From the December 2015 issue

Top space stories of 2015: Scientists spot youngest cluster of galaxies

Scientists spotted four distant quasars -- bright centers of active galaxies -- in close proximity to each other, making them the earliest galaxy cluster yet.
By | Published: December 10, 2015 | Last updated on May 18, 2023
These four quasars, the bright centers of active galaxies, are all the same huge distance from Earth. Their proximity to one another makes them the earliest galaxy cluster yet.
Hennawi and Arrigoni-Battaia (MPIA)
The process of forming clusters of galaxies is not one that astronomers can watch in real time because it takes billions of years. Instead, they look for galaxy clusters at different stages in their development. Because light travels at a constant speed, the light collected from more distant objects means scientists are seeing those objects further back in time. In 2015, astronomers reported they had found the youngest cluster yet, still in an early stage of formation.

To find this protocluster, Joseph F. Hennawi of the Max Planck Institute for Astronomy in Heidelberg and colleagues searched for the extremely bright centers of galaxies hosting actively feeding supermassive black holes. These quasars, as they are known, are used in two ways: first, as markers for large galaxies, and second, as flashlights to see through nearby gas clouds. Such gas clouds glow because they absorb the active galaxy’s light and then re-emit it. The researchers were looking for a specific color of light that energized hydrogen throws out, called Lyman alpha.

They spied four active galaxies near to one another on the sky. When they studied their light in more detail, they saw all four lie the same distance from Earth and the light from these objects has been traveling for 10.6 billion years. No one had ever seen, nor expected to find, four quasars in the same gravitationally bound group, so this discovery was a surprise.

The team also saw these galaxies embedded in an enormous cloud of hydrogen. The conglomeration existed when the universe was just about 3.2 billion years old, and the gas clump stretches about 1 million light-years across. “It’s 100 percent clear that it’s a protocluster,” says team member J. Xavier Prochaska of the University of California, Santa Cruz. “It’s a structure that will evolve into something like [the] Virgo [Cluster] today.”