Astronomers in the UK have found new evidence that black holes tear stars apart 100 times more often than previously thought. These stellar-swallowing smorgasbords, scientifically known as tidal disruption events (TDEs), were thought to happen only every 10,000 to 100,000 years, but a recent discovery has researchers from the University of Sheffield rethinking that rate of occurrence.
The researchers found a star being destroyed by a supermassive black hole in a survey of just 15 so-called "starburst" galaxies, considered an extremely small sample size by astronomy standards. Starburst galaxies have rates of star formation that are far higher than most other galaxies.
"Each of these 15 galaxies is undergoing a 'cosmic collision' with a neighboring galaxy," said Dr. James Mullaney, Lecturer in Astronomy at the University of Sheffield and co-author of a study reporting the findings. "The rate of TDEs dramatically increases when galaxies collide. This is likely due to the fact that the collisions lead to large numbers of stars being formed close to the central supermassive black holes in the two galaxies as they merge together."
Of course, spotting one instance of anything, especially from such a small sample size, can always be chalked up to dumb luck. New Atlas asked astrophysics professor Clive Tadhunter, who led the study, what made his team so sure they didn't just get lucky in picking the right sample of fifteen galaxies.
"If the rate were truly one event every 10,000 years per galaxy, in a 10 year period we'd stand a ~1.5% chance of observing a TDE in a sample of 15 galaxies and a ~0.1% chance in a sample of 1 galaxy," Tadhunter responded. "Therefore, although we can't absolutely rule out the idea that we 'got lucky,' this is unlikely."
Tadhunter says he believes that what he refers to as an "enhancement" in the occurrence of TDEs "only occurs in systems that are undergoing, or have recently undergone, galaxy-galaxy mergers that lead to major nuclear starbursts."
The study involved observations and data from the William Herschel Telescope in the Canary Islands, the Hubble Space Telescope and the Catalina Sky Survey. The team first observed the small sample of galaxies in 2005, and then when they took another look in 2015, something was different.
"We noticed that one galaxy - F01004-2237 - appeared strikingly different," said Rob Spence, University of Sheffield Ph.D. student and study co-author, in a statement. "This led us to look at data from the Catalina Sky Survey, which monitors the brightness of objects in the sky over time. We found that in 2010, the brightness of F01004-2237 flared dramatically."
After analyzing the flare and changes in the galaxy over time, the team determined it was not likely to be explained by a supernova or active galactic nucleus, but did have the tell-tale signs of a TDE.
Spotting TDEs can be helpful to astronomers because they basically act like a huge arrow pointing at the presence of a supermassive black hole that would otherwise be hard to spot since they don't "shine" in the conventional sense, holding even light itself in their grasp.
Tadhunter says the notion of more common TDEs could be bad news for our own galaxy, although not for quite a while.
"We expect that TDE events will become common in our own Milky Way galaxy when it eventually merges with the neighboring Andromeda galaxy in about 5 billion years," he explained in a statement.
That should be right around the time our sun is swelling up into a red supergiant and swallowing our planet. But if we manage to relocate to another, safer part of the Milky Way before then we might be in for quite a show, according to Tadhunter.
"Looking towards the center of the Milky Way at the time of the merger we'd see a flare approximately every 10 to 100 years. The flares would be visible to the naked eye and appear much brighter than any other star or planet in the night sky."
Source: University of Sheffield