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February 28, 2017

Black holes eat stars more often than we thought

The supermassive black holes that lie in the center of most galaxies may have far more voracious appetites than experts previously believed, according to a new study that has uncovered evidence that these behemoths shred stars 100 times more often than earlier research had suggested.

Such incidents of stellar cannibalism, also known as tidal disruption events or TDEs, previously had been found only in surveys that observed thousands of galaxies at once. However, new work by astronomers at the University of Sheffield has discovered an incident of a star being destroyed by a supermassive black hole in a much smaller sample size – a group of just 15 galaxies.

According to Popular Science, Dr. James Mullaney from the University’s Department of Physics and Astronomy and his colleagues were observing the 15 galaxies in 2015 when they noticed one that had changed since it was last studied 10 years beforehand. They detected a flash of light that is a sign that a TDE had occurred, and while such a discovery by itself is nothing to unusual, that they found it in such a small group of galaxies was unexpected.

“The results came out purely by chance really, quite like a lot of scientific discoveries. We were initially looking to find out what happens when galaxies collide,” Dr. Mullaney told the website. “Until now, such events have only been detected by surveying thousands or tens of thousands of galaxies. And even then, you might not always find one.”

Findings could help explain why supermassive black holes grow

While it is possible that his team was simply extremely lucky, he said that his group calculated that there was just a one percent chance of this being nothing more than a happy accident. The more likely explanation, he said, is that they’ve found a series of conditions – that is, a collision of galaxies – that make stellar cannibalism events occur more frequently.

“It’s a little bit like if you surveyed a population of non-smokers – you might only identify that one had lung disease in a pretty large group. Whereas if you sampled a population of smokers, you’d find one in fifteen had lung disease, because smoking causes lung disease,” Dr. Mullaney told Popular Science. “We found one of these events taking place in a much smaller group than we'd expect,” making it likely that colliding galaxies are more at risk for TDEs.

Each of the 15 galaxies the Sheffield team studied was in the process of colliding with another, nearby galaxy, they explained in a statement. Based on their observations, they have concluded that the rate of a TDE occurring increases “dramatically” when two galaxies are colliding, most likely due to the fact that such events cause a large number of stars to be formed near the central supermassive black holes of the merging systems.

“Our team first observed the 15 colliding galaxies in the sample in 2005, during a previous project,” Rob Spence, University of Sheffield PhD student and co-author of the study, said in a statement. “However, when we observed the sample again in 2015, we noticed that one galaxy – F01004-2237 – appeared strikingly different. 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.”

This galaxy, which is located 1.7 billion years from Earth, contained a combination of variability and post-flare spectrum that was characteristic of a TDE, the researchers said. Additional studies are needed to verify their findings, but if the results hold true, Dr. Mullaney believes that it could help researchers better understand how supermassive black holes continue to grow.

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Image credit: NASA