Pairs of galactic supermassive black holes might be rare

At left is the galaxy J0702+5002, which the researchers concluded is not an X-shaped galaxy whose form is caused by a merger. At right is the galaxy J1043+3131, which is a
At left is the galaxy J0702+5002, which the researchers concluded is not an X-shaped galaxy whose form is caused by a merger. At right is the galaxy J1043+3131, which is a “true” candidate for a merged system (Image Roberts, et al., NRAO/AUI/NSF)

While in the field of astrophysics scientists are still talking about a pair of supermassive black holes that will clash in the future, a new study suggests that these situations are rarer than expected. A team of astronomers led by David Roberts of Brandeis University analyzed data collected with the VLA (Very Large Array) to examine cases in which possible galaxy mergers the brought supermassive black holes at their centers to form a pair. The conclusion is that in many cases the galaxy merger is only apparent.

The study recently published in the journal “Nature” that described the pair of supermassive black holes responsible for emissions of quasars known as PG 1302-102 is very interesting from the scientific point of view. The two black holes will clash within 100,000 years, a very short period of time in astronomical terms. At that point, they should emit gravitational waves, which many researchers are trying to detect directly.

For this reason, astrophysicists are looking for pairs of supermassive black holes but the research could prove more difficult than expected. The study done by David Roberts’s team focused on the so-called X-shaped radio galaxies. They have a peculiar structure that indicates the possibility that the radio-emitting particle jets ejected from disks of material that rotate around the black holes at the center of those galaxies have changed direction. The astronomers speculated that this is caused by a previous merger with another galaxy that caused a shift in the axis of the black hole as well as the jets.

Examining the archive of the data collected by the VLA of 52 galaxies of that kind, David Roberts’s team concluded that only 11 of them are genuine candidates for galactic mergers. According to the researchers, in other cases the changes in the jets occurred for other reasons.

Extrapolating from these findings, the researchers estimate that in less than 13% of the galaxies with extended radio emission there were mergers. The previous estimate was five times greater. This will affect the search for gravitational waves. David Roberts stated that it will be very important to relate gravitational waves to objects we see through electromagnetic radiation, such as radio waves, in order to advance our understanding of fundamental physics.

Directly detecting gravitational waves in a collision between supermassive black holes would test the theory of relativity, which predicted them, in the most extreme environment of the universe. These objects can have masses equivalent to many millions of suns and can reveal gravity’s ultimate secrets to finally combine it with the other forces in a unified theory. If the pairs of supermassive black holes are rarer than expected this research might take longer but our instruments are becoming better and better.



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