Chemistry of the Cosmos: Quasars illuminate the young universe

Measuring the composition of some of the earliest structures in the universe, two teams of astronomers have unveiled new findings about star formation when the cosmos was young.

In one study, astronomers used the Hubble Space Telescope to examine three of the most distant quasars known. These brilliant beacons are so remote that it takes their light 12.8 billion years to reach Earth. The observations therefore show how the quasars appeared when the universe was only 900 million years old.

Even so, spectra taken with Hubble’s near-infrared camera and multiobject spectrograph reveal that quasars back then already contained iron and magnesium.

Because heavy elements such as these can be made only inside stars, their presence requires that an early generation of stars preceded the quasars. More significantly, the much higher abundance of iron relative to magnesium provides a time marker for when these first stars would have blazed into existence, notes Wolfram Freudling of the European Southern Observatory in Garching, Germany.

Iron can be produced by two classes of stars: massive ones that last only a few million years and intermediate-mass stars that live a hundred times longer. When stars of these masses die, they spew their contents into space.

However, only intermediate-mass stars produce a high abundance of iron relative to magnesium, so these longer-lived stars are the likely source of the metals found in the quasars, say Freudling and his colleagues Michael R. Corbin of the Space

Telescope Science Institute in Baltimore and Kirk T. Korista of Western Michigan University in Kalamazoo. Their study appears in the April 20 Astrophysical Journal Letters.

The data suggest that the first stars in the universe were already in place when the cosmos was only 200 million years old. That’s extraordinarily early in cosmic history yet it’s consistent with recent signs of early stars in measurements from the Wilkinson Microwave Anisotropy Probe, a satellite that examines the radiation left over from the Big Bang (SN: 2/15/03, p. 99: Cosmic Revelations: Satellite homes in on the infant universe).

Fred Hamann of the University of Florida in Gainesville says the new report is intriguing but not conclusive. Theorists aren’t sure how to link the light emitted from iron and magnesium ions to the actual abundances of these elements, he notes.

“The result is nonetheless tantalizing,” Hamann says.

In a second study looking at the composition of the early universe, reported in the May 1 Nature, Jason X. Prochaska of the University of California, Santa Cruz and his colleagues used the light from a background quasar to probe the composition of a dense gas cloud–a probable galaxy in the making–that lies directly between the quasar and Earth. The so-called protogalaxy resides about 11 billion light-years from Earth.

Quasar light absorbed by gas in the protogalaxy has revealed the fingerprints of 25 star-forged elements. These include atoms heavier than iron, such as germanium and lead, that had never before been detected in such a distant body. The relative abundance of some of these elements could only have arisen in massive stars.

The protogalaxy probably has since coalesced into a giant elliptical galaxy, says David N. Spergel of Princeton University. “These observations reveal [that galaxy’s earliest] star-formation history,” he notes.

Prochaska says the findings suggest that his team can use the same quasar technique to measure the composition and trace the star-formation history of 100 other distant galaxies.

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