An international team led by astronomers from the University of Hawaii
reported discovery of a galaxy which gives the earliest glimpse of
the epoch when galaxies and stars formed in the universe.
"This galaxy is forming stars at a time speculated to be in the `Dark Ages'
of the universe when galaxies begin to `turn on'," said University of Hawaii
professor Esther Hu, who led the team.
According to the generally accepted picture, the universe started with the
Big Bang some 14-16 billion years ago. As the universe expanded and
cooled over the next half million years, the glowing plasma of which
it was composed recombined into atoms of neutral gas -- mostly hydrogen
and some helium. The glow from this era of recombination has been
observed as the cosmic microwave background radiation, and used to
study the large-scale geometry of the universe.
Over the next nearly half billion years, termed the Dark Ages, the cold gas
began to assemble into the first galaxies. The Dark Ages ended as the light
from the newly formed galaxies and quasars reionized and changed the
character of the surrounding neutral gas.
Till now the earliest probes of the Universe have been quasars, which
are extremely luminous distant objects, believed to be powered by
black holes.
To capture an early snapshot of galaxies, which are typically a thousand times
fainter, researchers concentrated on a bright hydrogen emission
line dubbed "Lyman alpha" that is strongly excited during star formation.
Since a large fraction of the light from early forming galaxies emerges
in this line, distant galaxies can look prominent viewed through
filters which only pass wavelengths near the Lyman alpha emission but
appear faint or undetected when seen through other filters.
The method of discovering distant galaxies by searching for objects
identified as "Lyman alpha emitters" by the sharp increase in their
detectability in narrow bandpass filters has been very successful,
and team members had previously used one of the giant Keck 10-m
telescopes to find the most distant galaxy previously known, an object
whose light took some 15.3 billion years to reach us.
In order to reach fainter and yet more distant galaxies in the present
work, Hu and her colleagues used a gravitational lens, in the form of
a massive cluster galaxies to further amplify the light. According to
Einstein's theory of general relativity, very massive objects can bend and
focus light in much the same way as a magnifying glass. The astronomers
used the cluster Abell 370, which is 6 billion light years away and whose
core contains the mass of several hundred galaxies, to magnify light
from a galaxy behind the cluster that is 15.5 billion light years distant.
The discovery images made with the 10-m Keck I telescope were confirmed
with spectra obtained later on the same telescope which showed
there was indeed a strong Lyman alpha emission line.
"It's significant that you can see the line," said Peter Capak, a
University of Hawaii graduate student and team member. "If only a few
galaxies had turned on by this point the emission would have been smothered
by the surrounding hydrogen gas and the light would never have made it
out to us."
Len Cowie, another Hawaii astronomer and team member added, "The fact
that this is a galaxy, and not a quasar, is also important. When the
first galaxies form, it's like turning on lights to clear out a fog bank.
Quasars are really bright though rare, so they can make large clear
cavities around themselves, but the fact that light from the fainter
but much more numerous galaxies is getting out means that a significant
amount of early star formation has already taken place and much of the
general fog has already dissipated."
The newly discovered galaxy has a redshift of 6.56, and samples the
universe when it is about 780 million years old. This is about 50
million years earlier than the view supplied by the most distant quasar
(redshift = 6.28), and 80 million years earlier than the speculated period
of reionization (redshift ~6.1).
Since most of the light from these galaxies has been redshifted to
infrared wavelengths, the team followed up their discovery with
infrared images on the Subaru 8.3-m Telescope, also on Mauna Kea,
to estimate the star formation rate -- finding that 40 times the mass of
the Sun is being turned into new stars each year.
"You want to catch galaxies in their infancy and see how they
develop", commented Hu.
"Scaling the age of the Universe to a person's lifetime, we're
showing you baby pictures. The last snapshot showed a toddler
just past his fourth birthday. This one is three and a half."
"This is good news for the Next Generation Space Telescope
planned for launch in the next decade,"
she concluded. "It means that there should be plenty of these
distant galaxies bright enough to observe, using a large telescope with good
infrared detectors, above the strong airglow of our atmosphere."
Images and additional information about distant galaxy searches are
available at:
http://www.ifa.hawaii.edu/~cowie/z6/z6.html
The paper will appear in the April 1 issue of the Astrophysical Journal
Letters. It has been posted to the public archive and should appear this
Friday as
http://xxx.lanl.gov/abs/astro-ph/0203091
CONTACTS:
Esther M. Hu
Institute for Astronomy, University of Hawaii
Phone: 808-956-7190
hu@ifa.hawaii.edu
Lennox L. Cowie
Institute for Astronomy, University of Hawaii
Phone: 808-956-8134
cowie@ifa.hawaii.edu
Richard G. McMahon
Institute of Astronomy, University of Cambridge
Phone: 44-1223-337548
rgm@ast.cam.ac.uk
