Black Hole Collisions Tell a Tale of Repeating Mergers

• Black holes can undergo "hierarchical merging," where black holes created by supernova explosions collide repeatedly to form increasingly massive objects.
• Researchers are using gravitational wave data to track black hole lineage by analyzing spin rates, which reveal whether a black hole has merged previously.
• Analysis suggests that approximately 14% of merging black holes are likely second-generation objects resulting from at least one prior collision.

The Hierarchical Merging Process

• Black hole mergers are high-energy events that emit gravitational waves, providing a signature of the objects involved.
• Hierarchical merging is most probable in dense stellar environments, such as clusters, where black holes and massive stars are packed together.

Identifying Provenance via Spin

• First-generation black holes, formed directly from supernovas, typically have little to no spin because the progenitor star loses most of its mass and angular momentum upon death.
• Second-generation black holes, formed from the collision of two smaller black holes, exhibit rapid rotation—roughly 70% of their maximum possible spin.
• Astronomers identify potential repeaters by looking for pairs where one partner displays significantly higher spin than the other.

Observations and Implications

• The team analyzed data from the GWTC-4.0 catalog, looking for orbital "wobbling" caused by non-perpendicular spins, which serves as a marker for hierarchical mergers.
• Findings indicate that black holes in the 20-to-40 solar mass range (and above) are frequently second-generation.
• This process helps explain the existence of black holes exceeding 45 solar masses, which standard stellar evolution theory suggests should not be possible through direct supernova collapse alone.