An article published in the journal “Nature” reports evidence that the globular cluster Omega Centauri contains an intermediate-mass black hole. A team of researchers led by Maximilian Häberle of the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, used two decades of observations conducted with the Hubble Space Telescope to find that evidence. They did this by tracing the orbit of seven stars at the center of Omega Centauri that are moving fast due to an object with a mass that is at least 8,200 times the Sun’s. This also confirms the hypothesis that this star cluster is what remains of a dwarf galaxy absorbed by the Milky Way.
The image (ESA/Hubble & NASA, M. Häberle (MPIA)) shows from the left, the globular cluster Omega Centauri, its central area, and the area where the intermediate-mass black hole is probably located.
As the name suggests, intermediate-mass black holes are halfway between the ones born from the cores of dead stars and the supermassive black holes at the centers of galaxies. It could be a phase in the evolution of supermassive black holes, and astronomers are therefore very interested in finding them. Various candidates have been discovered over the years but they seem really rare and difficult to study.
The candidate in the globular cluster Omega Centauri has also been known for some time since it was the focus of a study published in April 2008 in the journal “The Astrophysical Journal”. However, that discovery was at the center of various controversies, confirming the difficulty in studying that area even if it’s practically in the cosmic neighborhood given that it’s about 17,700 light-years from Earth.
The seven stars moving around an invisible object were discovered by examining more than 500 images captured over two decades by the Hubble Space Telescope. Most were taken to calibrate Hubble’s instruments but were still useful for measuring the speed of 1.4 million stars in the Omega Centauri cluster.
Maximilian Häberle used the classic metaphor of the needle in the haystack regarding the research he conducted together with his collaborators. Despite this, they eventually found seven needles in the haystack of Hubble’s archive, seven fast-moving stars at the center of Omega Centauri. A comparison was made with some fast-moving stars around Sagittarius A*, the supermassive black hole at the center of the Milky Way.
The examination of the movements of the seven stars allowed Maximilian Häberle’s team to calculate that at the center of Omega Centauri there is an invisible object with a mass that is at least 8,200 times the Sun’s. This is the most solid evidence in favor of a candidate intermediate-mass black hole.
According to models of galaxy formation and evolution, supermassive black holes were born from the growth of those of intermediate mass thanks to the absorption of enormous quantities of materials present in primordial galaxies. Omega Centauri has long been considered what probably remains of a primordial dwarf galaxy absorbed in remote times by the Milky Way. Its central black hole remained of intermediate mass almost certainly precisely because the Milky Way stole a lot of gas from it.
Information regarding the location and mass of the at the center of Omega Centauri remains approximate. To eliminate any doubt, the researchers intend to conduct follow-up observations with the James Webb Space Telescope. In the future, it will be possible to conduct observations also with other instruments under construction. The stars close to this black hole can have orbital periods of over a century, so precisely reconstructing their orbits is a truly long-term task.
