Scientists have observed the most massive black hole merger to date, with masses that are incompatible with standard stellar formation.
The most recent black hole merger, the largest on record, is making news and challenging astronomers’ assumptions. The groundbreaking event involved black holes with masses approximately 100 times that of the Sun, a range where the conventional stellar collapse formation process is considered impossible. The new observations, reported earlier this week, suggest that some black holes may form through a multimerger scenario.
Since the first merger detection in 2015, the LIGO-Virgo-KAGRA (LVK) Collaboration, a network of scientists operating gravitational-wave detectors in the US, Europe, and Japan, has observed around 300 black hole mergers. When two black holes spiral into each other, they create ripples in spacetime that can be recorded on Earth using highly sensitive interferometers. Most of these black holes weigh around 10 solar masses, a size that can be explained within a scenario where a massive star runs out of fuel and collapses into a black hole.
However, not all stars undergo this process. Very massive stars of around 200 solar masses are unstable, according to theoretical models. The energy within the core of such a star would be so high that it would produce electron-positron pairs, a matter-antimatter combination that reduces the star’s self-sustaining pressure. The drop in pressure leads to a destructive implosion, and no chance for the star to reach a black hole finale. The implication is that stellar collapse cannot produce black holes in the mass range between about 60 and 130 solar masses.
In 2019, the LVK Collaboration detected a merger of black holes whose measured masses—65 and 85 solar masses—were at the lower end of the gap. This surprising observation led to speculations about alternative black hole formation scenarios, but since the predicted mass range for the gap was uncertain, it remained possible that these black holes formed through some modified stellar-collapse scenario.
Now the LVK Collaboration has raised the stakes by detecting two heavier black holes, with one squarely in the gap. “This event provides good evidence that black holes in the mass gap can exist,” says LIGO Scientific Collaboration spokesperson Stephen Fairhurst from Cardiff University in the UK.
The new event, named GW231123, was observed in November 2023 by the two LIGO observatories in the US. The gravitational-wave signal was in the low-frequency range of the detectors, which implies that the black holes were on the heavy side. The signal was also short—just one-fifth of a second—which complicated the interpretation, says LVK member Sophie Bini from Caltech. The collaboration uses so-called waveform models to determine parameters such as mass, orbital orientation, and distance. “The analysis is more uncertain because you have less information to start with,” Bini says.
The best fit turned out to be a merger of two black holes with masses of 103 and 137 solar masses, with the resulting black hole weighing in at 225 solar masses. Due to the signal’s brevity, the researchers could not pinpoint the location in the sky where the event happened. Bini presented these results on Monday at the GR–Amaldi meeting in Glasgow, Scotland.
Davide Gerosa, an astrophysicist at the University of Milano-Bicocca in Italy, who was not involved in the study, hailed the finding as “exciting news,” noting the “spectacular” discovery of black holes exceeding 100 solar masses. He recalled that a decade ago, the existence of 30-solar-mass black holes was a surprise; now, even larger ones have emerged.
The crucial question is, how do these colossal black holes form? According to Bini, one plausible scenario is “hierarchical mergers,” a model suggesting that earlier black hole merger events produce increasingly massive black holes, which then participate in further merges – a process Bini likens to “a family tree of black holes.” A key challenge for this ‘family tree’ model, however, is how these merged black holes would encounter new partners. Bini explains that a merger typically imparts a ‘recoil kick’ to the resulting black hole, potentially ejecting it from star-dense environments where subsequent mergers would be likely.
The observed ‘spin’ – or rotation – of these black holes around their axes offers a potential clue. In the conventional stellar-collapse theory, black holes forming in a binary system are expected to have small, aligned spins; most LVK Collaboration observations align with this. However, Gerosa clarifies that the final black hole resulting from such a merger should possess a relatively high spin, as it absorbs the significant orbital angular momentum of its rapidly orbiting predecessors.
Intriguingly, the GW231123 event revealed two merging black holes with exceptionally high spins, despite significant uncertainties (ranging from 20% to 50%). One spun at 80% of the maximum allowed value, the other at 90%. Such high rotational rates strongly imply that these objects are products of previous mergers. Yet, Gerosa notes, these observed spins are slightly higher than the typical 70% predicted by the hierarchical merger scenario. He suggests, however, that ongoing data analysis might lead to a slight downward revision of these estimates.
Another hypothesis considered by the LVK Collaboration involves a massive star merging directly with another star, thus bypassing the ‘pair-creation instability.’ However, these models struggle to account for the high spin values observed by the team.
While the mechanism of formation remains an open question, scientists may be able to pinpoint where these black holes originate, according to theorist Rosalba Perna from Stony Brook University in New York, who was also not involved in the study. She suggests the GW231123 merger likely happened in a region with either a high density of stars, fostering frequent black hole interactions, or a sufficiently dense surrounding medium that allows significant accretion of additional material. Perna specifically points to “the disks of active galactic nuclei” as a prime candidate for such environments.
