Dec. 7, 2004 — Sensitive instruments on the Hubble Space Telescope have allowed astronomers to measure the age of what could be the youngest galaxy ever seen.

The galaxy, called I Zwicky 18, may be as young as 500 million years old, a toddler of a galaxy created as complex life was just springing up on Earth, according to a press release from the Space Telescope Science Institute.

The finding, reported in the Dec. 1 issue of the Astrophysical Journal, provides new insight into how galaxies first formed, and what the early Milky Way may have looked like 13 billion years ago.

Our galaxy is, at about 12 billion years old, about 20 times older than I Zwicky 18.

Scientists have long suspected that the galaxy was young, given its chemical composition. I Zwicky 18 was photographed in the 1930s as part of a northern sky census undertaken by Swiss astronomer Fritz Zwicky.

But it was only recently, with Hubble's sensitive eyes, that astronomers took a close look at all of the stars in the galaxy and gauged the upper limits of their ages.

I Zwicky 18 is only about 45 million light-years away from Earth, much closer than other young galaxies. Other galaxies collapsed under the weight of gravity very soon after the Big Bang, but not I Zwicky 18.

In a process astronomers don't understand, the galaxy remained a baby — a cold gas cloud of hydrogen and helium — for most of the 14-billion-year evolution of the universe, only beginning active star formation in the past 500 million years.

"I Zwicky 18 is a bona fide young galaxy," said Trinh Thuan, professor of astronomy at the University of Virginia, who co-authored the study with Yuri Izotov from the Kiev Observatory.

"This is extraordinary because one would expect young galaxies to be forming only around the first billion years or so after the Big Bang, not some 13 billion years later. And young galaxies were expected to be very distant, at the edge of the observable universe, but not in the local universe," Izotov said in the press release.

Among clues to the age of I Zwicky 18 were that the researchers could find no old stars, which would have been expected in an ancient galaxy. Also, the interstellar gas in the galaxy is "nearly pristine," Thuan said in the press release.

The gas is composed mostly of hydrogen and helium, the primary two light elements created during the first three minutes of the universe's existence. Only a sprinkling of the heavier elements created during star formation — carbon, nitrogen or oxygen — were present.