Astronomers measure the most massive pair of black holes ever found
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Astronomers recently discovered the heaviest pair of supermassive black holes ever identified using archival data from the Gemini North telescope. This discovery sheds light on the long-standing mystery surrounding the merger of these objects, an event theoretically predicted but never observed in the Universe.
Two ogres 24 light years apart
At the heart of most large galaxies lies a supermassive black hole. When two galaxies merge , their central black holes can come together and form a gravitationally linked binary pair. Although these pairs are expected to eventually merge, no cases of merger have been directly observed , sparking debate among astronomers. A study published by an international team of astronomers offers new insights into this enigma.
The researchers focused on a binary pair located within the elliptical galaxy B2 0402+379 . This pair is unique in that they represent the only example of supermassive black holes whose separation could be measured directly, revealing an incredibly short distance of just 24 light years between them .
Despite this proximity, which would indicate an imminent merger, data shows that the pair remained in this state for more than three billion years , raising questions about the obstacles to their merger. To investigate further, the team used data from Gemini North's multi-object spectrograph, which made it possible to measure the speed of the stars around these two black holes.
A pair of black holes with a mass of 28 billion suns
As a reminder, Newton's universal gravitation tells us that the greater the mass of the central object (in this case, the pair of supermassive black holes), the greater the gravitational force it exerts on surrounding objects (like stars ) is important. This greater gravitational force results in higher orbital speeds for nearby stars. By measuring the speed at which stars move around black holes, astronomers were able to use the laws of physics to calculate the mass of the object exerting the gravitational force.
These analyzes revealed that the combined mass of the pair reaches about 28 billion times that of the sun , making them the most massive pair ever identified . This colossal mass provides clues to the dynamics of the pair and explains why the merger is so difficult to achieve.
Not enough material left to fuse
The study suggests that the formation of this supermassive binary pair resulted from multiple galaxy mergers. B2 0402+379 is in fact identified as a “fossil cluster” resulting from the merger of several ancient galaxies into a single massive entity. The black holes' large mass indicates that they likely swallowed up several other, smaller black holes during these mergers.
The researchers explain that supermassive black holes do not collide directly during a galactic merger. Rather, they enter into a gravitational dance and gradually lose energy with each orbit through interaction with surrounding stars until gravitational radiation forces dominate and cause merger.
However, for this particularly massive pair, the process appears to have stopped . The enormous amount of stars and gas needed to bring the black holes to their current proximity appears to have been entirely consumed, leaving the galaxy's core devoid of material and preventing any progress toward a merger .
The future of this pair therefore remains uncertain. They could potentially merge if a new galactic merger brings in enough material to restart the process, although this seems unlikely given B2 0402+379's status as a fossil cluster. The researchers hope that additional studies will provide more information about the presence of gas in the galaxy's core, which could shed light on the possibility of future supermassive black hole mergers.
Details of the study are published in The Astrophysical Journal .
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