Major merger: An illustration shows what the sky might look like from a hypothetical planet around a star tossed out of an ongoing four-way collision between big galaxies. This cosmic smash-up is the largest known galactic merger. While three of the galaxies are about the size of our Milky Way galaxy, the fourth (centre of image) is three times as big.
Credit: NASA/JPL-Caltech
CHICAGO: Four galaxies are slamming into one another in the largest galaxy merger ever observed – throwing billions of stars off into space in a remarkable fireworks display.
Eventually – in a process that may take up to 100 million years to play out – the galaxies will merge to form a galactic monster up to 10 times as massive as our own Milky Way.
Though the galaxies have remained intact, gravitational forces have contorted them, flinging billions of stars into space. These are visible in a plume emanating from the central, largest galaxy.
Formation clues
"This merger tells us an interesting story about how the biggest galaxies in the universe are made," said Kenneth Rines of the U.S.'s Harvard-Smithsonian Centre for Astrophysics in Cambridge, Massachusetts. "The biggest galaxies in the universe are the ones found in the centres of groups and clusters, just like this quadruple merger."
Rines heads a team of astronomers who spotted the merger by chance while using Arizona's WIYN Observatory to image clusters of galaxies five billion light-years away. He said he noticed an odd plume in the centre of one of the clusters, which further analysis proved to be a massive galactic merger. The find was confirmed using data from the U.S. space agency NASA's Spitzer Space Telescope.
The team reports the cosmic collision in the current Astrophysical Journal Letters.
Galaxy mergers are relatively common – many of the universe's large galaxies were formed when smaller galaxies collided in the first few billion years following the Big Bang. In five billion years, the Milky Way itself will combine with our neighbour, the Andromeda Galaxy, producing a slew of new stars.
'Minor mergers' where a large galaxy and several small ones combine occur more frequently. One elaborate example has been observed around the distant Spiderweb galaxy, write the researchers. In this case, the galaxy's gravity is drawing in around a dozen dwarf galaxies. While experts have also observed many 'major mergers' of pairs of similarly sized galaxies, this new merger is the first ever seen between larger groups of galaxies of such massive proportions.
"Dry merger"
The quadruple merger is also unusual in that it is resulting in the formation of surprisingly few new stars. Typically during a galactic merger, gas clouds become so dense that they collapse into new stars and astronomers would expect to observe a "fossil record" of the merging process, said Rines. This would reveal a sequence of stars of different ages.
However, the giant merger adds to evidence that galaxies can collide without producing new stars. These so-called dry mergers, which are gas-poor, occur without leaving a trace of evidence in the form of new stars.
Risa Wechsler, a theoretical cosmologist and astrophysicist from Stanford University in California, commented that while dry mergers have been observed before, this paper supplies "clear evidence" for one of the largest ever seen. "The dynamical information from the stars in this kind of merging event can provide powerful information on the formation history of galaxies and clusters."
