By Paul Rincon
Science reporter, BBC News, Hatfield
The water maser resides in a distant galaxy
Astronomers have found the most distant evidence of water in the Universe, a major conference has been told.
The vapour is thought to be present in a jet ejected from a supermassive black hole at the centre of a galaxy that is billions of light-years away.
The discovery, by a US-European team, was announced at the European Week of Astronomy and Space Science meeting.
The water was emitted from the black hole when the Universe was only about 2.5 billion years old.
This is about one fifth of the Universe's current age, astronomers say. The water's signature, seen at radio wavelengths, is only now being detected because of the huge distance in space between the black hole and Earth.
The vapour is observed as a "maser", in which molecules in the gas amplify and emit beams of microwave radiation.
"We have been observing the water maser every month since the detection and seen a steady signal with no apparent change in the velocity of the water vapour in the data we've obtained so far," said Dr John McKean of the Netherlands Institute for Radio Astronomy.
"This backs up our prediction that the water is found in the jet from the supermassive black hole, rather than the rotating disc of gas that surrounds it."
The faint signal was detected using a technique called gravitational lensing.
This is where the gravity of a massive galaxy in the foreground acts as a cosmic telescope, bending and magnifying light from the more distant galaxy.
"The radiation that we detected has taken 11.1 billion years to reach the Earth," explained Dr McKean.
"However, because the Universe has expanded like an inflating balloon in that time, stretching out the distances between points, the galaxy in which the water was detected is about 19.8 billion light years away."
The researchers said it was likely there were many more galaxies like this one in the early Universe.
But surveys of nearby - and hence younger - galaxies show that only about 5% have powerful water masers associated with active galactic nuclei.
In addition, studies show that very powerful water masers are extremely rare compared with their less luminous counterparts.
"We found a signal from a really powerful water maser in the first system that we looked at using the gravitational lensing technique," said Dr McKean.
If these phenomena were just as rare in the early Universe as they are today, he said, the chances of detecting one would have been vanishingly small - about one in a million, in fact.
"This means that the abundance of powerful water masers must be much higher in the distant Universe than found locally because I'm sure we are just not that lucky," he added.
The research team was led by Dr Violette Impellizzeri, from the Max Planck Institute for Radio Astronomy in Germany, using the 100m German Effelsberg radio telescope from July to September 2007.
The data was confirmed by observations with the Expanded Very Large Array in the US in September and October 2007.