Black holes are getting caught in “traffic jams”

A new study reveals the complex dynamics of black holes in the centres of galaxies, how they slow down and interact with each other.

This could help explain when and where black hole mergers occur.

Led by researchers at Melbourne’s Monash University, the study is published in the Monthly Notices of the Royal Astronomical Society.

Unlike conventional astronomy, which is based on the emission of different wavelengths of light, the research used data from gravitational wave emissions resulting from black hole mergers.

Gravitational waves, ripples in the fabric of space time caused by the most energetic events in the universe, were first observed in 2015 at the Laser Interferometer Gravitational-wave Observatory, or LIGO, in the US.

When black holes get too close to each other, they disturb space-time, sending gravitational waves out through the universe before merging.

The researchers say it’s like a black hole “traffic jam.”

“We looked at how many and where we’d have these busy intersections,” says lead author Dr Evgeni Grishin, an astrophysicist from Monash.

The team focused their attention on the massive discs at the centres of galaxies where black holes tend to congregate due to the gravitational pull of a central supermassive black hole. As the black holes suck in spinning gas and dust, it leads to bright accretion discs. These regions are called Active Galactic Nuclei (AGN).

As the smaller black holes move through the accretion disc, they clump together in “migration traps.” When black holes enter these busy intersections, it increases the likelihood of close encounters and mergers.

“We don’t see migration traps occurring in active galaxies with large luminosity,” Grishin notes.

“Despite these significant findings, much about the physics of black holes and their surrounding environments remains unknown,” Grishin adds. “We’re thrilled with the results, and we now are one step closer to discovering where and how black holes merge in galactic nuclei.

“The future of gravitational wave astronomy and active galactic nuclei research is exceptionally promising.”

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