A team of astronomers using data from the Gemini North telescope in Hawaii has found interesting insights about the heaviest pair of supermassive black holes ever identified. These insights shed light on the dynamics of black hole binaries and their potential merger.
Sizing Up the Heaviest Pair of Black Holes
Supermassive black holes are common features at the centers of massive galaxies. When galaxies merge, their black holes can form binary pairs, orbiting each other in a bound system. However, observing such binaries and understanding their behavior has been a longstanding challenge for astronomers.
The astronomers' study focused on a supermassive black hole binary located within the elliptical galaxy B2 0402+379. This binary is unique as it is the only one ever resolved in enough detail to distinguish both objects separately.
Despite being in close proximity - just 24 light-years apart - the binary has remained stalled at this distance for over three billion years, raising questions about why its merger has not progressed.
To understand the system's dynamics clearly, the team analyzed archival data from the Gemini Multi-Object Spectrograph (GMOS) to identify the speed of stars near the black holes.
This analysis allowed them to estimate the binary's total mass, which was a staggering 28 billion times that of the sun, making it the heaviest binary black hole ever measured.
"The excellent sensitivity of GMOS allowed us to map the stars' increasing velocities as one looks closer to the galaxy's center," said Roger Romani, Stanford University physics professor and co-author of the paper. "With that, we were able to infer the total mass of the black holes residing there."
The mass of the black hole binary is a crucial factor in determining whether a merger will occur. In this case, the huge mass suggests that the binary formed through multiple galaxy mergers, resulting in a depletion of stars and gas in the vicinity of the black holes, according to the astronomers.
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Merging of Black Holes
As supermassive black hole binaries orbit each other, they interact with surrounding stars, transferring energy and gradually reducing their separation.
Eventually, gravitational radiation takes over, causing the black holes to merge. While this process has been observed in pairs of stellar-mass black holes, it has never been directly observed in supermassive binaries.
The team's analysis indicates that the black hole binary in B2 0402+379 has depleted much of the surrounding material, leaving it stalled in its final stages of merger.
Whether the binary will overcome this stagnation and merge remains uncertain. If a merger does occur, it could produce gravitational waves significantly more powerful than those generated by stellar-mass black hole mergers.
The possibility of another galaxy merger injecting additional material into the system could facilitate the binary merger. However, given B2 0402+379's status as a fossil cluster resulting from past mergers, such an event is unlikely, according to the astronomers.
"We're looking forward to follow-up investigations of B2 0402+379's core where we'll look at how much gas is present," said Tirth Surti, Stanford undergraduate and the lead author on the paper.
"This should give us more insight into whether the supermassive black holes can eventually merge or if they will stay stranded as a binary," he added.
The team's findings were published in The Astrophysical Journal.
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