A research team has proposed a new method to reconstruct the “family tree” of black holes. Published in The Astrophysical Journal, this research offers a way to infer the properties of the black hole progenitors of these mergers, one of the most brutal events that can be observed in the universe.
As a result of these mergers, gravitational waves are generated, a kind of “wrinkle” in spacetime that travels at the speed of light, and that can currently be detected through the detectors developed by international collaborations such as Virgo, Kagra or LIGO.
By analyzing gravitational waves, it is possible to obtain information about merging black holes, such as their masses, the direction of their spin, and other clues about their origins. In most cases, black holes are formed from the remnants of massive stars that have collapsed under their own gravity after exhausting their nuclear fuel.
However, according to astrophysical theories, there is a kind of vacuum in which black holes cannot form directly from stellar collapse, and which is known as the pair instability mass interval. Black holes within this interval are believed to originate from hierarchical mergers, i.e., successive mergers of smaller “ancestral” black holes, each of which forms a progressively more massive black hole. They thus form a sort of family tree in which this research intends to delve.
Although this explanation seems straightforward, the process is not trivial. For a black hole to participate in successive mergers, it must remain bound to its host environment, such as a galaxy or a dense star cluster. However, black holes produced in mergers acquire a recoil velocity, or kick, that can reach thousands of kilometers per second, often enough to eject them from most host environments.
For example, in globular clusters, which are considered key hosts for black hole mergers, the escape velocity is only about 50 km/s. Although the spin and mass of black holes can be measured directly from gravitational wave signals, the recoil velocity depends on the properties of the “ancestors” of merging black holes, which cannot be observed directly.
“With this type of study, we can not only guess the ancestors of the black holes we observe. We can also guess what kind of environment (if any) this process could have taken place in. If no environment is viable and these black holes cannot be the result of previous mergers, we may have to rethink stellar evolution or consider that we may not be observing black holes at all,” says Prof. Juan Calderón Bustillo, Ramón y Cajal fellow at IGFAE, joint center of the University of Santiago de Compostela and Xunta de Galicia (Spain), and co-author of the study.
Analysis of the mysterious GW190521 signal
The team applied this technique to the mysterious gravitational-wave signal GW190521, which involves a black hole that falls in the forbidden mass gap.
“We have found that, according to the properties certain groups have found for this black hole, it is unlikely it formed in a Globular Cluster due to the large kicks that this black hole may have inherited,” says Carlos Araujo, Masters Student at the Instituto de Astrofisica de Canarias and former undergrad student at University of Santiago de Compostela.
“Indeed, environments with larger escape velocities, like Active Galactic Nuclei or Nuclear Star Clusters seem more plausible, due to their ability to retain black-holes with large kicks. This aligns with existing studies suggesting that GW190521 happened in an Active Galactic Nucleus,” says Henry Wong, former undergrad at CUHK and now a data scientist in the private sector.
“We found that we can access the birth kick of the black hole because it is closely tied to its spin. Unfortunately, we cannot nowadays measure spins with much precision, which is one of the limiting factors of our study. As LIGO and Virgo keep increasing their sensitivity and new third generation detectors come online, our method will provide more detailed insights into the genealogy of the black holes we observe,” says Ania Liu, co-author of the study and Ph.D student at CUHK.
More information:
Carlos Araújo-Álvarez et al, Kicking Time Back in Black Hole Mergers: Ancestral Masses, Spins, Birth Recoils, and Hierarchical-formation Viability of GW190521, The Astrophysical Journal (2024). DOI: 10.3847/1538-4357/ad90a9
Provided by
Galician Institute of High Energy Physics
Citation:
Black hole genealogy: A new way to discover ‘ancestors’ of cosmic phenomena (2024, December 17)
