Two new papers have provided some exciting new insights into a rare phenomenon in the universe: The mergers of three galaxies, and the interactions that are happening between the supermassive black holes at the center of these bodies.
When galaxies merge, there are two main types of increased activity. The first is new star formation, as clouds of gas are pushed together by the strong tidal forces between two galaxies. The second one is the emergence of an “Active Galactic Nucleus”, or AGN. Material thrown towards the core of the galaxy awakens the supermassive black hole. Most of this material is gobbled up, and the rest is thrown out in an energetic frenzy.
The work showed that there are differences between the level of activity in these seven galaxy triplets. One doesn’t seem to have any AGN. In one, they found only one AGN. So only one of the three supermassive black holes is “switched on”. Four of them are dual AGN, with two black holes feeding. The last one is an incredible triple-AGN, which was already known to astronomers.
The team found that the supermassive black holes in question were located between 10,000 and 30000 light-years apart. They also found that the galaxies with growing supermassive black holes were the ones richer in dust and gas, matching the theoretical prediction very well.
"There have been many studies of what happens to supermassive black holes when two galaxies merge," lead author Adi Foord of Stanford University, said in a statement. "Ours is one of the first to systematically look at what happens to black holes when three galaxies come together."
The insights from the study are important to better understand these complex objects. Interactions and the possible merger between two supermassive black holes is a complex and hotly debated field in astronomy – mostly because we don’t know how it happens.
Unlike stellar-sized black hole mergers that have been detected with gravitational wave observatories, supermassive black holes don’t just come together and collide easily in calculations. Theory suggests that two supermassive black holes coming together on the right trajectory will begin to orbit one another, and getting closer, eventually find a nice little stable orbit in which to settle.
The only way to make them collide from there is if they are losing orbital energy. Few mechanisms are suggested to make it so, but there are no certainties. A third supermassive black hole would definitely work, as the gravitational interactions will favor collisions.
The work was possible thanks to the data from the Sloan Digital Sky Survey, NASA’s Chandra X-ray detections of the AGN, and archival data from from the WISE mission and the Two Micron All Sky Telescope providing information about the gas and dust in these galaxies, all of which are less than one billion light-years from Earth.
The findings were presented at the 237th American Astronomical Society Meeting, and they are accepted for publication in The Astrophysical Journal (the preprints are available here and here).