May 5, 2003

Messages from the early universe shed light on how elements form

By Shawna Williams

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 UCSC 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, reported 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.

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