Black Holes: Why "Megamergers" Can Form in Stellar Clusters

Space is the Disneyland of death. It’s cold, unfeeling, and inarguably heartwarming all at once. When stars die, they can give rise to black holes, the most terrifying and beautiful things in the universe. These giant voids can find another black hole to form a binary with, and according to a new study, this cosmic pairing could happen multiple times in increasingly terrifying ways.

Researchers at the Massachusetts Institute of Technology suggest dense stellar clusters in space could be a perfect location for black holes to form. These “globular clusters” can be found in galaxies across the universe; our Milky Way has about 200 of them within its borders. A high concentration of stars means that when those stars supernova, more black holes could hypothetically rise. Black holes partner up, forming binaries that eventually collide — but that’s not always the end of the story.

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With his team, Carl Rodriguez, a Pappalardo fellow in MIT’s Department of Physics and the Kavli Institute for Astrophysics and Space Research, posit black holes in these regions can not only form binaries, but “megamergers.” This is when the product of a black hole binary partners with another black hole, creating something truly massive. Their findings were published on April 10 in the Physical Review Letters.

“How they merge depends on where the first binaries were formed,” Rodriguez tells Inverse. “If the two first-generation BHs were formed through gravitational interactions in the center of a dense star cluster (what we study), then the second-generation BH they form could find another companion and merge again. Our simulations show that up to 20% of the mergers from dense star clusters are these second-generation mergers (though not all of them will be above 50 solar masses).”

Artist's rendition of a black hole binary.

Northwestern Visualization/Carl Rodriguez

Rodriguez and his team created simulations of 24 globular clusters that model their evolution over a span of 12 billion years. Using a supercomputer called Quest based at Northwestern University, the team was able analyze the formation of these stellar regions and the black holes within them. The simulations confirmed that black hole binaries could indeed partner with another black hole in the stellar cluster.

“I want to better understand how the spins of the BHs effect this,” Rodriguez explains. “A large assumption in this work was that the BHs were not spinning very rapidly (which is consistent with the LIGO detections so far), but if the first generation of BHs is spinning, then it will be harder to retain the second generation BHs in the cluster.”

When black hole binaries merge, they send off ripples in the fabric of space-time that we can detect here on Earth. These gravitational waves, which have been detected a few times over the last few years by the Virgo and LIGO collaborations, are inarguably one of the most important concepts scientists will study over the course of the next few decades. With Virgo’s third observation run set for fall 2018, perhaps scientists will be able to detect gravitational waves from black holes in one of these stellar clusters.

Until then, it’s at least comforting to consider all the nice friends black holes get to hang out with in these distant clusters. I like to think they’re happy together.

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