When galaxies collide, black holes eat

Artist’s concept of a Tidal Disruption Event, in which a black hole eats a star, in the distant galaxy F01004-2237. As the black hole swallows the star, there’s a release of gravitational energy from the star’s debris. The result is a visible flare. Image via Mark Garlick.

What’ll our sky look like 5 billion years from now, when our Milky Way galaxy merges with the nearby Andromeda galaxy? If there are any people left to look then, they’ll be able to see flares about every 10 to 100 years, each time our Milky Way’s central supermassive black hole swallows a star. The flares will be visible to the unaided eye. They’ll appear much brighter than any other star or planet in the night sky. That’s according to astronomers at the University of Sheffield in England, who say that central, supermassive black holes in colliding galaxies swallow stars some 100 times more often than previously thought.

Their study was published March 1, 2017 in the peer-reviewed journal Nature Astronomy.

The study is based on a survey of just 15 galaxies, a very small sample size by astronomical standards. However, in that small sample, the astronomers were surprised to see a black hole swallow a star. Astronomers call this sort of event a tidal distruption event, or TDE. They’d been only been only seen before in surveys of many thousands of galaxies, leading astronomers to believe they were exceptionally rare: only one event every 10,000 to 100,000 years per galaxy.

Artist’s concept of Earth’s night sky in 3.75 billion years. The Andromeda galaxy (left) will fill our field of view then, astronomers say, as it heads toward a collision, or merger, with our Milky way galaxy. Image via NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger. Read more about the eventual merger of the Milky Way and Andromeda galaxies.

The 15 galaxies of the University of Sheffield study are doing something those other thousands of galaxies weren’t doing. They’re undergoing collisions with neighboring galaxies. Study co-author James Mullaney said in a statement:

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.

Another study co-author, Rob Spence, said:

Our team first observed the 15 colliding galaxies in the sample in 2005, during a previous project.

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.

Galaxy F01004-2237 – which is 1.7 billion light years from Earth – had flared in a way characteristic of TDEs. These events are known to cause flaring due to energy release, as a star edges toward a galaxy’s central, supermassive black hole.

Read more from the University of Sheffield

NGC 2207 and IC 2163 are two spiral galaxies in the process of merging, or colliding. If the new study from University of Sheffield is correct, there is a much greater chance for stars to be eaten in these galaxies by their central, supermassive black holes. Read more about this colliding galaxy pair from Chandra.

Bottom line: A study from the University of Sheffield shows that collisions – like that predicted for our Milky Way galaxy and neighboring Andromeda galaxy – cause black holes to eat stars some 100 times faster than previously thought.

March 12, 2017

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