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Received 17 May 2003

University of California-Santa Cruz

Contact:
Shawna Williams (831) 459-2495, sewillia{at}ucsc.edu
Tim Stephens (831) 459-2495, stephens{at}ucsc.edu

EMBARGOED: Not for release until 2 p.m. Eastern Time on Wednesday,
April 30 

Messages from the early universe shed light on how elements form

SANTA CRUZ, CA -- New information from a distant corner of the
universe may lead to a fuller understanding of how the elements of
the periodic table -- which make up all the familiar matter in the
universe -- come to be. A team of astronomers has used light from a
powerful quasar to analyze the composition of a young galaxy in
unprecedented detail, measuring elements never before detected in
such a far-off galaxy. 

"I never thought we'd find one where we could measure boron, tin, and 
lead," said Jason X. Prochaska, the University of California, Santa 
Cruz, astronomer who led the project. "This opens up a whole new area
of research."

The technique only works for very distant galaxies that happen to be
in the line of sight between Earth and a quasar, said Prochaska, an
assistant professor of astronomy and astrophysics. 

"The quasar provides a little window where we can do this
observation," he said.

Prochaska and his collaborators, J. Christopher Howk and Arthur M.
Wolfe of UC San Diego, report in the May 1 issue of the journal
Nature that galaxies in this window provide valuable clues about
nucleosynthesis, the process by which elements form.

By determining the relative amounts of elements in different cosmic 
objects, astronomers learn about how various astrophysical processes 
stock the periodic table. Only the lightest elements -- hydrogen, 
helium, and lithium -- are thought to have formed in the first
moments after the Big Bang. Other elements come together inside
stars, where extreme heat and density encourage lighter elements to
fuse together. 

Stars produce different elements at different stages of their life 
cycles. When stars burst into supernovae, the explosions forge still 
more elements. Supernovae spew out newly formed elements as
interstellar gas, which eventually condenses into new stars and
planets. Other processes, such as the action of cosmic rays, account
for further nucleosynthesis.

Most information on nucleosynthesis to date has come from studies of 
stars in our home galaxy, the Milky Way, and a handful of other
nearby galaxies. Each element absorbs and gives off light at a
certain wavelength. By analyzing the intensity of light coming from
stars at specific wavelengths, astronomers can determine the relative
amounts of the elements they contain. In this traditional approach,
the star both emits and absorbs the light that astronomers analyze.

An alternative technique uses the absorption of light by interstellar 
gas to measure elemental abundances in the gas that fills the Milky
Way and other galaxies. For example, analyzing the light from a
bright star in the Milky Way reveals absorption signatures that tell
astronomers about the composition of the gas between the star and the
Earth. 

This technique can be used on other galaxies by identifying a distant 
quasar that lies behind the galaxy. Quasars are extremely bright
objects astronomers think are related to massive black holes. The
technique has been applied to the Milky Way and its nearest
neighbors, but the observations are difficult because the majority of
the absorption signatures lie at ultraviolet wavelengths. Earth's
atmosphere filters out ultraviolet light, so the observations require
expensive space-based telescopes.

Ironically, this technique is more easily carried out on very distant 
galaxies. That's because the expansion of the universe causes
galaxies to move further apart so fast that the light they emit is
shifted toward longer wavelengths. Galaxies that are extremely far
away -- say, 10 billion light-years -- are moving at such a pace that
the absorption signatures from their elements are shifted out of the
ultraviolet and into the visible range. By analyzing the signature of
an intervening galaxy on light from a distant quasar, astronomers
gain vital information on galaxies that are generally too faint to
observe directly. 

In the Nature paper, Prochaska and his coauthors describe a young
galaxy in which they were able to study the signatures of many
different elements. The galaxy is so far away that the light from it
has taken billions of years to reach Earth, thereby giving the
astronomers a glimpse back in time.

Prochaska's group first found the galaxy by identifying a
characteristic dip in the quasar signal caused by hydrogen gas in the
galaxy. The researchers then looked for the signatures caused by
other elements. By measuring the dips in light intensities at the
corresponding wavelengths, they determined the relative amounts of 25
different elements in the galaxy. Previous observations of such
distant galaxies have yielded information on only a handful of
elements. 

"Many of the additional elements give us new information on how stars 
are forming, how elements form, and the age of the galaxy," Prochaska 
said. "Each of those is a key area of astrophysics, so to be able to
do all three is particularly exciting."

Scientists constantly look for new astronomical data to confirm or 
refine their models for how nucleosynthesis occurs. In this galaxy,
the ratios of elements to each other is similar to that in our own
galaxy, which Prochaska said was comforting because "it appears there
is nothing too weird going on here." The differences between the two
galaxies are also instructive, putting the age of the young galaxy at
about one to two billion years, compared with the 10-billion-year-old
Milky Way. 

The researchers hope to study many more galaxies in the same way.
They have already found another promising galaxy along the same sight
line as the one described in the paper.

"What is exciting is that this discovery suggests we can repeat the 
analysis for 100 other galaxies," Prochaska said. "That's 100
different galaxies walking down unique paths for the formation of the
elements." 

The researchers made their observations at the W. M. Keck Observatory
on Mauna Kea, Hawaii. The initial observations were made with the
Echellette Spectrograph Imager on the Keck II Telescope, and
follow-up observations with the High Resolution Echelle Spectrograph
(HIRES) on the Keck I Telescope.

Note to reporters: You may contact Prochaska at (831) 459-2135 or 
xavier{at}ucolick.org. He will be observing at the W. M. Keck
Observatory from April 26 to May 2 and can be reached there at
(808)885-7887.

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