Galaxy found with multiple black holes

A runaway black hole

A distant dwarf galaxy, located some 7.5 billion light years away (which means it is so far away its light is taking that long to get here), has ejected a supermassive black hole, with a mass 20 million times that of the Sun, at almost six million kilometres an hour.

As it speeds away from the galaxy it is passing through clouds of gas and dust, stirring them up so they are collapsing to form new stars. It is intriguing that a black hole, the most effective destruction machine in the universe, is triggering the birth of many new stars.

Most galaxies, including ours, have supermassive black holes (SMBH) in their centres, which have millions of times the mass of the Sun. These spend their time snacking on any star, gas cloud or other object that comes too close.

What gets pulled in gets broken down to its component atoms, then the atoms are torn apart. The debris gets sucked in for a one-way trip through the event horizon of the black hole, and then it is gone.

These black holes probably formed at the same time as their host galaxies. It is hard to conceive of any other concentration of mass in a galaxy substantial enough to move an SMBH. It would be like trying to shift a bowling ball by throwing feathers at it.

The only thing we know of that can shift a supermassive black hole is a close interaction with another supermassive black hole. However, this would require a runaway black hole of similar mass and moving around twice as fast to pass close by.

Where would this one come from? In addition, we would now see two black holes moving out from that galaxy in opposite directions. A possible explanation comes from a rather strange thing called the Three-Body Problem.

Newton's concept of gravity proved so useful in explaining what we see going on in the Solar System that many scientists got into conceiving various combinations of bodies orbiting each other to see what happened.

Everything worked smoothly for a big star with lots of small planets orbiting around it, such as our Solar System. It also worked really well in understanding two bodies orbiting around each other. However, when three or more similar bodies were orbiting each other, calculating how the bodies would orbit around each other proved impossible.

Computer simulations show chaotic orbits where eventually one of the bodies gets thrown out. Changing the situation very slightly leads to completely different chaotic orbits, which however still ends up with one of the bodies being ejected at high speed.

So the most likely scenario based upon what we know at the moment is that the host galaxy hosted three black holes. Maybe this was a rare situation where a galaxy was born with more than one central black hole. However, we know that galaxies grow by absorbing other galaxies.

Maybe two galaxies merged, leading to a new, larger galaxy with two black holes in its core, orbiting around one another. Given time, the two would lose orbital energy by radiating gravitational waves and slowly spiral together, combining to make one, larger black hole.

However, before this happened, another galaxy merged, leading to a larger galaxy ruled by a trio of black holes. These got involved in a chaotic orbital dance leading to one of them being ejected. Over time, the two remaining black holes will move closer and merge.

Providing galaxies merge one pair at a time, with enough time before the next merger for the two black holes to combine, growth should go smoothly.

Having possibly seen one case where mergers occurred so frequently that the combined galaxy had three black holes, with the loser being booted out, it will be useful to see if any more runaway black holes turn up.


Venus and Jupiter still lie close together, low in the southwest after sunset. Mars lies high in the south. The Moon will be full on March 7 and will reach its last quarter on March 14.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory near Penticton.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.

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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]

The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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