The most distant cluster of galaxies ever found has been revealed by astronomers 鈥� and it bears an uncanny resemblance to those nearby. The technique used to discover the cluster promises further discoveries at similar distances, which would help constrain cosmological models.
The cluster of galaxies spotted by astronomers lies 9 billion light-years away. That beats the 8.5 billion light-years鈥� distance of the previous record holder 鈥� a jump that represents a 鈥渟ignificant fraction of a galaxy鈥檚 lifespan鈥�, says Christopher Mullis, at the University of Michigan in Ann Arbor, US, who led the team.
The universe鈥檚 first galaxies probably formed a few hundred million years after the big bang. They then began assembling into 鈥減roto-clusters鈥�, or groups of several hundred, within a billion years. The new-found cluster, which may harbour thousands of galaxies, appears to have started growing when the universe was roughly 2.5 billion years old and stopped about 2 billion years later.
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At that time, it had already grown very massive. Only colossal objects can contain the searing, 70-million-degree gas detected by astronomers 鈥� the gas is heated as the galaxies fall toward each other. Images from Europe鈥檚 XMM-Newton spacecraft, launched to study X-rays in 1999, also reveal the gas has taken on a spherical shape 鈥� suggesting it has settled into portly middle age.
鈥淲e would have thought characters [from so early in the Universe] would have been a bit more youthful,鈥� Mullis told New 杏吧原创. 鈥淏ut this guy looks quite old.鈥�
Richard Mushotzky, a galaxy cluster expert at NASA鈥檚 Goddard Space Flight Center in Greenbelt, Maryland, agrees. 鈥淔inding such massive, evolved systems at such a great distance is somewhat unexpected,鈥� he says. Computer simulations show such systems are rare, he adds.
Quick snapshots
Indeed, only five other clusters have been found further than 7.5 billion light-years away, compared with about 1000 within 3.5 billion light-years of Earth. That results from the fact that the universe had not yet grown many gargantuan structures 7.5 billion years ago, as well as the challenges of observing such faint objects, says Mullis.
But Mullis and his colleagues developed a technique which makes observing the objects much easier. They ran archived XMM-Newton images through a software program that picks out candidate clusters by searching for large, diffuse sources of X-rays.
Then, they took relatively quick snapshots of about 30 candidates through blue and red filters on the Very Large Telescope in Chile. Because the accelerating expansion of the universe stretches light toward redder wavelengths, they picked out only the reddest objects for further study, as these would be the most distant and oldest.
One cluster, called XMMU J2235.3-2557, appeared to be 鈥渁lmost unbelievably distant鈥�, says Mullis. Follow-up observations of twelve of its brightest galaxies showed they were 9 billion light-years away.
鈥淭he relative ease with which these authors obtained their results is stunning,鈥� Mushotzky told New 杏吧原创. He adds the result is exciting because 鈥渋t represents the first steps towards the discovery of even more distant objects鈥�.
Mullis agrees and will use the technique to search for more clusters. The number of clusters found would provide a check of cosmological models, including those governing how galaxies form, he says.
The study of XMMU J2235.3-2557 will appear in a future issue of the Astrophysical Journal.
