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Binary black holes reveal some of their secrets

In the vast expanse of the universe, nearly every massive galaxy houses a supermassive black hole at its core. These cosmic behemoths often come into close contact during galaxy mergers, potentially forming binary black holes locked in a mutual orbit.

The fate of such pairs — whether they eventually merge — has puzzled astronomers for decades. Recent research published in The Astrophysical Journal sheds light on this cosmic conundrum, offering new insights into the behavior of supermassive black hole binaries.

Unveiling a cosmic enigma

A fascinating study utilizing the Gemini North telescope, part of the International Gemini Observatory operated by NSF’s NOIRLab, has focused on a supermassive black hole binary within the elliptical galaxy B2 0402+379.

This binary is notable for being the only one resolved with enough detail to observe both black holes separately, boasting the smallest separation ever directly measured at just 24 light-years.

Despite this close proximity, which typically indicates an imminent merger, the binary black hole has remained at a standstill for over three billion years. This unusual phenomenon prompted researchers to investigate what might be causing this delay.

The team analyzed archival data from the Gemini Multi-Object Spectrograph (GMOS) to study the velocity of stars near the black holes, revealing crucial information about the system’s dynamics.

Why binary black holes don’t always merge

Professor Roger Romani of Stanford University, a co-author of the study, highlighted the instrumental role of GMOS’s sensitivity in mapping the stars’ velocities, enabling the team to estimate the binary’s total mass at a staggering 28 billion solar masses.

This makes it the heaviest binary black hole ever measured, offering key insights into the binary system’s formation and the history of its host galaxy.

The massive combined mass of the binary system supports theories suggesting that the weight of supermassive black holes plays a significant role in delaying mergers.

Martin Still, NSF program director for the International Gemini Observatory, emphasized the wealth of untapped scientific discovery within the data archive, with the mass measurements of this binary serving as a prime example of the potential breakthroughs waiting to be uncovered.

The formation of a supermassive binary

The formation of this binary is believed to be the result of multiple galaxy mergers, with B2 0402+379 being a ‘fossil cluster’ — the product of a galaxy cluster merging into a single massive galaxy. The binary’s significant mass implies a history of amalgamations from smaller black holes belonging to multiple galaxies.

The process leading up to a merger involves supermassive black holes slingshotting past each other, gradually losing energy and drawing closer until gravitational radiation triggers the final merge. While this has been observed in stellar-mass black holes, a supermassive binary merger remains unrecorded.

The study suggests that a vast number of stars would have been necessary to decelerate the binary’s orbit to its current proximity, with the black holes expelling nearly all surrounding matter in the process. This has left the galaxy’s core devoid of stars and gas, stalling the merger.

Professor Romani explained that lighter black hole pairs typically have enough surrounding mass to drive a quick merger. However, the exceptional mass of this binary required a much larger amount of stars and gas, ultimately scouring the central galaxy of such materials and leaving the merger in limbo.

What lies ahead for B2 0402+379

The future of this binary, whether it will eventually merge or remain in orbital stagnation, is still uncertain. A successful merger would produce gravitational waves far more powerful than those from stellar-mass black hole mergers.

The possibility of another galaxy merger or the introduction of a third black hole could provide the necessary material to prompt the merger, though B2 0402+379’s status as a fossil cluster makes this scenario unlikely.

The research team, led by Stanford undergraduate Tirth Surti, plans to conduct follow-up investigations to determine the presence of gas in B2 0402+379’s core. This could offer further insights into whether the supermassive black holes will ultimately merge or remain eternally separated.

Implications and future study of binary black holes

In summary, the study of the supermassive black hole binary within galaxy B2 0402+379 has expanded our understanding of the cosmic forces at play in the universe by focusing on the invaluable role of archival data in unlocking the mysteries of space.

By meticulously analyzing the dynamics of this stalled binary system, astronomers have shed light on the intricate dance of gravity and mass, offering insights into the potential future of such colossal entities.

This research poses new questions about the nature of galactic mergers and the fate of supermassive black holes while setting the stage for future discoveries that could further unravel the fabric of our cosmos.

The full study was published in the journal The Astrophysical Journal.


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