There are black holes that never move, as well as ones that crawl across galaxies invisibly. Every active galactic center is thought to contain a supermassive black hole that feeds on an apparently limitless supply of gas and dust (AGN). What isn't immediately apparent is how many black holes were required for the beast to grow to that size. Galaxy mergers crushed black holes together in the early cosmos, but not all of them survived intact.

The study's lead author, Harvard & Smithsonian Center for Astrophysics astronomer Angelo Ricarte, utilized simulations to discover that some were too obstinate to merge with their AGN's supermassive black hole. Hence, they went rogue.

Researchers published their study, titled "Origins and Demographics of Wandering Black Holes," in the Monthly Notices of the Royal Astronomical Society.

ROMULUS Investigates Black Hole Behaviors said Ricarte and his colleagues used the ROMULUS algorithm to investigate how black holes might merge and interact with other things, including other black holes, as the cosmos grew after the Big Bang.

Black hole collision and merger releasing gravitational waves
(Photo: Simulating eXtreme Spacetimes Lensing (SXS) via Wikimedia Commons)
Collision and merger of two black holes, resulting in the first detection of gravitational waves, GW150914, by LIGO.

Another study titled "A Smooth Particle Hydrodynamics Code to Model Collisions Between Solid, Self-Gravitating Objects" pointed out that ROMULUS applies a smooth particle hydrodynamics algorithm. The said system treats the universe's objects as particles.

Everything is a distinct particle, from the first gas cloud through stars, planets, black holes, and even dark matter. Hence, researchers then sent astrophysical data into the computer, which subsequently evolved this digital cosmos.

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Black holes aren't generally given much thought by other programs, especially when they're fast-forwarding through 14-billion-year simulations of the whole universe. But ROMULUS paid special attention to them. Because the early cosmos was essentially one gigantic cosmic pinball machine, more of these rogues appeared.

Things zipped, soared, and collided with one another all the time in what was once a turbulent environment. So it's no wonder that newborn galaxies collided and merged far more than they do now.

Gravity's Role in Rouging Stars Merging With AGNs

Many black hole masses in the early cosmos have come from black holes that failed to combine and went out independently. SciNews said that even now, at least according to ROMULUS, wandering black holes account for roughly 10 percent of all black hole mass. But they are not dead. Some rogues roam on the fringe of their galaxy, snatching up unlucky stars in their path. But other more massive rogues may eventually fuse with the AGN's enormous black hole due to gravity.

Black holes with higher mass will face more drag as they travel through space with all their objects and dark matter. That is especially if they are still in the galactic center, which may become highly dense. Dynamical friction is the term for this type of drag. The black hole also has greater gravity, so it will continue to take up stars and dark matter as it travels, boosting its energy and momentum while slowing down. They eventually draw it backward, sometimes all the way back to the AGN, where it may combine with an existing supermassive black hole.

Researchers are looking forward to seeing what else the roaming black holes may disclose. They might provide more information about gravitational waves (produced by mergers) than a moon-based observatory could, as well as teach us more about themselves.

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