Massive primordial galaxies found nestling in vast halo of dark matter

Two giant galaxies seen when the universe was only 780 million years old, or about 5% of its current age, have been identified by an international team involving UCL scientists using the Atacama Large Millimeter/submillimeter Array (ALMA). The latest observations push back the epoch of massive-galaxy formation and reveal that these uncommonly large galaxies are nestled inside an even larger cosmic structure, a halo of dark matter several trillion times bigger than the sun. The two galaxies are in such close proximity - less than the distance from the Earth to the center of our galaxy - that they will shortly merge to form the largest galaxy ever observed at that period in cosmic history. This discovery provides new details about the emergence of large galaxies and the role that dark matter plays in assembling the most massive structures in the universe. "We expect the earliest galaxies that formed just a few hundred million years after the Big Bang to be 'nuggets' of stars and gas - perhaps with some similarities to dwarf galaxies we see in the nearby Universe. This system is anything but that, however.