Recent studies show that wandering, nomadic black holes smaller than 10 billion solar masses greatly outnumber the central supermassive black holes in the universe at large, which would make for interesting, if not danger-fraught, future starship explorations of our Milky Way!
Typically, a supermassive black hole (SMBH) exists at the core of a massive galaxy. But sometimes, according to ROMULUS simulations by Harvard’s Angelo Ricarte and colleagues, supermassive black holes may “wander” throughout their host galaxy, remaining far from the center in regions such as the stellar halo, a nearly spherical area of stars and gas that surrounds the main section of the galaxy.
Creatures of Galactic Mergers
Astronomers theorize that this phenomenon often occurs as a result of mergers between galaxies. A smaller galaxy will join with a larger, main galaxy, depositing its own, central SMBH onto a wide orbit within the new host. If the trajectory is right, the smaller SMBH will leave a high-density wake of stars and gas behind it as it orbits, yielding a net torque that causes the smaller SMBH to quickly inspiral inward and merge with the more massive SMBH. But in some cases, the smaller SMBH can orbit indefinitely without merging.
Wanderers Able to Affect our Solar System Should Occur Every 100 Billion Years
In a 2018 study published in the Astrophysical Journal Letters, researchers from Yale, the University of Washington, Institut d’Astrophysique de Paris, and University College London predicted that galaxies with a mass similar to the Milky Way should host several supermassive black holes. They used a new, state-of-the-art cosmological simulation, Romulus, to predict the dynamics of SMBHs within galaxies with better accuracy than previous simulation programs.
“It is extremely unlikely that any wandering supermassive black hole will come close enough to our Sun to have any impact on our solar system,” said lead author Michael Tremmel, a postdoctoral fellow at the Yale Center for Astronomy and Astrophysics. “We estimate that a close approach of one of these wanderers that is able to affect our solar system should occur every 100 billion years or so, or nearly 10 times the age of the universe.”
Could be Detectable
Tremmel said that since wandering SMBHs are predicted to exist far from the centers of galaxies and outside of galactic disks, they are unlikely to accrete more gas — making them effectively invisible. “We are currently working to better quantify how we might be able to infer their presence indirectly,” Tremmel said.
These wanderers may betray their locations, suggests American Astronomical Society NOVA The Hunt for Wandering Black Holes as:
Hyper-luminous X-ray sources: Some nearby, accreting, wandering black holes should be detectable as exceedingly bright X-ray sources.
Dual active galactic nuclei: The simulations predict that galaxies will often host more than one dramatically accreting supermassive black hole — particularly at higher redshifts.
X-ray halo: If the black holes are too distant or dim to resolve individually, we can identify wanderers by stacking images of galaxies of similar mass. The “halo” of excess X-ray radiation can then be used to describe the wandering black hole population.
Tidal disruption events: wandering supermassive black holes can tear apart stars that come too close! These disruptions should produce transient signals offset from galactic centers.
The Last Word –A Primordial Object in Our Solar System?
We might have a black hole in our galactic neighborhood, suggested Dennis Overbye at the New York Times:
“You don’t need a star to die to make a black hole”, Overby wrote. “In 1971, Stephen Hawking, drawing on an idea earlier suggested in 1966 by the Russian physicists Yakov Borisovich Zel’dovich and Igor Dmitriyevich Novikov,” explains Overbye, “theorized that intense pressures during the Big Bang could have collapsed matter directly into black holes. Those primordial black holes could be of any size and could be anywhere. A black hole as massive as Earth would be about the size of a Ping-Pong ball and would be exceptionally hard to see.
“No such primordial black holes have been detected yet,” noted Overbye. “But neither has their existence been ruled out.” An experiment called OGLE, for Optical Gravitational Lensing Experiment, based at the University of Warsaw in Poland, Overbye says” had detected the presence of a half-dozen dark objects in the direction of the center of our Milky Way galaxy.”
Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via New York Times Science and Yale University and Population of Wandering Black Holes via Electromagnetic Signatures
Image credit: Shutterstock License
Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona. Max can be found two nights a week probing the mysteries of the Universe at the Kitt Peak National Observatory. Max received his Ph.D in astronomy from Harvard University in 2015.