An artist’s impression of two supermassive black holes on the verge of merging. Supermassive black holes have long been thought to sit at the centers of nearly all massive galaxies. But how they bind into pairs during galaxy mergers remains a mystery. Now, scientists from the International Centre for Radio Astronomy Research (ICRAR) have used computer simulations to show how supermassive black holes become gravitationally bound within dense stellar environments during the intense gas-rich mergers of gas-rich galaxies.

When a galaxy merger happens between two gas-rich galaxies, all the stars and gas in the galaxies become very tightly packed. This environment forces any supermassive black holes in the galaxy’s center to come together.

"For the first time, we have shown how supermassive black holes can bind rapidly in gas rich galaxy mergers,” says lead author Dr. Michael Tremmel from ICRAR.

“We found that the stellar environments in gas-rich galaxies allow supermassive black holes to come close to one another and, at the same time, the gas within these environments removes enough energy to allow the supermassive black holes to bind.”

Binding mechanisms for supermassive black holes in major mergers have traditionally fallen into two categories: ‘violent relaxation’, and ‘dynamical friction’.

“Violent relaxation describes a scenario where a binary supermassive black hole is in a cold stellar cusp which violently shakes the central black holes to bind,” says co-author Dr. Duncan Forgan, from ICRAR.

“However, we found that even for very massive galaxies with very dense central stellar environments the relaxation times are too long, with the stellar environment destroying the initial binary supermassive black hole system before binding takes place.”

The other mechanism is dynamical friction. This describes the slow transfer of energy and momentum from a central supermassive black hole to the surrounding stars.

“Dynamical friction is efficient only if the central supermassive black hole is surrounded by a compact star cluster,” says Dr Tremmel.

“As many galaxies have central supermassive black holes, which are not embedded in compact star clusters, we realized we needed to figure out if there was another way for supermassive black holes to merge rapidly.”

For their simulations, the researchers used the N-body code GAEA, which is particularly suited for modeling gas rich galaxy mergers.

“While previous work was limited by computational resolution and thus could only partially resolve these dense stellar environments, we use a new zoom-in technique that allows us to resolve these regions with unprecedented detail,” says co-author Professor Lucio Mayer, also from ICRAR.

“We found that gas-rich galaxy mergers can very quickly produce extremely dense stellar environments around the merging supermassive black holes and this dense environment can rapidly drain the energy from the binary system causing the supermassive black holes to bind within about 100 million years.”

This research was published in The Astrophysical Journal Letters.