The centre of the Milky Way is a very crowded region, hosting a dense and compact cluster of stars—the so-called nuclear star cluster—and a supermassive black hole (SMBH) weighing more than 4 million solar masses.
A star cluster is an ensemble of stars kept together by their own force of gravity. These large systems are found in the outskirts of every type of galaxy, being comprised of up to several million stars. Nuclear clusters constitute a special variety, since they are found at the very centre of galaxies, including the Milky Way, and they are much heavier and denser than ordinary clusters.
Nuclear clusters are thought to form, at least in part, from repeated collisions of several star clusters, which spiralled slowly toward the centre of the host galaxy due to a process called dynamical friction. During the past 12 billion years, the Galactic Centre may also have been the site of multiple collisions among massive star clusters.
Massive star clusters are also ideal nurseries of stellar black holes (BHs), with masses of 10-100 solar masses, and represent the most likely birthplaces for intermediate-mass black holes (IMBHs), truly enigmatic objects with putative masses in the range 100-100000 solar masses. Intermediate-mass black holes leave little signs of their presence and their detection remains one of the outstanding challenges of modern astrophysics.
If the Galactic nuclear star cluster was formed by a series of collisions of massive clusters, the Galactic Centre might have been contaminated with their “dark” content. Some fallen clusters might have delivered tens to hundreds of stellar black holes and a few intermediate-mass black holes to the centre of the Milky Way.
A recent study published by the Monthly Notices of the Royal Astronomical Society has modelled the evolution of a massive star cluster spiralling toward the centre of the Milky Way, assuming that it hosts either an intermediate-mass black hole or population of stellar black holes. These models have one of the highest resolution levels ever achieved, thanks to the use of accurate numerical simulations and exploiting the advantages offered by the Graphic Processing Units computing power. The whole set of 12 simulations required nearly two years to be completed, but it would have required much more time with a normal computer—up to 100 years.
As the black holes are deposited close to the super massive black hole they interact strongly, emitting gravitational waves—ripples of spacetime predicted by Einstein’s General Theory of Relativity, and recently observed by the international LIGO/VIRGO detector.
A stellar black hole passing sufficiently close to the super massive black hole can remain trapped in a tight binary system. Due to the large difference in mass between the stellar black hole (~10 Msun) and the supermassive black hole (10^5 – 10^6 Msun), these exotic systems are called Extreme Mass Ratio Inspirals (EMRIs). As time passes, the EMRI’s orbit slowly shrinks due to gravitational waves emission, eventually leading to the black hole-super massive black hole pair merging. EMRIs are the most promising sources of gravitational waves to be seen with the next generation of space-born observatories, like LISA or the TianQin experiments.
Moreover, if some stellar black holes were paired in a binary system, the presence of a nearby super massive black hole can affect their evolution significantly, driving their merging efficiently. The echoes of the resulting emission of gravitational waves can potentially be “heard” by the LIGO and VIRGO experiments.
Dr. Gualandris, lecturer in physics, added “If the black holes delivered in the Galactic Centre form tight binaries, they can coalesce in a very short time due to the presence of the super massive black hole. This is the first time in which the disruption of a star cluster is directly connected with the coalescence of black hole binaries.”
When the spiralling cluster contains an intermediate-mass black hole, things are slightly different. The strong gravitational forces suffered by the cluster leads to its disruption as it approaches the super massive black hole and, like an oyster revealing its pearl, the cluster leaves its intermediate-mass black hole wandering around the SMBH. The two massive black holes pair together, forming a tightly bound system that may merge in 1-10 billion years.
Dr. Arca-Sedda, postdoctoral fellow, said: “We find that the Milky Way may have witnessed the coalescence between the super massive black hole and an intermediate-mass black hole a few billion years ago, resulting in a burst of gravitational waves. The SMBH may also have captured thousands of stellar mass black holes, forming tight binaries which will be detected by future observatories like LISA.”
The simulations performed by Arca-Sedda and Gualandris show that these processes can be part of the Milky Way past, and can potentially be observed in galaxies similar to the Milky Way.
Feature image credit: Hubble-Spitzer composite of the galactic centre (full-field) by NASA, ESA, Q.D. Wang (University of Massachusetts, Amherst) and S. Stolovy (Caltech). Creative Commons Attribute 4.0 via Spacetelescope.org.