In brief: Astronomers at MIT have discovered previously unidentified galaxy clusters that were overlooked by earlier studies. Their results, which were recently published in The Astrophysical Journal, suggest that as many as one percent of galaxy clusters could be misidentified as a single bright galaxy.

Clusters of galaxies containing hundreds or even thousands of individual galaxies are held together by gravity. As MIT highlights, they move through a sea of hot gas called the intracluster medium, and give off X-ray radiation that we can see using space-based telescopes.

This radiation creates a “fuzzy halo” around galaxy clusters, making them easier to identify versus an object with a single source of X-rays, like a star or quasar.

As MIT Associate Professor Michael McDonald discovered in 2012, however, not all clusters adhere to this general principal. The cluster he discovered, dubbed the Phoenix cluster, contains a black hole that emits X-rays bright enough to drown out the radiation from the intracluster medium. Thus, it looked like a single X-ray source and was misclassified for decades.

Armed with this new possibility, the Clusters Hiding in Plain Sight (CHiPS) survey came to life. During its six-year run, the survey identified three new galaxy clusters, one of which is similar to the Phoenix cluster. That’s notable considering astronomers only know of just a few Phoenix-style clusters.

McDonald hopes that the CHiPS survey results will help others, like those working with the eROSITA X-ray instrument, better understand how to search for clusters.

“The people that are building out the cluster searches for this new X-ray telescope need to be aware of this work,” McDonald said. “If you miss one percent of the clusters, there's a fundamental limit to how well you can understand the universe.”

Images courtesy Triff