Like a giant game of billiards, black holes in galactic centers attract and collide with each other, forming spectacular collisions that astronomers on Earth can now study by analyzing the gravitational waves they cause. Until now it was thought that they mostly merged in circular orbits, but in 2019 when the LIGO and Virgo observatories were studying one of these colossal mergers, they made an unexpected discovery that quickly became a mystery for astronomy.

The event was called GW190521, the merger of two black holes that were not only heavier than has typically been detected before by aLiGO, but also did not orbit each other along a circle orbit in the moments leading up to the merger. "This is the most surprising discovery to date. The masses and spins of the black holes were already surprising, but even more surprising was that they appeared not to have a circular orbit before the merger," says the research published in the journal Nature, which involved an international team of astronomers, including the Center for Astrophysics and Related Technologies (CATA) researcher, Nathan Leigh.

The paper was then oriented to discover how these kinds of mergers, known as “eccentric", can occur, which led scientist to explore the harsh environment of galactic centers that host a supermassive black hole -of millions of times the mass of the Sun-, surrounded by a flat and rotating gas disk. It is in this interaction that the key to the mystery is hidden.

Black hole billiards

According to Nathan Leigh, who is also astronomer of the Universidad de Concepción and also a researcher at the "Titans Millennium Nucleus" , the typical velocity and density of black holes are so high that the smallest black holes bounce around, as in a giant game of pool, while binary holes with wide circular orbits are rare. The research reveals that the gas disk plays an important role in capturing smaller black holes, which eventually move closer to the center and also to each other.

This not only implies that they meet and form pairs, but also that that pair could interact with another third black hole, often leading to a "chaotic tango" with three black holes flying around, Leigh explains. "There can also be hierarchies, for example two stars can be bound together in a binary and a third star interacts by undertaking a long journey, almost escaping. But it doesn‘t have quite enough energy to escape, so it eventually returns to interact with the binary again," concludes the PhD in Astrophysics at McMmaster University, Canada.

Black holes are among the most fascinating objects in the Universe, but our knowledge of them is still limited, especially because they do not emit light. In 2015, however, the Laser Interferometer Gravitational-Wave Observatory (aLIGO) made its groundbreaking observation of gravitational waves from the merger of two black holes, opening a new era of findings thanks to gravitational-wave astrophysics.

In addition to understanding how black holes form and merge in our universe - they are not just nearby stars that collapse and merge on circular orbits - the research provides new information for understanding flat gas disks and how black holes move inside them.

To access the original Nature paper, click here

Image Credit: J. Samsing/Niels Bohr Institute