Finding a baby galaxy in a grown-up universe

With the help of the Hubble Space Telescope, University of Virginia astronomer Trinh Thuan has identified what may be the youngest galaxy ever seen in the universe. By cosmological standards it is a mere toddler.

I Zwicky 18 may be as young as 500 million years old. Comparatively, our Milky Way galaxy is more than 20 times older — or about 12 billion years old, the typical age of galaxies across the universe. The galaxy offers a glimpse of how the early Milky Way may have looked.

“I Zwicky 18 is a bona fide young galaxy,” said Thuan. “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.”

To prove that I Zwicky 18 is a new galaxy, Thuan and his collaborator, Yuri Izotov of the Kiev Observatory in Ukraine, needed to show that it was devoid of stars from the first several billion years after the Big Bang, the period when a large fraction of stars in the universe were formed. They had to wait for Hubble to provide the sensitivity necessary to detect whether older stars existed within the dwarf galaxy. The presence of old stars in the galaxy would have indicated that the galaxy itself was old, similar to all other known galaxies in the universe. They found no old stars.

Further evidence for the youth of I Zwicky 18 is the fact that its interstellar gas is “nearly pristine,” Thuan said, and composed mostly of hydrogen and helium — the primary two light elements created in the Big Bang during the first three minutes of the universe’s existence.