The Chandra X-Ray Observatory has discovered a quartet of X-ray cavities in the hot gas surrounding a single galaxy. The four distinct bubbles suggest that the galaxy hosts two supermassive black holes, both of which erupted and generated two pairs of jets at about the same time. “The quartet of cavities might have also been created by a single black hole during two different episodes of activity,” said astronomer Francesco Ubertosi of the University of Bologna, who led the Chandra study told The Daily Galaxy. “If this turns out to be the case, then it will be interesting to investigate how the jets of the black hole have quickly turned around by approximately 90 degrees.”
The large massive galaxy is at the center of a galaxy cluster, known as RBS 797, located about 3.9 billion light years from Earth. If confirmed, the two supermassive black holes would be among the closest pair ever discovered, with a separation of about 250 light years.
Scientists have seen these X-ray cavities before in other galaxy clusters. A pair of cavities is thought to be a byproduct of eruptions from a supermassive black hole in a large galaxy at the center of a cluster. The eruptions power jets in opposite directions, which push gas away to create a pair of cavities. However, to produce four cavities each roughly pointing 90 degrees away from one another, a more complex phenomenon must be at play.
The left panel of the graphic below shows an optical image of the galaxy cluster called RBS 797, from NASA’s Hubble Space Telescope. Hot gas that envelopes the individual galaxies is invisible in optical light, but it is detected in X-rays by Chandra (right). One pair of cavities can be seen towards the left and right of center in the Chandra image as black oval-shaped regions. The other pair is less distinct, but can be found above and below the center of the image.
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, which typically hosts an active supermassive black hole that generates relativistic jets in opposite directions. The jets deposit heat and energy into the surrounding intergalactic medium, regulating the flow of gas and star formation within the galaxy.
“We think we know what a pair of cavities represents, but what is going on when a galaxy cluster has two pairs in very different directions?” said Ubertosi.
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 National Science Foundation’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.
Rare and Extraordinary
How was this quartet of cavities created? The most likely answer, according to Ubertosi and his colleagues, is that RBS 797 contains a pair of supermassive black holes that have launched jets in perpendicular directions at almost the same time.
“Our best idea is that one pair of supermassive black holes has led to a pair of a pair of cavities,” said Myriam Gitti, a co-author also of the University of Bologna. “While we think supermassive black holes can form binary systems, it is extremely rare that both of them are observed in an active phase – in this sense the discovery of two close active black holes inflating cavities in RBS 797 is extraordinary.”
Indeed, previously a radio observation with the European VLBI Network (EVN) discovered two radio point sources separated by only about 250 light-years in RBS 797. If both sources are supermassive black holes, they are among the closest pair ever detected. The two black holes should continue to spiral toward each other, generating huge amounts of gravitational waves, and eventually merge.
The Alternative Scenario
There is another possible explanation for the four cavities seen in RBS 797. This scenario involves only one supermassive black hole – with jets that somehow manage to flip around in direction quite quickly. Analysis of the Chandra data shows that the age difference for the east-west and north-south cavities is less than 10 million years.
“If there is only one black hole responsible for these four cavities, then we will have to trace the history of its activity. Key aspects are how the jets’ orientation changed quickly, and whether this is related to the galaxy cluster environment or to the physics of the black hole itself – or even a combination of both,” said Fabrizio Brighenti, a University of Bologna co-author.
The Last Word –Confirming Their Existence
When asked about the possibility of detecting the massive black hole in the central galaxy, Francesco Ubertosi replied in an email to The Daily Galaxy: “In order to confirm the presence of binary supermassive black holes, we need high resolution images of the core of the galaxy, which will be possible thanks to our deep VLBI radio data. If we find two close radio point source with the typical properties of active black holes – namely, a flat radio spectral index – then we might be able to confirm the presence of the binary system.”
Can these black hole bubbles stop the formation of new stars in the galaxies by altering the temperature and density of the gas throughout the cluster? Fabrizio Brighenti answered in an email: “Current observations and simulations of galaxy cluster support the picture that black hole inflated bubbles contribute to the heating of the cluster gas, which in turn prevent strong star formation to occur. In the near future, we will study how the inflation of four, nearly coeval cavities has influenced the formation of new stars in the central galaxy of RBS 797.”
“Previous radio observations of RBS 797 had revealed perpendicular lobes of the central radio galaxy, thus it was interesting to discover X-ray depressions coincident with the radio lobes. Once we measured the age of the perpendicular pairs, it was fascinating to find that the cavities are almost coeval,” Ubertosi concluded.
A paper describing these results appears in The Astrophysical Journal Letters and is available online: https://arxiv.org/abs/2111.03679
Image credit: X-ray: NASA/CXC/Univ. of Bologna/F. Ubertosi; Optical: NASA/STScl/M.Calzadilla