A Tour of RBS 797

Scientists have found four enormous cavities, or bubbles, at the center of a galaxy cluster using NASA's Chandra X-ray Observatory. This unusual set of features may have been caused by eruptions from two supermassive black holes closely orbiting each other.

Galaxy clusters are the largest structures in the Universe held together by gravity. They are a mixture of hundreds or even thousands of individual galaxies, enormous amounts of hot gas, and unseen dark matter. The hot gas that pervades clusters contains much more mass than the galaxies themselves, and glows brightly in X-ray light that Chandra detects. An enormous galaxy is usually found at the center of a cluster.

A new Chandra study of the galaxy cluster known as RBS 797, located about 3.9 billion light years from Earth, uncovered two separate pairs of cavities extending away from the center of the cluster.

These types of cavities have been seen before in other galaxy clusters. Scientists think they are the result of eruptions from regions near a supermassive black hole in the middle of the massive central galaxy. As matter flies away from the black hole as jets in opposing directions, it blows cavities in the hot gas. The revelation in RBS 797 is that there are two sets of jets directed perpendicular to each other.

Astronomers previously observed the pair of cavities in the east-west direction in RBS 797, but the pair in the north-south direction was only detected in a new, much longer Chandra observation. The deeper image uses almost five days of Chandra observing time, compared to about 14 hours for the original observation. The NSF's Karl G. Jansky Very Large Array had already observed evidence for two pairs of jets as radio emission, which line up with the cavities.

How was this quartet of cavities created? The most likely answer, according to the researchers from the new study, is that RBS 797 contains a pair of supermassive black holes that have launched jets in perpendicular directions at almost the same time.

Another possible explanation for the four cavities is that there is only one supermassive black hole — with jets that somehow manage to flip around in direction quite quickly. The researchers determined it would have had to do this in less than 10 million years — a short time span in cosmic terms.

Astronomers will continue to analyze this system. Whatever the ultimate explanation is, it will provide new information about how some of the Universe's biggest black holes behave and how they affect their surroundings.