Supermassive Black Holes Reveal Merger Slowdown

Gemini North’s data offers a potential elucidation for the cessation of the merger between two supermassive binary black holes.

Astronomers have utilized archival data from the Gemini North telescope to ascertain the weightiest duo of supermassive black holes ever discovered. The fusion of two supermassive black holes has been foreseen for a considerable time, yet it has never been witnessed. This colossal pair provides insights into the reasons behind the apparent rarity of such an occurrence in the vast expanse of the Universe.

Almost every colossal galaxy contains a supermassive black hole situated at its core. When two galaxies combine, their black holes can create a binary pair, indicating they are in a mutually bound orbit. The merging of these binaries is believed to be inevitable, although no direct observation of this phenomenon has been made. Astronomers have been debating the possibility of such an event for many years. In a recently published article in The Astrophysical Journal, a group of astronomers has provided fresh perspectives.

The group utilized information from the Gemini North telescope in Hawai‘i, one portion of the International Gemini Observatory managed by NSF’s NOIRLab, which is supported by the US National Science Foundation, to examine a supermassive black hole binary situated within the elliptical galaxy B2 0402+379. This marks the sole supermassive black hole binary ever observed in adequate detail to distinguish both entities separately, and it boasts the distinction of possessing the smallest measured separation on record—a mere 24 light-years.

Despite the proximity hinting at an imminent merger, further investigation uncovered that the duo has remained at this distance for more than three billion years, prompting the question: what is causing the delay?

To better understand the workings of this system and the paused merger, the team examined archival data from Gemini North’s Gemini Multi-Object Spectrograph (GMOS). This data enabled them to calculate the velocity of the stars near the black holes.

The excellent sensitivity of GMOS allowed us to map the stars’ increasing velocities as one looks closer to the galaxy’s center. With that, we were able to infer the total mass of the black holes residing there.

Roger Romani, Professor and Study Co-Author, Physics, Stanford University

The team has calculated that the binary's mass is an impressive 28 billion times greater than that of the Sun, making it the heaviest binary black hole ever recorded. This measurement not only provides important insights into the creation of the binary system and the background of its host galaxy but also reinforces the established idea that the mass of a supermassive binary black hole is crucial in preventing a potential merger.

The data archive serving the International Gemini Observatory holds a gold mine of untapped scientific discovery. Mass measurements for this extreme supermassive binary black hole are an awe-inspiring example of the potential impact from new research that explores that rich archive.

Martin Still, Director, NSF Program, International Gemini Observatory

By comprehending the formation process of this binary, it becomes possible to anticipate the occurrence and timing of its merger. Several indications suggest that the pair was formed through multiple galaxy mergers. Firstly, B2 0402+379 is classified as a “fossil cluster,” indicating that it originated from the merging of an entire galaxy cluster's worth of stars and gas into a single colossal galaxy. The existence of two supermassive black holes and their significant combined mass also implies that they emerged from the consolidation of numerous smaller black holes from various galaxies.

Following a galactic merger, supermassive black holes do not collide head-on. Instead, they start sling-shooting past each other as they settle into a bound orbit. Energy is transferred from the black holes to the surrounding stars with each pass they make. As they lose energy, the pair is pulled closer and closer until they are just light-years apart, where gravitational radiation takes over, and they eventually merge. This process has been observed directly in pairs of stellar-mass black holes, with the first recorded instance in 2015 through the detection of gravitational waves. However, it has never been observed in a binary of the supermassive variety.

The team reached a conclusion based on their newfound understanding of the system’s immense mass: a significantly high quantity of stars would have been required to decelerate the binary’s orbit to such an extent that it brought them into such proximity.

As a result, the black holes appear to have expelled nearly all the matter in their vicinity, resulting in a dearth of stars and gas in the core of the galaxy. Since no additional material is accessible to impede the pair’s orbit further, their merger has come to a halt during its ultimate phases.

Romani says, “Normally, it seems that galaxies with lighter black hole pairs have enough stars and mass to drive the two together quickly. Since this pair is so heavy, it required lots of stars and gas to get the job done. But the binary has scoured the central galaxy of such matter, leaving it stalled and accessible for our study.”

Whether the pair will overcome their stagnation and merge on timescales of millions of years or remain in orbital limbo indefinitely is still pending. If they merge, the resulting gravitational waves would be a hundred million times more potent than those generated by mergers of stellar-mass black holes. The pair could bridge the final distance through another merger with a galaxy, which would introduce additional material into the system.

Alternatively, a third black hole could potentially be involved to slow down the pair's orbit and facilitate the merger. However, considering B2 0402+379's classification as a fossil cluster, the occurrence of another galactic merger is unlikely.

We’re looking forward to follow-up investigations of B2 0402+379’s core, where we’ll look at how much gas is present. This should give us more insight into whether the supermassive black holes can eventually merge or if they will stay stranded as a binary.

Tirth Surti, Undergraduate and Study Lead Author, Stanford University

Journal Reference:

Surti, T., et al. (2024) The Central Kinematics and Black Hole Mass of 4C+37.11. The Astrophysical Journal. doi/10.3847/1538-4357/ad14fa.