At the center of almost every galaxy, there is a supermassive black hole. But it seems that a large fraction of these black holes do not like to stay still at the center. New work looks at these wandering black holes, the causes behind them, and how such a population changed throughout the history of the cosmos.
The team defines wandering black holes as those that are not at the center of their galaxies. As reported in the Monthly Notices of the Royal Astronomical Society, if you consider the whole mass budget of black holes in the visible universe, about 10 percent will be in wandering black holes. And the number appears to have been much higher in the past.
The supermassive black holes at the center of galaxies weigh millions to billions of times the mass of our Sun. Their gargantuan bulk keeps them pretty steadily at the center of their host galaxy. You've got to have something pretty forceful to cause them to shift away. And that is usually a collision with another galaxy.
When such a collision happens, the black holes in each galaxy can merge into a single one, or one can be pushed around to move through its galaxy. There are many unknowns on this process and the team used the ROMULUS simulation, to work out what would happen to supermassive black holes when they are kicked about.
The computer model suggests that for some black holes, wandering around the galaxy becomes their way of life and even after one billion years they remain separated from the core. It also shows that the earlier epochs of the universe had a lot more wandering black holes.
When the universe was younger, there were a lot more galaxy collisions taking place. Our own Milky Way has been known for snacking on some of its smaller neighbors. Other galaxies have had much more dramatic collisions and the team estimates that two billion years after the Big Bang, the wanderers were a larger population. They contained most of the black hole mass in the universe.
ROMULUS didn’t just give an estimate about the size of this type of black hole, the team was also able to get several ways to actually spot these black holes. Black holes are by their very nature “black” as no lights escape them, so we only see them indirectly with light and directly with gravitational waves.
If these black holes are actively feeding on interstellar gas or ripping a star apart, they would emit light. Such a luminous signature, coming not from the center of a galaxy, would make them stand out as a wandering black hole.