Largest merger of galaxies discovered

Four massive galaxies are colliding in the largest galactic merger ever seen, new observations reveal. The smash-up is shedding light on how the biggest galaxies in the universe form � and why many of them stopped giving birth to stars billions of years ago.

Astronomers classify mergers according to the relative sizes of the galaxies involved. Minor mergers unite galaxies of vastly different size � marrying a 'dwarf' galaxy with one the size of the Milky Way, for example � while major mergers join those of roughly equal size.

Now, researchers led by Kenneth Rines of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US, have found the largest major merger ever seen. It involves a quartet of galaxies at the centre of a galactic cluster known as CL0958+4702, which lies about 5 billion light years from Earth.

Three of the merging galaxies are the size of the Milky Way, while the other is about three times as massive. "This is the largest major merger in terms of total stellar mass," Rines told New Scientist, adding that the next runner-up is the merger of two Milky Way-sized bodies known as the Antenna Galaxies.