature.

Nearly 14 billion years ago, the Big Bang filled the universe with hot
matter in the form of electrons and hydrogen and helium ions. As space
expanded and cooled, electrons and ions combined to form neutral atoms.
Those atoms efficiently absorbed light, yielding a pervasive dark fog
throughout space. Astronomers have dubbed this era the "Dark Ages."

The first generation of stars began clearing that fog by bathing the
universe in ultraviolet radiation. UV radiation splits atoms into
negatively charged electrons and positively charged ions in a process
called ionization. Since the Big Bang created an ionized universe that
later became neutral, this second phase of ionization by stars is known
as the "epoch of reionization." It took place in the first few hundred
million years of existence.

"We want to study this time period because that's when the primordial
soup evolved into the rich zoo of objects we now see," said Loeb.

During this key epoch in the history of the universe, gas was not only
ionized, but also heated. While cool gas easily clumps together to form
stars and galaxies, hot gas refuses to be constrained. The hotter the
gas, the more massive a galactic "seed" must be to attract enough matter
to become a galaxy.

Before the epoch of reionization, galaxies containing only 100 million
solar masses of material could form easily. After the epoch of
reionization, galaxies required more than 10 billion solar masses of
material to be assembled.

To determine typical galaxy masses, Wyithe and Loeb looked at light from
quasars -- powerful light sources visible across vast distances. The
light from the farthest known quasars left them nearly 13 billion years
ago, when the universe was a fraction of its present age. Quasar light
is absorbed by intervening clouds of hydrogen associated with early
galaxies, leaving telltale bumps and wiggles in the quasar's spectrum.

By comparing the spectra of different quasars along different lines of
sight, Wyithe and Loeb determined typical galaxy sizes in the infant
universe. The presence of fewer, larger galaxies leads to more variation
in the absorption seen along various lines of sight. Statistically,
large variation is exactly what Wyithe and Loeb found.

"As an analogy, suppose you are in a room where everybody is talking,"
explains Wyithe. "If this room is sparsely populated, then the
background noise is louder in some parts of the room than others.
However if the room is crowded, then the background noise is the same
everywhere. The fact that we see fluctuations in the light from quasars
implies that the early universe was more like the sparse room than the
crowded room."

Astronomers hope to confirm the suppression of dwarf galaxy formation
using the next generation of telescopes -- both radio telescopes that
can detect distant hydrogen and infrared telescopes that can directly
image young galaxies. Within the next decade, researchers using these
new instruments will illuminate the "Dark Ages" of the universe.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists, organized into six research divisions, study the origin,
evolution and ultimate fate of the universe.

Note to editors:

High-resolution artwork to accompany this release is available online at
http://www.cfa.harvard.edu/press/pr0615image.html