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LONG BEACH, CALIF.—You might think that the universe 11.5 billion years ago was in a more primitive state than it is today. Barely two billion years had passed since the big bang, our Milky Way galaxy was still taking shape, and billions more years would pass before the sun pulled itself together. Yet astronomers have come to realize that the universe was actually quite precocious. Even by that early stage, much of it had already seen many cycles of stellar birth and death.
The latest hint of its precocity came yesterday when astronomers announced that cosmic gas in that period, seen when backlit by a gamma-ray burst (a gigantic stellar explosion), contained molecular hydrogen and carbon monoxide—the first time astronomers have discovered molecules, as opposed to isolated atoms or ions, in the light of a gamma-ray burst. The molecules’ presence indicates that the galaxy where the burst occurred was nearly as chemically developed as the present-day Milky Way. Jason Xavier Prochaska of the University of California, Santa Cruz, and his colleagues announced the discovery at the American Astronomical Society’s meeting being held here this week.
NASA’s Swift satellite discovered the burst on June 7—hence its designation GRB 080607—and Prochaska’s team followed its progress with the Keck telescope in Hawaii and other instruments. It was the second-brightest gamma-ray burst ever recorded, and among the most distant. Its light spectrum, like that of other celestial bodies, was etched with "lines" representing colors of light that were filtered out by intervening material. GRB 080607 had an especially intricate pattern of lines, bearing imprints of all the gas the light passed through on its long journey to Earth: ionized gas in the immediate vicinity of the burst, a mix of thick clouds and wispy ambient gas in the interstellar space of the host galaxy, and the even more tenuous gas of intergalactic space. The molecular hydrogen and carbon monoxide resided in the thick clouds.
Some of the spectral lines are utter mysteries; they do not match up with the known spectral lines of any chemical element or compound, presumably because the light from GRB 080607 was so intense that it brought out some unfamiliar optical properties of the gas it illuminated. "It’s the most impressive spectrum I’ve ever seen," Prochaska says. Agrees Neil Gehrels of NASA’s Goddard Space Flight Center, leader of the Swift project: "It is beautiful spectroscopic data."
In addition to the thicket of spectral lines, the overall light level was attenuated and reddened by clouds of dust. A lesser burst might not have shone through the dust at all. And indeed many past bursts have been dark or "ghost" bursts that showed up in gamma rays but not in visible light. GRB 080607 supports arguments that these bursts were dark not because they failed to emit light but because dust hid them.
Thick interstellar clouds of dust and molecular gas are the raw material for new stars, so it seems that GRB 080607 went off in a region of space where stars were actively forming. That is just where you’d expect to find short-lived, explosion-prone stars.
The fact the burst’s host galaxy was so chemically enriched, even in ancient times, makes sense in the emerging picture of galaxy evolution, in which large galaxies begin cranking out stars very early on.
Composite image showing the afterglow of GRB 080607 as a white dot in the center, comprising Swift optical data (blue, green) and x-ray images, courtesy of NASA/Swift/Stefan Immler
