Galaxy mergers not needed to fuel supermassive black holes

With the help of Hubble Space Telescope, astronomers at the University of California, Santa Cruz, have confirmed that galaxy mergers are not the main trigger for turning quiescent supermassive black holes into radiation-blasting active galactic nuclei.

They examined the morphology and structure of distant galaxies hosting active central black holes, and found that these galaxies were no more likely to be involved in an ongoing merger than non-active galaxies of similar mass.

“Theoretical models show that a merger is a great way to trigger an active galactic nucleus, because it funnels a lot of gas to the center of the galaxy. But we found that most of the host galaxies did not look disturbed. They look like disk galaxies, and a disk would be destroyed by a merger,” said first author and postdoctoral researcher Dale Kocevski.

A supermassive black hole (millions or even billions of times the mass of the sun) sits at the core of most, if not, all large galaxies. But only a fraction of these are the bright radiation sources known as active galactic nuclei, which light up when the central black hole is actively gobbling up nearby gas clouds.

The galactic centre glows brightly at all wavelengths because the gas heats up as it falls into the black hole and emits intense radiation.

“The problem has been how to get the gas into the centre of the galaxy, rather than orbiting around in the disk,” said co-author Sandra Faber, University Professor of astronomy and astrophysics at UC Santa Cruz.

Faber said new images from Hubble’s WFC3 infrared camera include galaxies at distances of nearly 11 billion light years.

Looking deep into space gives astronomers a window back in time, revealing earlier stages in the evolution of the universe.

This study covered a period when the brightest active galaxies, known as quasars, were at their peak, and theoretical models predicted that galaxy collisions would be an important trigger, Faber said.

“Theory says that collisions should be more important, but we did not find that,” she said.

These are not the first findings to suggest that violent galactic mergers are not needed to fuel active black holes.

But only Hubble’s WFC3 could provide images showing the morphologies of galaxies at that distance.

If galaxy collisions are not responsible for triggering active galactic nuclei, it suggests less violent processes within galaxies may play a greater role in feeding black holes, said Kocevski.

The findings will be published in the Astrophysical Journal.