The earliest known galaxies in the universe, which formed during the universe’s "dark age" nearly 13 billion years ago, have been spied by two teams of astronomers.
The discoveries, reported separately in this week’s issue of the journal Nature, suggest that galaxies were forming just 700 million years after the birth of the universe.
Theory holds that the universe formed 13.7 billion years ago when an extremely dense concentration of mass rapidly expanded in an known as the big bang.
The universe has been expanding ever since, so astronomers are able to age galaxies by computing how much the wavelength of their light has stretched—or redshifted--as the expansion takes the galaxies farther from Earth. The redder the light is, the older and more distant the galaxy is.
The detection of such ancient galaxies adds intrigue(神秘色彩) to theories of how the very first galaxies formed, according to astronomers.
Were there many large, young galaxies in the early universe that are obscured from astronomers’ view by abundant gases absorbing their light Or were galaxies rare and small way back then, as a prevailing theory suggests, and later clumped together to form larger galaxies such as the Milky Way
"We believe that we need both these processes to explain what we see," Masanori Iye, a professor at the National Astronomical Observatory of Japan, said in an email.
The galaxy, called IOK-1, formed about 750 million years after the big bang—60 million years closer to the than the previous record holder.
Given the number of galaxies found during a later epoch about 810 million years ago, the researchers had expected to find as many as six galaxies like IOK-1. But the comparative rarity(稀有) of objects like IOK-1 means that the universe must have changed significantly over the 60 million years that separate the two epochs, the team suggests.
Iye and colleagues believe that they are witnessing the last phase of a process known as reionization(再次电离).
According to Iye, about 380,000 years after the fiery hot big bang, the universe cooled so much that protons and electrons recombined to form neutral hydrogen. This is known as the beginning of the dark age of the universe, because neutral hydrogen absorbs the light from stars.
As more galaxies stared to form about 300 million years later, the hot stars heated the intergalactic(银河间的) medium and gradually reionized the neutral hydrogen back to protons and electrons. The ionized hydrogen then became more transparent, allowing the galaxies light to pass through.
Iye said the new results support the idea that neutral hydrogen was still abundant 750 million years after the big bang, blocking even older galaxies from view. "We are starting to see the last phase of cosmic reionization, or the dawn of the cosmic dark age," he said.
Iye added that the discovery also supports the "hierarchical" theory of galaxy formation, which suggests that big, bright galaxies formed as smaller galaxies collided and merged. "The epoch we have probed is yet in this critical stage," he said.
What happened when the ionized hydrogen became more transparent