How the dark ages ended is a matter of hot debate. From 300 million years to 1 billion years after the Big Bang, figures from NASA's Wilkinson Microwave Anisotropy Satellite show that the hydrogen inspace was re-ionized and split back into electrons and protons by radiation from stars or, perhaps, black holes.

Calculations suggest that the first stars to form, out of hydrogen and helium produced in the Big Bang, would be 100 times as massive as the Sun and would rapidly explode, scattering heavier elements like oxygen, carbon, nitrogen and iron — the stuff of planets and life — into space to serve as material for a new generation of stars.

The stars forming in the newly discovered galaxies are probably of the second type, Illingworth said.

"It is not known which type of stars ionized the universe or when the transition between the two populations took place," Loeb said.

Comparing data from surveys like the ones being reported with theoretical models could help resolve such questions, as well as sharpen theories of galaxy formation.

Alan Dressler of the Carnegie Observatories in Pasadena, Calif., cautioned that it would take bigger telescopes and observations of even smaller objects to discern what occurred in the early universe. "This is a good solid step," he wrote in an e-mail message. "However, it doesn't come close to settling anything."

Nor is the Japanese record likely to last long. Richard Ellis of the California Institute of Technology said in an e-mail message that he had used the Keck Telescope on Mauna Kea and a quirk of Einsteinian gravity to find protogalaxies even farther in the past, less than 500 million years after the Big Bang. These objects, too feeble and small for the Hubble to have seen them, have been amplified by the gravitational fields of intervening galaxies.