International team uses powerful cosmic lens
to find galactic building block in early universe
PASADENA, Calif.- Exploiting a phenomenon known as gravitational
lensing, an international team of astrophysicists has detected a very
small, faint stellar system in the process of its formation during
the first half billion years or so of the universe's existence.
The discovery is being reported in an upcoming issue of the
Astrophysical Journal. According to lead author Richard Ellis, a
professor of astronomy at the California Institute of Technology, the
faint object is an excellent candidate for the long sought after
"building blocks" thought to be abundant at early times and which
later assembled to make present-day galaxies.
The discovery was made possible by examining small areas of sky
viewed through a massive intervening cluster of galaxies, Abell 2218,
2 billion light-years away. The cluster acts as a powerful
gravitational lens, magnifying distant objects and allowing the
scientists to probe how distant galaxies assembled at very early
times.
Gravitational lensing, a dramatic feature of Einstein's theory of
general relativity, means that a massive object in the foreground
bends the light rays radiating from one in the background because
mass curves space. As a result, an object behind a massive
foreground galaxy cluster like Abell 2218 can look much brighter
because the foreground object has bent additional photons toward
Earth, in much the same way that glass lenses in binoculars will bend
more photons toward the eyes.
In the case of the system detected by Ellis and coworkers, the effect
makes the image at least 30 times brighter than would be the case if
the Abell 2218 cluster were not in the foreground. Without this
boost, neither the Keck 10-meter Telescopes nor the Hubble Space
Telescope would have detected the object.
Ellis explains, "Without the benefit of the powerful cosmic lens, the
intriguing source would not even have been detected in the Hubble
Deep Fields, historic deep exposures taken in 1995 and 1998."
Using the 10-meter Keck Telescopes at Mauna Kea, the collaboration
found a faint signal corresponding to a pair of feeble images later
recognized in a deep Hubble Space Telescope picture. Spectroscopic
studies made possible with the superior light-gathering power of the
Keck confirmed that the images arise via the magnification of a
single source diagnosed to be extremely distant and in the process of
formation.
"The system contains about a million or so stars at a distance of
13.4 billion light-years, assuming that the universe is 14 billion
years old," claims Ellis. "While more distant galaxies and quasars
have been detected with the Keck Telescopes, by virtue of the
magnification afforded by the foreground cosmic lens, we are
witnessing a source much smaller than a normal galaxy forming its
first generation of stars."
"Our work is a little like studying early American history," says
team member Mike Santos, a Caltech graduate student in astronomy.
"But instead of focusing on prominent individuals like George
Washington, we want to know how everyday men and women lived.
"To really understand what was going on in the early universe, we
need to learn about the typical, commonplace building blocks, which
hold important clues to the later assembly of normal galaxies. Our
study represents a beginning to that understanding."
The precise location of the pair of images in relation to the lensing
cluster allowed the researchers to confirm the magnification. This
work was the contribution of team member Jean-Paul Kneib of the
Observatoire Midi-Pyr=E9n=E9es near Toulouse, France, an expert in the
rapidly developing field of gravitational lensing.
The team concludes that the star system is remarkably young (by
cosmic standards) and thus may represent the birth of a subcomponent
of a galaxy or "building block." Such systems are expected to have
been abundant in the early universe and to have later assembled to
form mature large galaxies like our own Milky Way.
Santos explains, "The narrow distribution of intensity observed with
the Keck demonstrates we are seeing hydrogen gas heated by newly
formed stars. But, crucially, there is not yet convincing evidence
for a well-established mixture of stars of different ages. This
suggests we are seeing the source at a time close to its formation."
In their article, the researchers infer that the stars had been
forming at a rate of one solar mass per year for not much longer than
a million years. Such a structure could represent the birth of a
globular cluster, stellar systems recognized today to be the oldest
components of the Milky Way galaxy. The work represents part of an
ongoing survey to determine the abundance of such distant
star-forming sources as well as to fix the period in cosmic history
when the bulk of these important objects formed.
Additional information and images are available at:
http://www.astro.caltech.edu/~rse/firstlight/