500 million years old and still a tot.
Using 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.
Called I Zwicky 18, it 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 finding, reported in the Dec. 1 issue of the Astrophysical Journal, provides new insight into how galaxies first formed. The galaxy I Zwicky 18 offers a glimpse of how the early Milky Way may have looked.
The baby galaxy managed to remain in an embryonic state as a cold gas cloud of primeval hydrogen and helium for most of the universe's evolution. As innumerable galaxies blossomed all over space, this late-bloomer did not begin active star formation until some 13 billion years after the Big Bang and went through a sudden first starburst only about 500 million years ago.
Because it is located 45 million light-years away -- much closer than other young galaxies in the nearly 14 billion light-year span of the universe -- I Zwicky 18 might represent the only opportunity for astronomers to study in detail the building blocks from which galaxies are formed. It remains a puzzle why the gas in the dwarf galaxy, in contrast to that in other galaxies, took so long -- nearly the age of the universe -- to collapse under the influence of gravity to form its first stars.
"I Zwicky 18 is a bona fide young galaxy," said Thuan, professor of astronomy at U.Va. and co-author of the study with Yuri Izotov from the Kiev Observatory in Ukraine. "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 Izotov 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. Though astronomers previously had suspected that the galaxy was exceptionally young, Thuan and Izotov had to wait for Hubble to provide the sensitivity necessary to detect whether older stars existed within the dwarf galaxy. Hubble's Advanced Camera for Surveys (ACS) needs a very long exposure, requiring 25 Hubble orbits to look for the faintest stars in the 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.
Large galaxies such as the Milky Way are thought to grow hierarchically, with smaller galaxies merging into bigger galaxies, similar to tributaries merging into large rivers. I Zwicky 18 is prototypical of this early population of small dwarf galaxies. "These building block dwarf galaxies are too faint and too small to be studied without the most sensitive instruments, even in the local universe, let alone in the far reaches of the cosmos," Thuan said.
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. The dwarf galaxy includes only a sprinkling of the other heavier elements such as carbon, nitrogen or oxygen that are created later as stars develop. The near absence of such heavy elements suggests that much of the primordial gas in the dwarf galaxy has not managed to form stars that subsequently manufacture heavy elements.