By Paul Rincon
Science reporter, BBC News, Austin
Astronomers probing the distant Universe have found the building blocks of spiral galaxies like our Milky Way.
Galaxy M74, a spiral galaxy that evolved in the early universe
They discovered ancient galaxies, about one-tenth the size of the Milky Way, which were among the first to form in the Universe.
Over billions of years, galaxies like these merged to form much bigger spiral galaxies such as our own.
The findings were outlined at a meeting of the American Astronomical Society in Austin, Texas.
The ancient objects – known as Lyman alpha-emitting galaxies - are fertile breeding grounds for new stars.
Astronomers see them as they looked when the Universe was just two billion years old.
According to cosmological theory, the small galaxies which existed in the early Universe grew into larger ones by merging with one another.
Other early galaxies had previously been observed, but these were much bigger, and were destined to evolve not into spiral galaxies, but into a different type known as elliptical galaxies. The "ancestors" of galaxies like our own remained elusive.
Lyman alpha-emitting galaxies, breeding grounds for new stars
While he was a postdoctoral fellow at Yale University, Eric Gawiser embarked on a search for the ancient precursors of Milky Way-type spirals.
His colleague Caryl Gronwall, from Penn State University, worked on characterising the distinictive Lyman alpha-emitting galaxies.
The researchers used a combination of computer simulations and cosmological theory to predict how these galaxies would evolve.
"This determined that the typical present-day descendent of the galaxies we identified is a spiral galaxy like the Milky Way," said Eric Gawiser, now a professor of astronomy at Rutgers University in New Jersey.
The research was carried out using the Blanco 4m telescope at the Cerro Tololo Inter-American Observatory in Chile, the Magellan Telescope at Las Campanas Observatory, also in Chile, and Nasa's Spitzer and Hubble Space Telescopes.
But it appears that not all galaxies grow through mergers.
In other findings released at the meeting, astronomers report that a class of massive, disc galaxies that existed in the early Universe must have formed from a massive gas cloud collapsing in on itself.