Many Mergers Might Fill the Mass Gap – Image for illustrative purposes only (Image credits: Unsplash)
Black holes with masses that stellar evolution theory says are impossible continue to appear in observations, raising questions about how the universe actually assembles its most extreme objects. A new analysis of more than 200 binary black hole mergers detected by the LIGO-Virgo-KAGRA network offers fresh evidence that many of these objects form not from single stars but through chains of earlier collisions. The findings strengthen the case that repeated mergers can populate the so-called pair-instability mass gap, a forbidden range between roughly 45 and 120 times the mass of the Sun.
The Forbidden Range in Black Hole Masses
Stars that end their lives in pair-instability supernovae leave no remnant at all, creating a predicted gap in black hole masses. Theory holds that stars between about 130 and 250 solar masses explode so violently that their cores are completely disrupted. This process should prevent black holes from forming in the 45-to-120 solar mass window. Yet detectors have recorded mergers involving black holes squarely inside that range, forcing astronomers to look beyond single-star death for an explanation.
The gap is not merely an academic curiosity. It marks a clear boundary in how nature converts stellar material into compact objects. Any black hole found inside it must have assembled its mass through a different channel, most likely by swallowing other black holes that already existed.
Building Heavier Black Holes Through Successive Collisions
Black holes can grow by merging with one another when they orbit in tight pairs. A first-generation black hole forms directly from a star’s collapse. When two such objects merge, the result is a second-generation black hole that is more massive and often carries a different spin signature. That second-generation object can later merge with another first-generation black hole, and the process can repeat. Each step adds mass while also randomizing the spin directions because the new black hole is kicked into a new orbital plane.
These hierarchical mergers leave measurable traces. The heavier black hole tends to be noticeably more massive than its partner, and the spins point in less predictable directions compared with pairs that formed together from the same binary star system. Gravitational wave signals encode both the mass ratio and the spin orientations, giving observers a way to distinguish the two populations without needing to watch the mergers in real time.
Spin and Mass Patterns in the Latest Catalog
Researchers modeled the distribution of two spin parameters across the entire GWTC-4 catalog. One parameter tracks how aligned the spins are with the orbital plane; the other captures the component of spin that lies in the plane itself. They then compared the observed values against two separate templates: one for ordinary first-generation mergers and one for hierarchical events. A mixture model allowed the fraction of hierarchical mergers to change smoothly with the mass of the heavier black hole.
The fit revealed three clear trends. Above roughly 46 solar masses, nearly all mergers appear hierarchical, exactly where the mass gap begins. A smaller but distinct group of hierarchical events clusters around 16 solar masses, consistent with the expected output of certain dense star clusters. Between 20 and 40 solar masses, hierarchical mergers become rare, suggesting that most black holes in that window still come from single stars.
What the Patterns Reveal and What Remains Unknown
The mass-dependent rise in hierarchical mergers matches earlier studies that used only one spin parameter, yet the new work incorporates an extra spin component and therefore reduces one source of systematic uncertainty. The result increases that repeated mergers really do operate inside the mass gap. At the same time, the model cannot yet say how often these chains occur in different galactic environments or whether some black holes inside the gap arrive through entirely different routes, such as accretion from surrounding gas.
- Most mergers above 46 solar masses show hierarchical signatures.
- A secondary population appears near 16 solar masses, likely from cluster dynamics.
- A dip between 20 and 40 solar masses points to a larger share of ordinary first-generation black holes.
- Spin modeling now includes both alignment and in-plane components, tightening earlier conclusions.
Next Steps in Mapping the Black Hole Population
Future observing runs will add hundreds more events, allowing the same mixture model to be tested at higher precision and across different redshift ranges. If the hierarchical fraction continues to climb inside the mass gap, the case for repeated mergers will become still stronger. Until then, the current catalog already shows that nature finds ways to assemble black holes where single stars cannot, and gravitational waves are the tool revealing how those pathways work.