Four gigantic galaxies have been seen crashing into one another in one of the biggest cosmic collisions ever seen.
A US team of astronomers observed the four-way cosmic smash-up using Nasa's Spitzer and Chandra space telescopes along with ground-based observatories.
The clashing galaxies are expected to eventually merge into a single, behemoth galaxy up to 10 times as massive as our own Milky Way.
Details of the research appear in Astrophysical Journal Letters.
The rare observation offers an unprecedented look at how the most massive galaxies in the Universe are formed.
Collisions, or mergers, between galaxies are common in the Universe. Mergers between one large galaxy and several small ones, called "minor mergers", have been well documented.
Astronomers have also observed "major" mergers among pairs of galaxies that are similar in size. But, until now, no major mergers between multiple large galaxies have been seen.
"It's the first one that I know about. So far, nobody has written to me to say they've found another four-way merger," said co-author Kenneth Rines, of the Harvard-Smithsonian Center for Astrophysics, in Cambridge, US.
Plume of stars
Dr Rines likened the collision to "four sand trucks smashing together, flinging sand everywhere". The new quadruple merger was discovered serendipitously during a survey of massive galaxy clusters consisting of tens to hundreds of galaxies.
The Spitzer Space Telescope spotted an unusually large fan-shaped plume of light emerging from a gathering of four elliptical galaxies in the cluster CL0958+4702, which is located nearly five billion light-years from Earth.
"The galaxies that live in the middle of clusters like this are the biggest galaxies anywhere in the Universe," Dr Rines told the BBC News website.
"When this merger finishes, the giant galaxy left at the end will be one of the biggest ones in the Universe. This shows how these giant galaxies get assembled."
All the galaxies in the merger are categorised as large; three are about the size of our own Milky Way, while the biggest one is about three times the size.
Analysis of the plume coming from the merger revealed it was made up of billions of stars flung out and abandoned in the ongoing clash. About half of the stars in the plume will later fall back into the galaxies.
"It seems as if there are several galaxies-worth of stars being thrown out by this collision," said the Cambridge-based astronomer.
"There are more stars in that plume than there are in the Milky Way. So an incredible amount of material is being tossed out."
The stars studied so far from the merger all appear to have formed within the first three billion years after the Big Bang. The quadruple merger itself took place some nine billion years after the Big Bang.
The observation that large galaxies contained many old stars used to be problematic for a popular theory of galaxy assembly - the hiearchical model.
This proposed that smaller structures underwent successive mergers to form larger ones. Under this model, the largest galaxies should be sites of star formation and therefore contain young stars.
One way to resolve the problem is through the idea of gas-rich and gas-poor mergers. In gas-rich mergers, the galaxies are soaked with gas that ignites to form new stars. But in gas-poor mergers, no new stars are formed.
Gas-poor mergers, then, were one way that large galaxies might merge without accompanying star formation.
Indeed, the Spitzer observations demonstrate that gas is a missing component in the new quadruple merger, perhaps explaining why only old stars have been found.
In addition to Spitzer, the team used Nasa's Chandra X-ray Observatory to weigh the mass of the giant cluster of galaxies in which the merger was discovered. Two ground-based observatories were also used in the study: the MMT and WIYN observatories, both of which are based in Tucson, Arizona.