Posted on Apr 6, 2019 in Science

Astronomers are beginning to understand what happens when black holes get the urge to roam through the Milky Way. Typically, a supermassive black hole (SMBH) exists at the core of a massive galaxy. But sometimes SMBHs 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.

Astronomers theorize that this phenomenon often occurs as a result of mergers between galaxies in an expanding universe. A smaller galaxy will join with a larger, main galaxy, depositing its own, central SMBH onto a wide orbit within the new host.

“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.”

In the 2018 study published in the Astrophysical Journal Letters, researchers from Yale, the University of Washington, Institut d’Astrophysique de Paris, and University College London predict that galaxies with a mass similar to the Milky Way should host several supermassive black holes. The team 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.

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.

On March 9th, we reported that after conducting a cosmic inventory to calculate and categorize stellar-remnant black holes, astronomers from the University of California concluded that there are probably tens of millions of the enigmatic, dark objects in the Milky Way – far more than expected.

In stark contrast to the predictions of the Yale team, is physicist George Chapline’s conjecture that there has never been direct evidence of a central black hole,” while acknowledging there are objects that general relativity would predict are black holes at the centers of galaxies. “Ironically, Einstein also didn’t believe in black holes even though he created general relativity.”

Compared to the supermassive black holes in the centers of other galaxies, our black hole, Sagittarius A*, is strangely quiet. But Chapline thinks it’s more than quiet: he predicts that we’ll soon find that it does not exist.

Chapline, at the Lawrence Berkeley National Laboratory, expects to have his prediction that black holes don’t exist confirmed with the release of findings by the Event Horizon Telescope—really a virtual telescope with an effective diameter of the Earth—that has been pointing at the Milky Way’s central supermassive black hole for the last several years.

The Daily Galaxy, Max Goldberg, via Yale University