Huge set of galaxies is set to form largest cluster in known universe

One of the largest quasars in the early universe has helped to reveal a vast “protocluster” of nascent galaxies that is expected to grow more massive than any galaxy cluster we know of.

Protoclusters are the primordial beginnings of the galaxy clusters we see in our region of the universe, before they are bound together by gravity. Astronomers usually look for them by casting their telescopes over wide swathes of space, hoping to find regions with an unusually high number of early galaxies.

Instead, Feige Wang at the University of Arizona and his colleagues decided to focus their attention on a quasar, a luminous galactic core where gases emit light as they fall into a supermassive black hole. They chose one of the most massive and brightest quasars known in the early universe, whose light is only now reaching us from 800 million years after the big bang, hoping it would act as a signpost for other massive objects around it.

They found a vast web of early galaxies surrounding the quasar. “This protocluster is huge,” says Wang. “If you look at it in the sky compared to other protoclusters, which are only a few arcminutes across the sky, we see this one is tens of arcminutes.” An arcminute is 1/60th of a degree.

The protocluster is expected to grow over time and eventually collapse into a galaxy cluster. Once this happens, Wang and his team calculate that it will have a combined mass around 7 quadrillion times that of the sun, potentially larger than the largest known galaxy cluster in our region of the universe.

The quasar at the centre of the cluster is probably a supermassive black hole with a mass about 3 billion times that of the sun. It is difficult to explain how this black hole grew so massive in such a short time after the birth of the universe, says Wang, but finding systems similar to this could help shed light on the problem, as well as on how such massive quasars affect nearby galaxies.

A protocluster is an exceptionally dense environment, and this provides key clues that might explain the quasar’s expeditious growth, says David Alexander at the University of Durham, UK. “Such a dense environment will give a large availability of gas to fuel the black hole and allow it to grow rapidly,” he says.

However, we would need to find many more objects similar to this before we can begin to explain how early supermassive black holes grow to be so large, he says.