Image caption Circles show the galaxies making up the protocluster far in the background

Astronomers have revealed the most distant cluster of galaxies ever observed, caught at a never-before-seen stage of development.

Cosmos-Aztec3 has been described as a "metropolis in the making", because such clusters are believed to grow like cities, absorbing outlying villages.

It lies 12.6 billion light years away, and appears to be just tens or hundreds of millions of years old.

Galaxy clusters discovered to date have been billions of years further along.

By contrast, the light from the "protocluster" Cosmos-Aztec3 left when the Universe itself was just one billion years old.

Galaxy clusters grow over billions of years, drawing together many galaxies and huge amounts of gas to form the largest structures in our Universe.

However, their earliest formative stages have remained a matter for speculation because they have not been caught this early in the process of formation.

Peter Capak, an astronomer from the California Institute of Technology, presented the result at the annual meeting of the American Astronomical Society in Seattle to coincide with the publication of the work in the journal Nature.

"We're seeing the seeds of a galaxy metropolis, a city of galaxies that will eventually grow into a large city like London," Dr Capak told BBC News.

"But we're seeing it when it was very small; we can see the conditions around the city that will eventually lead to it growing into a major metropolis."

Star production

The find came out of the Cosmos survey, a concentrated effort making use of many of the world's major telescopes, trained on a large chunk of the night sky.

Image caption Simulation shows how the growth of clusters is similar to urban sprawl

The find was surprising for a number of reasons, mostly because the cluster - a collection of objects with more mass than 400 billion Suns - is extraordinarily active.

It contains an unexpectedly high amount of activity, sporting a quasar spewing copious amounts of radio waves, a black hole 30 million times more massive than our Sun, and a large amount of cool gas.

What is more, the cluster is forming some 4,000 new stars a year.

"Not only is this city incredibly massive at this huge distance in the early Universe, but it's also growing at a very rapid rate," Dr Capak said.

Rogier Windhorst, an astronomer from Arizona State University, called the result "very exciting stuff", principally because such clusters are expected to be incredibly rare in the earliest days of the Universe.

"The city analogy is a good one here," he told BBC News.

"If you go looking for Los-Angeles-sized cities in the Mediterranean in the year zero, you won't find any; if you look in Europe in 1900, you're getting somewhere close, and today you can find them all over," Mr Windhorst said.

"The fact that the mass growth can be that high in such a region that early on is really remarkable."

Dr Capak and his collaborators have not yet fully sifted through the data that Cosmos-Aztec3 yielded, and on the basis of theoretical models they expect to find four more ancient clusters in the area of the sky that they studied.

As a result, this most distant early cosmic neighbourhood may not hold the record for long.