They reveal the origin of the merger of black holes in galaxies like ours

June 29 () –

A team of scientists from the University of Geneva (UNIGE) in Switzerland, and Northwestern University and the University of Florida, both in the United States, have revealed through simulation tools the origin of the merger of black holes in galaxies such as the Milky Way and sheds light on its enigmatic nature, as published in the journal ‘Nature Astronomy’.

Black holes have an immense gravitational pull so strong that not even light can escape. The pioneering detection of gravitational waves in 2015, caused by the coalescence of two black holes, opened a new window on the universe. Since then, dozens of such observations have sparked a search among astrophysicists to understand their astrophysical origins.

Thanks to recent advances in the POSYDON code in simulating populations of binary stars, the team of scientists has predicted the existence of massive binary black holes of 30 solar masses in galaxies similar to the Milky Waythus challenging previous theories.

Stellar-mass black holes are celestial objects born from the collapse of stars with masses from a few to a few hundred times that of our Sun. Their gravitational field is so intense that neither matter nor radiation can evade them, making detection extremely difficult.

So when the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected the tiny ripples in spacetime produced by the merger of two black holes in 2015, it was considered a defining moment. According to astrophysicists, the two merging black holes that caused the signal were about 30 times the mass of the Sun and 1.5 billion light-years away.

The POSYDON collaboration has made significant progress in simulating populations of binary stars. This work is helping to give more precise answers and to reconcile theoretical predictions with observational data.

“As it is impossible to directly observe the formation of merging binary black holes, it is necessary to rely on simulations that reproduce their observational properties. To do this, we simulate binary star systems from their birth to the formation of binary black hole systems,” explains Simone Bavera, postdoctoral researcher at the Department of Astronomy of the UNIGE Faculty of Sciences and lead author of this study.

Interpreting the origins of merging binary black holes, such as those observed in 2015, requires comparing the predictions of theoretical models with actual observations,” he continued. The technique used to model these systems is known as “binary population synthesis”.

He adds that “this technique simulates the evolution of tens of millions of binary star systems in order to estimate the statistical properties of the resulting population of gravitational wave sources. However, to achieve this in a reasonable time frame, researchers have so far relied on models that use approximate methods to simulate the evolution of stars and their binary interactions.“.

“Hence the oversimplification of simple and binary stellar physics leads to less accurate predictions.“, explains Anastasios Fragkos, assistant professor of the Department of Astronomy of the Faculty of Sciences of UNIGE.

POSYDON has overcome these limitations. Designed as open source software, it takes advantage of a large precomputed library of detailed simulations of single and binary stars to predict the evolution of isolated binary systems.

Each of these detailed simulations can take up to 100 CPU hours to run on a supercomputer, so this simulation technique is not directly applicable to the synthesis of binary populations.

“However, by precomputing a library of simulations covering the entire parameter space of initial conditions, POSYDON can use this vast data set in conjunction with machine learning methods to predict the complete evolution of binary systems in less than a second.. This speed is comparable to previous generation rapid population synthesis codes, but with higher precision,” explains Jeffrey Andrews, an adjunct professor in the UF Department of Physics.

“Models prior to POSYDON predicted a negligible rate of formation of merging binary black holes in galaxies similar to the Milky Way, and in particular did not predict the existence of merging black holes as massive as 30 times the mass of our sun. POSYDON has shown that such massive black holes could exist in galaxies similar to the Milky Way,” explains Vicky Kalogera, Daniel I. Linzer Distinguished Professor of Physics and Astronomy in Northwestern’s Department of Physics and Astronomy, director of the Center for Interdisciplinary Research and Exploration. in Astrophysics (CIERA) and co-author of this study.

Previous models overestimated certain aspects, such as the expansion of massive stars, which influences their mass loss and binary interactions. These elements are key ingredients that determine the properties of merging black holes.

Thanks to detailed and fully self-consistent simulations of stellar structure and binary interactions, POSYDON achieves more accurate predictions of properties of merging binary black holes, such as their masses and spins.

This study is the first to use the open source POSYDON software to investigate binary black hole merging. It provides new insights into the formation mechanisms of merged black holes in galaxies like ours.

The research team is currently developing a new version of POSYDON, which will include a broader library of detailed binary and stellar simulations, capable of simulating binaries in a broader range of galaxy types.