An international team of researchers has found the most remote cluster of galaxies yet--at a distance of 13 billion light-years. Light from this cluster has taken so long to reach Earth that astronomers now see the cluster in the process of being born, when the universe was only a tenth of its present age.
Immediately after the big bang, the universe was hot, dense, and homogeneous. But as it expanded, regions that were just slightly denser than average clumped together into small, compact clouds that later merged into galaxies. The galaxies subsequently arranged themselves into the large groups and clusters we see today--a process that took billions of years. No one knows when the very first protoclusters started to appear on the scene; the earliest detected previously were born when the universe was almost 3 billion years old.
The newly discovered cluster was born when the universe was only half that age. A team led by George Miley of Leiden Observatory in the Netherlands trained the European Very Large Telescope in Chile on a bright radio galaxy in the constellation Virgo, 13 billion light-years away. They suspected that the galaxy might be at the core of a protocluster, because many clusters at much smaller distances also have active galaxies at their cores. After surveying the surroundings of the distant galaxy, they found a large cluster of faint galaxies at the same distance. The team reports the discovery of the cluster, which is still forming, in the 10 April issue of The Astrophysical Journal Letters.
The discovery is "very, very interesting," says Neta Bahcall of Princeton University. The number of protoclusters at various distances tells astronomers about the amount of matter in the universe and the large-scale structure of galaxies, she says. Bahcall adds that the finding is consistent with current models of the makeup and evolution of the universe. But she adds that a larger sample of protoclusters is needed to really put the theoretical predictions to the test.