Stars regularly ripped apart by black holes in colliding galaxies

February 27 2017

UK astronomers have found that stars are ripped apart by supermassive black holes 100 times more often than was previously thought.

Up to now astronomers had thought that this stellar cannibalism – known as Tidal Disruption Events (TDEs) – was exceptionally rare, only occurring between every 10,000 to 100,000 years per galaxy. However new research by an astronomy team based at the University of Sheffield and funded by STFC, recorded a star being destroyed by a supermassive black hole in a survey of just 15 galaxies – an extremely small sample size by astronomy standards.

Depiction of the tidal disruption event in F01004-2237. The release of gravitational energy as the debris of the star is accreted by the black hole leads to a flare in the optical light of the galaxy.
(Credit: Mark Garlick)

“Each of these 15 galaxies is undergoing a ‘cosmic collision’ with a neighbouring galaxy,” said Dr James Mullaney, Lecturer in Astronomy at the University of Sheffield and co-author of the study.

“Our surprising findings show that 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.”

Supermassive black holes can be difficult to spot because they are so dark due to their gravity being so strong that nothing can escape, not even light. However, the release of energy when stars are ripped apart as they move close to the black holes leads to dramatic flares which shine very brightly. In this way, TDEs can be used to locate and study otherwise dim black holes.

The Sheffield team first studied these 15 galaxies in 2005, but when they looked at the sample again in 2015 they noticed that one galaxy looked completely different. Looking at data from 2010 it showed the brightness of this galaxy, which is 1.7 billion light-years away, flared dramatically in a manner which was characteristic of TDEs.

Clive Tadhunter, Professor of Astrophysics at Sheffield and leader of the study, said: “Based on our results, we expect that TDE events will become common in our own Milky Way galaxy when it eventually merges with the neighbouring Andromeda galaxy in about 5 billion years.”

The findings were based on observations made with the William Herschel Telescope, which is operated by STFC on the island of La Palma. The study also used data taken with NASA/ESA Hubble Space Telescope, and the Catalina Sky Survey.

END

Notes to editors

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Scientists at the University of Sheffield’s Department of Physics and Astronomy are exploring the fundamental laws of the universe and developing pioneering technologies. They look beyond our planet to tackle global challenges – from climate change to meeting energy demands.
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