Keck finding: did stars predate galaxies?

In astronomy, one maxim has always seemed to hold true: Stars reside in galaxies.

But new observations with the world's largest optical telescope suggest that the first generation of stars formed more than a billion years before galaxies.

Antoinette Songaila and her colleagues base this tentative conclusion on a study of heavy elements--atoms heavier than helium--in gas clouds that fill the space between galaxies several billion light-years from Earth. The team, from the University of Hawaii in Honolulu, finds that the clouds aren't pristine reservoirs of hydrogen and helium, the main elements forged in the Big Bang.

Instead, the clouds, observed as they appeared when the cosmos was less than one-third its current age, are contaminated with highly ionized carbon, an element that could only have been created inside stars.

The clouds appear too wispy to be fledgling galaxies. Unlike denser hydrogen clouds, which rank as prime candidates for galaxies in the making, these clouds do not seem to cluster together. In addition, most of the clouds the group studied had roughly the same abundance of carbon, regardless of their inferred density. This argues that the carbon represents a pollutant from some outside source.

These lines of evidence suggest, but do not prove, that the carbon came from a generation of stars that predated the clouds and the formation of galaxies, Songaila says.

"Scientists have long suspected that there might have been an early generation of stars that formed prior to the period of galaxy formation," says Lennox L. Cowie, who collaborated with Songaila. "Our observations could be the first evidence for this population."

Songaila, Cowie, Tae-Sun Kim, and Esther M. Hu report their work in the April Astronomical Journal.

The team used the 10-meter Keck Telescope atop Hawaii's Mauna Kea to measure the abundance of carbon in vast, low-density clouds of hydrogen. Like foreground trees silhouetted by a distant searchlight, these clouds make their presence known by absorbing light from a bright background quasar, forming a thicket of absorption lines known as the Lyman-alpha forest.

The team found that in the clouds, up to one atom in a million was ionized carbon. This carbon abundance, about one-hundredth the amount in the sun's vicinity, matches a prediction of cosmologist Jeremiah P. Ostriker of Princeton University and his colleagues.

How could stars have predated galaxies? Ostriker explains that some 300,000 years after the Big Bang, the universe had cooled enough for ions and electrons to recombine into atoms, allowing matter and radiation to go their separate ways. This enabled gases to begin clumping together. Because there were many more small clumps than large ones, star-size objects--rather than galaxy-size bodies--could have been the first to form.

When the massive members of this early generation of stars exploded as supernovas, they would have littered the cosmos with carbon and other heavy elements. By heating their surroundings, these stars would have prevented other, similarly small collections of matter from condensing. However, larger clusters of gas and dust would have had enough gravity to overcome this thermal pressure and eventually condense into galaxies.

"These stars may have modified the intergalactic gas to help create the conditions necessary for galaxy formation," says Cowie.

Ostriker likens the early groupings of stars to the clusters of stars in the Milky Way's Orion nebula. Excited about the Keck findings, he notes that the observations "are not direct evidence that stars predate galaxies, because we don't know when the first galaxies formed, but they certainly make it more likely."

However, Kenneth M. Lanzetta of the State University of New York at Stony Brook cites a caveat. He and his colleagues report in the April 1 Astrophysical Journal that some Lyman-alpha clouds, rather than residing between galaxies, actually constitute vast halos around galaxies.

The clouds examined by Lanzetta's team lie nearer the Milky Way than those in the Keck study and are observed as they appeared when the cosmos was about half its current age. But if more distant clouds also form the outer parts of galaxies, the carbon detected by Songaila's team may simply reflect galactic activity rather than a generation of stars without galaxies, he says.