Buzz
Globular clusters are awe-inspiring and filled with stars. They could also provide the ideal environment for black holes to repeatedly merge. One such cluster, NGC 362, is situated within the Milky Way. ESA/Hubble & NASAScientists are becoming more familiar with black holes colliding in the dark. In 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) made history by detecting the tremors in spacetime caused by two black holes colliding in a distant galaxy. This initial detection confirmed the existence of binary stellar-mass black holes, those formed from the explosive deaths of massive stars. Since then, numerous other mergers (including a neutron star merger) have been detected.
Now, in research published on April 10, 2018, in the journal Physical Review Letters, scientists suggest that black holes probably merge multiple times, creating black holes too massive to be produced by a single star. And globular star clusters could be the perfect setting for such objects to form and merge — repeatedly.
"We believe these clusters were formed by hundreds to thousands of black holes that quickly sank toward the center," stated Carl Rodriguez from MIT and the Kavli Institute for Astrophysics and Space Research in a statement. "These clusters act like factories for black hole binaries, where so many black holes are concentrated in a small area that two of them could merge to create a larger black hole. That new black hole could then pair up with another and merge again."
LIGO has yet to detect one of these "second-generation mergers." All of the mergers observed so far have involved stellar-mass black holes (likely formed by the collapse of single massive stars). However, if gravitational waves from a merger involving a black hole 50 times the mass of our sun are detected in the future, it would provide strong evidence for the repeated merging of black holes. And that would be a thrilling discovery.
"If we wait long enough, LIGO will eventually detect something that could only have originated from these star clusters, as it would be larger than anything that could come from a single star," Rodriguez added.
Most galaxies contain globular clusters, with larger galaxies having more clusters. For example, massive elliptical galaxies may host tens of thousands of clusters, while the Milky Way has around 200, with the closest one located 7,000 light-years from Earth. These clusters are packed with ancient stars in a small space, creating the ideal conditions for black holes within them to fall to the center and interact with any other black holes that may be nearby.
This artistic depiction of a black hole showcases how its immense gravity warps everything around it, bending the light in a way that mirrors the distortion seen in a carnival mirror.
Image courtesy of NASA's Goddard Space Flight Center/Jeremy SchnittmanIf two black holes happen to drift toward each other after originating from different regions of a cluster, relativistic calculations predict that they will generate gravitational waves, which will gradually reduce their speed and bring them closer together. Eventually, this will lead to the formation of a binary system. Their final act is inevitable: the black holes will continue to emit gravitational waves, causing them to spiral closer until they collide, merge, and release a massive burst of gravitational waves that travel outward at the speed of light. The newly formed black hole will then remain within the cluster, awaiting another black hole to drift by, beginning the cycle once again.
However, when Rodriguez and his team ran the simulations, they assumed the black holes would be spinning rapidly before merging, leading to results that were, well, unexpectedly dramatic.
"If the black holes are rotating when they merge, the new black hole they form will emit gravitational waves in a specific direction, similar to the thrust of a rocket. This would create a black hole that could be ejected at speeds of up to 5,000 kilometers per second — incredibly fast," explained Rodriguez. "It would only take a kick of a few tens to a few hundred kilometers per second to propel one of these black holes out of its cluster."
Based on this reasoning, if the merged black holes are being ejected from their clusters, they can't merge again. But after analyzing the spin rates of the black holes detected by LIGO, the team discovered that their spin is much slower than anticipated, which reduces the chances of a cluster releasing a newly merged black hole. With this adjustment, the researchers concluded that nearly 20% of black hole binaries would include at least one black hole that originated from a previous merger. They found that second-generation black holes would typically fall into a mass range between 50 and 130 solar masses. Such massive black holes could only be formed through mergers.
For now, it's up to the global network of gravitational wave detectors to capture a signal originating from a second-generation black hole.
NGC 362, the Milky Way's very own globular star cluster, is estimated to be between 10 and 11 billion years old, according to the European Space Agency. Meanwhile, the galaxy itself is over 13 billion years old.
