A Colossal Number of Rogue Supermassive Black Holes Are Eating Through the Universe

Once you go rogue, you can't go back.
Derya Ozdemir
An illustration of two colliding quasars. draco-zlat/iStock

Delightfully dark and perpetually hungry, supermassive black holes are the biggest black holes in the known cosmos, billions of times more massive than our Sun.

Scientists believe that at the heart of almost every large galaxy lies a supermassive black hole. No one really knows how they got there in the first place, and it appears that a "substantial" number of those don't like to remain stationary at the center.

These wandering supermassive black holes are probably floating through galaxies, eating and warping their way out, and a team of researchers led by Angelo Ricarte at the Center for Astrophysics at Harvard University has found that around 10 percent of the whole mass budget of black holes in the visible universe might actually be contained in these wandering individuals, as reported in the Monthly Notices of the Royal Astronomical Society.

And it looks that the figure was significantly greater in the past.

Supermassive black holes far from home

Supermassive black holes' gigantic size keeps them at the center of their host galaxy, which means that if you wanted one to wander away, you'd need something very forceful to compel them to flee. In some cases, a collision with another galaxy can do the trick.

When two galaxies collide, the black holes in each can either merge into a single one, or be pushed around to travel across their respective galaxies. The researchers utilized the ROMULUS simulation, which involves large-scale cosmological simulations with resolution on par with the best resolution simulations of this sort conducted so far, to determine what would happen to supermassive black holes when they are kicked around.

In addition to the 10 percent figure, researchers also discovered in the simulations that galaxies with masses equivalent to our Milky Way galaxy had an average of 12 rogue supermassive black holes distant from the galactic center.

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"Wandering supermassive black holes in the ROMULUS simulations originate from the centers of destroyed infalling satellite galaxies," according to the research, which also predicts "a substantial wandering supermassive black hole population".

A Colossal Number of Rogue Supermassive Black Holes Are Eating Through the Universe
An image from the ROMULUS computer simulation showing the locations (and velocities) of "wandering" supermassive black holes. Source: Ricarte et al, 2021

Moreover, the team was able to find numerous methods for spotting these wandering black holes. Since no light escapes black holes, we can only view them indirectly through light and directly through gravitational waves. Wandering black holes radiate light if they are actively feeding on interstellar gas or tearing a star apart, which means that, when scientists detect a bright signal that isn't originating from a galaxy's core, it might be the indicator of a wandering black hole.

The team's next step is to dig further into the observational fingerprints of these wandering black holes.

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