The Hubble Space Telescope has photographed what might be the youngest galaxy ever seen, a hotbed of star birth that formed within the past one billion years.
The galaxy, named I Zwicky 18, might have been born as recently as 500 million years ago -- about the time complex life began to appear on Earth. The uncertain age for the galaxy's birth is the result of two separate studies.
Either way, I Zwicky 18 is a toddler compared to most galaxies, astronomers said today. Our own Milky Way, for example, is at least 12 billion years old.
Galaxies are created when a cloud of hydrogen and helium collapses under its own weight. The denser areas form knots that become stars. But much of the process is poorly understood.
Studies of the new observations could refine this understanding. The young galaxy also provides a glimpse of what things were like shortly after the universe emerged 13.7 billion years ago from the Big Bang, the theoretical starting point in time.
Astronomers have known about I Zwicky 18 for years. But the galaxy, classified as a dwarf irregular, is surprisingly nearby for one so young. It is just 45 million light-years away. A light-year is the distance light travels in a year, about 6 trillion miles (10 trillion kilometers).
"I Zwicky 18 is a bona fide young galaxy," said Trinh Thuan, professor of astronomy at the University of Virginia. "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."
By that measure, a young galaxy ought to be far away in space and time, near the edge of the observable universe, Thuan explained.
Thuan and colleagues calculated Zwicky's age in party by looking for old stars. There were none. In addition, the gas between stars is nearly all hydrogen and helium, Thuan said. Had there been several generations of stars, as in more mature galaxies, then the explosions of stars would have loaded the interstellar gas with other elements.
Astronomers are therefore curious how Zwicky kept its natal gas cloud of hydrogen and helium in an embryonic state for all those eons.
Zwicky is in the lower left of the photograph. Star birth is concentrated in two blue-white knots. Wispy blue filaments around the galaxy are bubbles gas heated by the radiation of young, intense stars. Redder stars are older, but still less than 1 billion years old, the observations show.
A companion galaxy is visible to the upper right. Gravitational interaction between the two may have caused the burst of star formation captured by Hubble.
Thuan and colleagues report their findings in today's issue of the Astrophysical Journal. A separate study of the galaxy led Goran Ostlin of Stockholm Observatory will be published in Astronomy & Astrophysics.
