Astronomers have discovered the faintest known galaxy circling the Milky Way, containing only about a thousand stars, that is a fossil left over from the early universe.
The finding provides a new window into how the first galaxies were formed, researchers said.
Scientists including Carnegie Institution's Josh Simon analysed the chemical elements in the faintest known galaxy, called Segue 1, and determined that it is effectively a fossil galaxy left over from the early universe.
Using chemical analysis techniques, the team was able to categorise Segue 1's uniquely ancient composition.
The new analysis shows that Segue 1's star formation ended at what would ordinarily be an early stage of a galaxy's development.
Segue 1 likely failed to progress further because of its unusually tiny size.
"Our work suggests that Segue 1 is the least chemically evolved galaxy known," Simon said.
"After the initial few supernova explosions, it appears that only a single generation of new stars were formed, and then for the last 13 billion years the galaxy has not been creating stars," said Simon.
Because it has stayed in the same state for so long, Segue 1 offers unique information about the conditions in the universe shortly after the Big Bang.
Segue 1's uniquely low iron abundance relative to other elements shows that its star formation must have stopped before any of the iron-forming supernovae occurred.
This truncated evolution means that the products of the first explosions in Segue 1 have been preserved.
Intriguingly, very heavy elements like barium and strontium are nearly absent from Segue 1's stars.
"The heaviest elements in this galaxy are at the lowest levels ever found," said Anna Frebel of the Massachusetts Institute of Technology, the leader of the team.
"This gives us clues about what those first supernovae looked like," said Frebel.
Studying individual stars in dwarf galaxies can be difficult and Segue 1, which orbits our own Milky Way, is particularly puny, containing only about a thousand stars.
Just seven stars in the entire galaxy are in the red giant phase of their lives, making them bright enough for modern telescopes to detect the features astronomers use to measure the abundance of each chemical element.
Three of the seven red giants have heavy element abundances more than 3,000 times lower than that of the Sun, highlighting the primitive nature of the galaxy.
"Having found such a fossil galaxy is of enormous importance to astronomy, because it provides a new window into the first galaxies," Frebel said.
The study was published by The Astrophysical Journal.
