Cosmic Collision 12 Billion Years Ago Created Mega-Galaxy

The colossal merger of 14 galaxies more than 12 billion years ago has been captured by astronomers who used the world’s most powerful telescopes to peer 90% of the way across the observable universe. The cosmic pileup occurred 12.4 billion years ago, and the resultant gigantic galaxy will have continued to snowball in size ever since. Calculations suggest that by the present day, hundreds of more galaxies would have been swallowed up by the cluster, propelling it to a mass equivalent to 1,000 trillion suns, which would make it the largest known object in the universe.

The observations reveal at least 14 galaxies packed into an area only four times the diameter of the Milky Way’s galactic disk. An adjoining blob of light that has not yet been resolved into individual objects suggest that the total number of galaxies heading for a collision could be closer to 30. The light from the merger began travelling to us 1.4bn years after the big bang when the universe was just a tenth of its current age. “Having caught a massive galaxy cluster in the throes of formation is spectacular in and of itself,” said Scott Chapman, an astrophysicist at Dalhousie University in Halifax, Canada and co-author. “But, the fact that this is happening so early in the history of the universe poses a formidable challenge to our present-day understanding of the way structures forms in the universe,” he said.

During the first few million years of cosmic history, normal matter and dark matter began to clump together, eventually giving rise to galaxy clusters. In the present day, these clusters may contain as many as a thousand galaxies, vast swathes of dark matter, immense black holes, and clouds of gas that reach temperatures of over a million degrees. Current theories suggest that clusters as massive as the one observed, known as SPT2349-56, should have taken about twice as long to evolve, however. “How this assembly of galaxies got so big so fast is a bit of a mystery, it wasn’t built up gradually over billions of years, as astronomers might expect,” said Tim Miller, a PhD student at Yale University and co-author on the paper. Computer simulations of the galaxies predict that over time the cluster will have assembled into one of the most massive structures – perhaps the most massive known object – in the modern universe. “It would exceed anything we’ve seen in the local universe,” said Carlos De Breuck, an astronomer at the European Space Observatory.

The 14 galaxies seen heading towards each other are known as starburst galaxies due to the vigorous star formation. In such galaxies, thousands of stars are born every year, compared to just one in our own Milky Way. The galaxies would have blazed bright and then burnt out quickly because they consume their gas at an extraordinary rate. The galaxy cluster was first spotted as a faint smudge of light, using the South Pole Telescope and the Herschel space observatory. Astronomers then used the Atacama Large Millimetre/Submillimetre Array (Alma), a telescope comprising 66 antennas spread over 16km in the Chilean Andes, to make more detailed observations.

The findings are published in Nature.

Credit: Hannah Devlin for The Guardian, 25 April 2018.