A 9.6 billion-year-old cluster of about 60 galaxies is seen by separate teams of astronomers.
A cluster of galaxies has been dated 9.6 billion years old -- the oldest cluster ever found.
Since such clusters take time to form, they might contain some of the universe's oldest galaxies.
The evidence that the galaxies are bound together is X-ray emissions from their intergalactic gas.
Galaxies are a gregarious lot, even those that existed 9.6 billion years ago. Two separate teams of astronomers have detected signs of a surprisingly modern looking cluster of galaxies of that age using a combination of X-ray and infrared coming from the distant galactic gathering.
Galaxy clusters are the universe's largest structures held together by gravity. And the newly discovered cluster, dubbed ClG J02182-05102, is the oldest such structure ever found, which is a big help to researchers trying to piece together the evolution of galaxies and clusters throughout the universe.
"Their cluster is not only observed to be established at a very early time, but looks to be pretty extensive," said astrophysicist Jeff Cooke of Caltech.
Cooke made headlines in 2008 with the discovery of what he calls a "proto-cluster" that appears to be more than 11 billion years old. This larger cluster is an entirely different animal, he said.
"Such clusters haven't been discovered until now because they require the more powerful technology and instruments now available," said Cooke. "The X-ray observations are a nice confirmation."
Once the technology is in place, however, it's then a matter of measuring the distances of the galaxies and detecting something that suggests they are gravitationally bound together.
To gauge the distances, astronomers used the "red-shifts" of the galaxies. As light travels through the ever expanding universe, its waves gets stretched out, making it redder and redder.
What was once the bright white light of these galaxies has now become dim, mostly infrared light that could be captured and analyzed by such instruments as the Multi-Object InfraRed Camera and Spectrograph on the Japanese Subaru telescope.
"A cluster is a collection of galaxies gathered close to each other. So, it is important to make sure that galaxies have similar redshifts," explained Masayuki Tanaka of Japan's Institute for the Physics and Mathematics of the Universe. Tanaka is the lead author on a paper that has been accepted by Astrophysical Journal Letters confirming the cluster's existence.
Another team of astronomers, led by Casey Papovich of Texas A&M University, used an array of telescopes and astronomical survey data, including the orbiting Spitzer telescope, to discover the cluster, as well as estimate that the cluster probably contains around 60 galaxies.
Their paper will appears in the June 2010 issue of Astrophysical Journal.
"You don't expect the biggest things to form so quickly," commented astrophysicist David Koo of the University of California's Lick Observatory. "It takes time to assemble great big things like clusters, and the galaxies in them are some of the oldest. So they provide a sign post to find the oldest galaxies."
The clincher for calling this a cluster, however, is the X-ray observations. If the galaxies are bound by gravity, they should be swimming in a giant cloud of hot gases. Those gases should be hot enough to emit X-rays.
To see if this was the case, Alexis Finoguenov, of the Max Planck Institute for Extraterrestrial Physics, used the orbiting XMM-Newton observatory. Sure enough, the X-rays are there, forming a hot spot where the distant galaxies are located.