Light’s Debut: Good Morning, Starshine!

Astronomers have for the first time detected signs of one of the earliest and least understood eras in the universe, the murky time just before the first stars and quasars flooded the cosmos with light.

The new observations, announced independently by two teams, “are very important because they are beginning to probe the time [just after] the first massive stars and quasars came into being,” comments theorist David N. Spergel of Princeton University.

The findings reveal that these objects first illuminated the universe about 900 million years after the Big Bang, says codiscoverer Robert H. Becker of University of California, Davis and the Lawrence Livermore (Calif.) National Laboratory.

The era before light, dubbed the cosmic Dark Ages, began some 300,000 years after the Big Bang. That’s when the universe had dissipated enough of the Big Bang’s heat that electrons and protons could bind together to form hydrogen atoms. With electrons no longer free to scatter the radiation left over from the Big Bang, that light finally streamed freely into space. As this relic radiation faded, the cosmos plunged into darkness.

Within that murk, gas gathered into clumps and, over several hundred million years, formed fledgling galaxies. But even as the stars and quasars within these protogalaxies began to light up, the universe as a whole remained dark. That’s because the atomic hydrogen, which readily absorbs photons, quenched the radiation.

The Dark Ages came to an end only when these early stars and quasars generated enough ultraviolet light to reionize the hydrogen atoms, turning them back into protons and electrons. Unlike atomic hydrogen, ionized hydrogen doesn’t absorb radiation. This process of reionization, therefore, permitted the light from luminous objects to pervade the universe for the first time. Cosmic dawn had arrived.

It’s the transition from atomic to ionized hydrogen that Becker and his colleagues, as well as researchers led by S. George Djorgovski of the California Institute of Technology in Pasadena, say that they have now detected.

To do so, each team relied on light emitted by extraordinarily distant quasars. Recently found by the Sloan Digital Survey (SN: 6/9/01, p. 356), an ongoing, mammoth census of the heavens, the quasars hail from a time when the cosmos was less than one-tenth its current age. The astronomers reasoned that if the quasars were born before the universe had been reionized, their light would have encountered vast amounts of atomic hydrogen, and the radiation would have been absorbed.

Using one of the Keck telescopes atop Mauna Kea in Hawaii to examine spectra of the distant quasars, the teams both report that they’ve found this absorption. It shows features first described in 1965 by James E.

Gunn, now at Princeton University and Bruce Peterson, now at the Australian National University in Canberra.

Becker and his colleagues report finding the Gunn-Peterson effect (http://xxx.lanl.gov/abs/astro-ph/0108097), as do Djorgovski and his collaborators (http://xxx.lanl.gov/abs/astro-ph/0108069). Detecting the Gunn-Peterson effect “has been a Holy Grail, long sought after for decades,” notes cosmologist Jerry P. Ostriker of Princeton University.

Becker’s team examined the four most remote quasars known but found the Gunn-Peterson effect in the spectra of only the most distant one.

Djorgovski and his collaborators recorded the spectra of a single quasar–one of the three for which Becker’s team found no evidence of the effect. Djorgovski says he and his coworkers observed the quasar for a much longer time than Becker’s team did and that the extended study enabled them to view “the tail end” of the Dark Ages.

Rather than detecting complete absorption of the radiation associated with hydrogen atoms, Djorgovski says his team sees small amounts of light transmission mixed in with the absorption. He interprets this as chunks of atomic hydrogen embedded among regions that have already become ionized.

“We’re seeing the finale” of the Dark Ages, he says.

Djorgovski suggests that Becker’s team, looking at a more distant quasar, “is seeing reionization at an earlier stage,” when the cosmos was about 100 million years younger and had more atomic hydrogen.

Spergel finds the evidence of reionization presented by Becker’s team more convincing than that of Djorgovski’s. Although both teams may be probing the reionization era, it’s difficult to know for sure, he adds. Because atomic hydrogen absorbs light so well, scientists looking at quasar spectra can’t distinguish between a tiny bit of unionized hydrogen and a much larger amount. Therefore, researchers can’t tell whether they are examining the universe when it is almost fully reionized or has just begun the process, Spergel says.

The teams hope to confirm their results by examining other distant quasars.

Abraham Loeb and George B. Rybicki of the Harvard-Smithsonian Center for

Astrophysics in Cambridge, Mass., propose a search for the dim halo of scattered light expected to surround quasars born before reionization.

“All that we’re really seeing is the consequence of the turning on of quasars and stars,” notes Djorgovski. “We still haven’t seen the formation of the very first luminous objects. That will be a job for the next generation of big telescopes on the ground and in space.”