Once Upon a Time in the Cosmos: Using distant galaxies to study the early universe

Peering far back in time, two teams of astronomers say they’ve found some of the universe’s earliest galaxies. The findings suggest that less than a billion years after the Big Bang, some galaxies were already minting the equivalent of several suns a year, a prodigious rate of star formation.

GOING THE DISTANCE. The most distant galaxy (bottom arrow) and the third most distant galaxy (top arrow) identified so far. Adapted from Subaru

The data also hint that the average density of these galaxies is only about one-sixth that of similarly bright star-forming galaxies observed roughly a half-billion years later in cosmic history.

“We are seeing some of the first galaxies to be born,” suggests Richard G. McMahon of the University of Cambridge in England, a coauthor of one of the two studies. “A decrease in the density as we go back in time means we are approaching the ‘dark ages,’ the time when there were no galaxies.”

Other researchers say that could be an overstatement. The decline in the number of bright, star-forming galaxies may only indicate that earlier, the cosmos had many more small, faint galaxies that are difficult to detect, notes Charles C. Steidel of the California Institute of Technology in Pasadena. Astronomers generally assume that small galaxies coalesce to build larger ones. Steidel says, “It is nice to see [this assumption] backed up with some real data, over an area of sky larger than [astronomers have recently examined].”

A team of astronomers used the 8-meter Subaru telescope atop Hawaii’s Mauna Kea to identify 73 galaxies that might be extraordinarily distant. Spectra of two of these galaxies revealed that one of them is the most distant galaxy now known, residing 12.8 billion light-years from Earth. The other galaxy, the third most distant galaxy known, is only slightly closer to Earth, report Yoshiaki Taniguchi of Tohoku University in Sendai, Japan, and his colleagues in the April Publications of the Astronomical Society of Japan. The team found that the density of known star-forming galaxies decreases as scientists look at earlier times in cosmic history.

Taniguchi notes that he and his colleagues could identify a large group of distant galaxies because their camera covers an area of the sky the size of the full moon, about 20 times larger than either of the 10-m Keck telescopes on Mauna Kea, the biggest visible-light telescopes on Earth.

The researchers accomplished their feat without the aid of a so-called gravitational lens, an intervening cluster of galaxies whose mass boosts the light of distant objects on its way to earthly observers. The most distant galaxy previously identified, discovered by McMahon and his colleagues using Keck, required such a lens (SN: 5/27/00, p. 340: Newfound Galaxy Goes the Distance). That galaxy now ranks number 2.

In his newest work, McMahon and other colleagues analyzed some of the first images taken with the Hubble Space Telescope’s advanced camera for surveys. The scientists identified six galaxies that could be extremely distant, and spectra revealed that one of these candidates is about 12.7 billion light-years away, McMahon’s team reports in an upcoming Monthly Notices of the Royal Astronomical Society. In agreement with the Subaru team, his group finds a reduced density of star-forming galaxies early in the universe.

Steidel cautions that because today’s telescopes can only detect the brightest distant galaxies, “it is hard to extrapolate what [the new findings] mean about galaxy formation in general.”

Other astronomers expect to release new estimates of early galaxy densities and star-formation rates in the next few weeks.


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