When It Comes To Kickstarting A Supermassive Black Hole, Violent Galaxy Mergers Are Better

Merging Galaxy NGC 262.  Hubble Legacy Archive, ESA, NASA; Processing - Martin Pugh

Almost every galaxy has a supermassive black hole at its center and most of these behemoths are quiet and calm. The rest are actively feeding through a process that ends up throwing a lot of energy and material into space. This feeding frenzy can happen for several reasons and a new study suggests that violent collisions between galaxies are more effective at triggering it than less violent ones.

When supermassive black holes turn on they are known as Active Galactic Nuclei (AGNs) and they play an important role in the evolution of their host galaxies. The new research was presented at the 232nd meeting of the American Astronomical Society.

“A big question in modern astronomy is what are the primary routes by which these black holes grow,” lead researcher Scott Barrows, from the University of Colorado Boulder, said in a statement. “One idea is that galactic mergers play a major role.”

But not all mergers are the same. Astronomers divide them into two broad categories. Major mergers occur if the galaxies involved have roughly the same mass while minor mergers happen if the masses are mismatched. The team studied black hole activation rates in both major and minor mergers.

The mergers were picked before full coalescence, allowing the researchers to potentially see two supermassive black holes. Their analysis showed that two-thirds of major mergers led to the activation of both supermassive black holes, but only one in nine minor mergers did.

A minor “merger is less violent, and that leads to less gas and dust falling onto the black holes,” explained Barrows. “And the less material you have falling onto the black holes, the less likely you are to have two of them become AGNs.”

The situation is different for major mergers. These collisions are more violent, and as the two cores move towards each other, the gravitational pull of the black hole sends gas and dust spiraling down. The material rains on the black holes and due to the huge gravitational pull, is heated up to millions of degrees.

The research also uncovered something peculiar. In minor mergers, there wasn’t a clear rationale for the activation of a single black hole. Sometimes it was the one in the smaller galaxy, and sometimes it was the one in the bigger one.

The team will continue to work on mergers and their follow-up work will look into the effect of a supermassive black hole collision on the host galaxy, focusing on the potential for them to destroy or form stars.

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