Astronomers have discovered that the black holes created by the merger carry information about their progenitors

Astronomers believe that at the heart of most, if not all, galaxies resides a titanic black hole with a mass millions or even billions of times that of our Sun. These supermassive black holes cannot be directly created by the collapse of a massive star, as is the case with stellar black holes with masses ten times the mass of the Sun, because no star is large enough to give birth to such a massive object.

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

Paper published in Astroparticle Physics Imre Bartos and Oscar Barrera of the Department of Physics at the University of Florida detail how some of the “daughter” black holes created by such a merger could carry information about the “parent” black holes that collided to create them.

“We discovered that black holes that were born from the collision of other black holes carry with them information about the properties of their progenitors, including the spin of the progenitors and their mass,” says Bartoš. “A key new focus of our research is the reconstruction of the spins of progenitor black holes, building on previous work that focused on progenitor masses.”

Black holes have very few properties that can be used to distinguish them, only variations in mass, angular momentum or “spin”, and electric charge. Theoretical physicist John Wheeler of Princeton University described this by saying that “black holes don’t have hair”. Bartoš adds that even in the face of these few characteristics and the “hairless theorem,” it is still possible to use the rotation of a black hole to reveal details about its origin.

“For example, black holes that feed on the surrounding gas or previous collisions of ‘parent’ black holes could result in high spin, while black holes often have low spin when born through the death and collapse of stars,” continues Bartoš.

To conduct their study, Bartos and Barrera used a mathematical technique called Bayesian inference, which took the measured properties of black holes and their prior expectations of them as inputs and produced inferred distributions of the properties of the original black holes. The research is current because physicists are using tiny ripples in spacetime called gravitational waves to learn more about the collisions and mergers of black holes.

“Recent observations of black hole mergers suggest the possibility that black hole assembly lines—places where several black holes join together to form heavier and heavier black holes—may be common in the universe.

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