Astronomers say they've discovered the farthest galaxy ever observed, so distant that we're seeing it as it existed when the universe was just 5 percent of its current age.

The galaxy cataloged as EGS-zs8-1, one of the most massive and brightest objects in the early universe, is giving astronomers a glimpse 13 billion years into the universe's past.

Age and distance become inextricably linked when we observe the universe, astronomers point out; light from far-off galaxies takes billions of years to reach the Earth, so what we see is what a galaxy looked like all those billions of years ago in the past.

The newly discovered galaxy is surprisingly massive and forming stars very rapidly, about 80 times faster than our galaxy does today, the researchers say.

"It has already grown more than 15 percent of the mass of our own Milky Way today," says study lead author Pascal Oesch from Yale University. "But it had only 670 million years to do so. The universe was still very young then."

An international team of astronomers led by Yale and the University of California, Santa Cruz, was able to determine the distance to the far-off galaxy using the 10-meter telescope at the W.M. Keck Observatory in Hawaii.

It had first been detected by NASA's Hubble and Spitzer space telescopes.

"One of the most dramatic discoveries from Hubble and Spitzer in recent years is the unexpected number of these very bright galaxies at early times close to when the first galaxies formed," says study co-author Garth Illingworth, a UC Santa Cruz professor of astrophysics and astronomy.

"We still don't fully understand what they are and how they relate to the very numerous fainter galaxies."

Few galaxies from the very early universe have had their distances accurately measured, which makes the discovery of EGS-zs8-1 important, the astronomers say.

"Every confirmation adds another piece to the puzzle of how the first generations of galaxies formed in the early universe," says co-author Pieter van Dokkum at Yale. "Only the largest telescopes are powerful enough to reach to these large distances."

With new discoveries come new mysteries, the researchers say. The discovery of EGS-zs8-1 confirms that massive galaxies were already forming early in the age of the universe, but the Hubble, Spitzer and Keck observations also show such early galaxies possessed different physical properties compared with what we see in the universe today.

That may be the result of a rapid formation of massive young stars interacting with the elemental gas in those earliest galaxies at a time when the universe was undergoing an important change, as hydrogen between galaxies was transitioning from a neutral state to an ionized, or charged, state.

"It appears that the young stars in the early galaxies like EGS-zs8-1 were the main drivers for this transition, called reionization," says astronomer Rychard Bouwens of the Leiden Observatory.

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