When the universe was young, it didn't sport many giant galaxy clusters, which need time to grow their gravitational muscles.
So the discovery of a giant cluster nearly 12 billion light-years away is a rare find. Even more surprising, however, is that the cluster, known as CL J1449+0856, resembles mature galaxies of modern day, though it existed when the universe was less than a quarter of its present age.
The notion that fully grown galaxy clusters were in place during the universe's youth isn't at odds with currently accepted theories of how the universe formed -- at least not yet.
"We don't expect to find them frequently," Norbert Schartel, project scientist with the European Space Agency's XMM-Newton X-ray telescope, wrote in an email to Discovery News.
"If we would find them frequently, then we would have a problem. In such a sense, we can use them to test the current models for structure formation and the assumptions" on which they are based, Schartel said.
The discovery of CL J1449+0856 was complete serendipity, notes lead researcher Raphael Gobat, with the Commissariat a l'Energie Atomique in Paris.
"It was found in a field in a series of images taken for totally different purposes. The field was selected because there was no known structure like this," Gobat told Discovery News.
Astronomers measured the distance to the cluster with light-splitting spectrographs, and then comparing the spectrum with a similar object relatively nearby. From this data, they determined that the light from the cluster had shifted due to its great distance, much like the sound of a train whistle changes pitch as it recedes into the distance. Scientists determined the cluster exists at the time when the universe was about three billion years old.
Astronomers then found X-ray emissions coming from CL J1449+0856, believed to be caused by hot gas trapped between the galaxies. Most clusters at this time haven't amassed the gravity to trap pockets of gas in this way.
Scientists also determined the galaxy wasn't forming new stars anywhere near the rate expected in young clusters.
"This one evolved quicker," Gobat said, adding that the galaxy likely formed in a particularly dense region of space, which allowed it to form structures earlier.
Previous discoveries of galaxy clusters in the early universe are mostly proto-clusters, a type of object not found later in the universe's life.
"This is interesting because it puts a constraint on models of galaxy evolution," Gobat said. "I would be interested to see how they can make the models fit both proto-clusters and this."
The research appears in the journal Astronomy & Astrophysics.