Galaxy Cluster Spotted in Early Universe

Astronomers have discovered a vast collection of young galaxies in the early universe.

The newly discovered protocluster of galaxies located in the Bootes field of the NOAO Deep Wide-field Survey.
The green circles identify the confirmed cluster members. Density contours (white lines) emphasize the concentration of member galaxies toward the center of the image. The inset images highlight two example members that glow in the Ly-alpha line of atomic hydrogen.
Dr. Rui Xue (Purdue University)

Astronomers have discovered a collection of young galaxies, stretching more than 489 million by 244 million light-years whose light has taken 12 billion years to reach Earth, one of the most massive structures known at that distance. This “protocluster,” a precursor to giant galaxy clusters near us such as the Coma Cluster, exists in a universe only 1.7 billion years old.

Modern-day galaxy clusters may contain thousands of galaxies drawn together by gravity. But while nearby clusters can be studied in detail, their early history and formation is not well understood. With the discovery of 65 galaxies in this faraway protocluster, scientists can witness its formation directly.

Measurements put this baby cluster at a mass of a thousand trillion Suns — already gigantic. “The protocluster will very likely grow into a massive cluster of galaxies like the Coma cluster,” said Kyoung-Soo Lee (Purdue University), in a press release. Clusters this massive are extremely rare, with only a handful of candidates known at such a young age. And it’s not just that they’re difficult to observe — they’re rare even in cosmological simulations.

This system was the first to be confirmed using extensive spectroscopy. The team first obtained very deep images of a small patch of sky, about the size of two full Moons, using the Mayall telescope on Kitt Peak. Then they used the Keck II Telescope on Mauna Kea to measure the distance to the faint galaxies within the patch, to determine which galaxies live close to each other and which are chance alignments.

“Many of the faint galaxies in this patch lie at the same distance,” says Arjun Dey (National Optical Astronomy Observatory). “They are clumped together due to gravity and the evidence suggests that the cluster is in the process of forming.”

The team is now searching larger areas of the sky to uncover more examples of young and massive protoclusters. “The discovery and confirmation of one distant and massive protocluster is very exciting,” says Naveen Reddy (University of California at Riverside), “but it is important to find a large sample of these so we can understand the possibly varied formation history of the population as a whole.”

If astronomers can find even more clusters at these kinds of distances, their prevalence can help constrain the size and expansion history of the universe.

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