Black hole Dance

This is interesting because it changes what astronomers think might happen when Andromeda collides with the Milky Way galaxy.

A paper recently published in Nature Astronomy describes finding dual supermassive black holes in the center of a spiral galaxy designated NGC 7674, which is 400 million light years away. These two black holes are orbiting one another with less than a light year separation. The mass of the two black holes combined is 40 million solar masses. They orbit each other with a period of 100,000 years. This is similar to what binary star systems experience. This would suggest that these two supermassive black holes were formed from the supernovae of two close stars in a binary star system.

Recently, gravitational waves were detected from the collision of two supermassive black holes. It turns out that a binary pair of supermassive black holes creates gravitational waves of much lower frequency, but by combining radio telescopes from around the Earth, astronomers were able to detect the waves. This was an amazing feat because NGC 7674 emits very strong radio signals.

This work suggests that when two galaxies collide, like what our galaxy and Andromeda are expected to do, the two supermassive black holes in the center of the two galaxies don't necessarily have to collide, but could form a dance partnership, orbiting one another. The possibility for this to happen is problematic, but if the collision is not dead on - a glancing blow - this could happen.

As you might imagine, the collision of two supermassive black holes is very violent, causing the release of devastating amounts of radiation. There are two opposing theories as to why two black holes resulting from supernovae eventually collide. The old theory depends on the idea that massive stars grow into giant red stars when they run out of hydrogen. If such a star has a smaller companion, which is the usual case, the smaller star sucks the gas from its larger buddy.  When the time comes for the stars to supernovae, they end up closer because of the fact that the smaller star is slowed down by the extra gas of its larger companion. The result is that they form into black holes closer than they were when stars.

The new theory depends on the idea that the two stars never interact and simply spin faster, making them hotter and able to fuse higher elements. This ends up causing both to collapse into massive black holes, which gradually attract one another because of gravity until they merge.

No matter what, there are two separate outcomes when two galaxies collide. One is that their supermassive black holes collide, and the other is that they dance together for a much longer time.

Thanks for reading.