Globular Clusters Could Produce Binary Black Holes

Deep within globular clusters such as M15, black holes first orbit each other and then merge. Credit: ESA, Hubble, NASA

What is more awe-inspiring than an object you can’t see with a gravitational field so strong it can spaghettify entire stars? How about two of them whirling around each other? A new paper not only predicts that such scenarios are common at the heart of globular star clusters, but that we could soon detect many pairs of dancing black holes, including the moment when these great behemoths merge.

Some of the most spectacular events in the universe are believed to be the result of neutron stars colliding, or black holes devouring stars. So imagine the power of two black holes merging. We expect that this happens quite often – it is thought to be how medium-sized black holes grow to become the supermassive objects at the heart of galaxies.

However, given that we can’t detect black holes directly, observing these encounters is a challenge. Observing the dance that occurs as two black holes go into orbit around each other, the orbits of which decay until the two combine, is one of the major aims of the new generation of gravitational wave detectors under construction. In preparation for the detectors coming on line, theoretical astronomers have created models of where such events may occur.

"Up until now, all of our observations have been from telescopes, literally looking out at the universe. Detecting gravitational waves will change that. And the cool part is we can hear things we could never see, such as binary black hole mergers, the subject of our study," said Northwestern University Ph.D. student Carl Rodriguez.

Rodriquez is first author of a paper in Physical Review Letters that notes, “In dense stellar environments such as globular clusters, binary black holes form by well-understood gravitational interactions.” After comparing the results of their models with observations of the globular clusters around the Milky Way, the authors conclude, "Mergers of dynamically formed binaries could be detected at a rate of ∼100 per year.” 

"Our study indicates the observatories will detect more of these energetic events than previously thought, which is exciting,” said senior author Frederic Rasio.

The authors think that mergers occurring within globular clusters could be the most common examples we will find of black holes coming together, and also may have distinct characteristics that will allow us to determine the origins of a merger whose gravitational waves we observe.

If all this is sounding pretty exotic, consider that Rasio has previously proposed that at the heart of merging galaxies, it is not uncommon for a triple black hole to conduct a cosmic ménage à trois before either all coming together, or one being ejected from the whole galaxy.

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