Astronomers discover that black holes created by mergers contain information about their ancestors

Astronomers believe that at the heart of most, if not all, galaxies lies a gigantic black hole with a mass millions or even billions of times greater than that of our Sun. These supermassive black holes cannot be formed directly from the collapse of a massive star, as is the case with stellar black holes with masses tens of times greater than that of the Sun, since no star is large enough to sustain such a to produce a huge object.

This means that there must be processes that allow black holes to grow into such enormous masses. While the consumption of gas and dust and even stars around black holes can facilitate this growth, a faster way to accumulate mass is a chain of mergers of increasingly larger black holes.

An article published in Astroparticle physics by Imre Bartos and Oscar Barrera of the University of Florida’s Department of Physics explain how some ‘daughter’ black holes formed in such mergers could contain information about the ‘parent’ black holes that collided to create them .

“We find that black holes that form from the collision of other black holes carry information about the properties of their ancestors, including the rotation of the ancestors and their masses,” says Bartos. “The main new focus of our research is the reconstruction of the spins of ancestral black holes, building on previous work that focused on ancestral masses.”

Black holes have very few features that can be used to distinguish them from each other; they possess only variations in mass, angular momentum or ‘spin’ and electric charge. Theoretical physicist John Wheeler of Princeton University, US, described this by saying: “black holes have no hair.” Bartos adds that even with these few features and the ‘no hair theorem’, it is still possible to use a black hole’s spin to unravel details about its origins.

“For example, black holes feeding on surrounding gas, or the previous collisions of ‘parent’ black holes, can result in high spin, while black holes at birth through the death and collapse of stars often have low spin,” continues Bartos.

To conduct their research, Bartos and Barrera used a mathematical technique called Bayesian inference, which took measured black hole properties and their prior expectations as input and performed inferred distributions of the ancestral black hole’s properties. The research is timely as physicists use tiny ripples in spacetime, called gravitational waves, to learn more about black hole collisions and mergers.

‘Recent observations of black hole mergers point to the possibility that black hole assembly lines – places where multiple black holes merge in succession to form increasingly massive black holes – may be common in the universe.

“This raises the question of how we can determine the properties of ancestral black holes from measurements of the latest generation,” says Bartos. “I’m fascinated by the detective story of discovering what happened to these black holes in the past and finding the fingerprints of previous generations there.”