Astronomers have found the nearest massive black hole, which could be the missing link in understanding how massive black holes form.
This discovery was made by observing fast-moving stars in the star cluster Omega Centauri.
These stars provided strong evidence for a central black hole in the cluster, which has at least 8,200 times the mass of our sun.
This black hole is a candidate for a special class called intermediate-mass black holes, which are believed to have formed early in the evolution of galaxies.
The discovery supports the idea that Omega Centauri is the core of a small galaxy that was swallowed by the Milky Way billions of years ago.
After losing its outer stars, this galaxy nucleus has remained unchanged since then. The study was published in the journal Nature.
Omega Centauri is a stunning cluster of about 10 million stars, visible from southern latitudes as a fuzzy spot in the night sky. With a small telescope, it appears as a dense, spherical collection of stars, making it hard to see individual stars in the center.
Led by Maximilian Häberle from the Max Planck Institute for Astronomy, the new study confirms that Omega Centauri contains a central black hole.
This black hole is less massive than those typically found in the centers of galaxies, placing it in an intermediate stage of evolution. Omega Centauri seems to be the core of a small galaxy that merged with the Milky Way and stopped evolving.
Black holes come in different sizes. Stellar black holes have the mass of one to a few dozen suns, while supermassive black holes can have millions or even billions of solar masses. Astronomers believe early galaxies had intermediate-sized black holes that grew as the galaxies evolved, merging with other galaxies or consuming smaller ones. However, finding these medium-sized black holes has been challenging.
The new study used dynamical models to trace the origins of these dark comets. They found that the main asteroid belt is the most likely source. One dark comet, called 2003 RM, has an orbit that takes it close to Earth and then out to Jupiter, similar to comets influenced by Jupiter’s gravity. This suggests it might have been pushed inward from its original orbit.
Galaxies that have remained small, known as dwarf galaxies, are hard to observe with current technology. Observing their central regions to detect black holes is very difficult. While there have been promising candidates, there has been no definite detection of intermediate-mass black holes until now.
Omega Centauri is special because it might be the core of a separate galaxy that merged with the Milky Way. The remaining core and its central black hole would be “frozen in time,” providing a glimpse into the early stages of black hole evolution.
To confirm this, astronomers needed to detect a central black hole in Omega Centauri. Previous evidence was inconclusive, leaving room for doubt.
Nadine Neumayer from the Max Planck Institute for Astronomy and Anil Seth from the University of Utah designed a project to understand Omega Centauri’s formation history. They aimed to find fast-moving stars around a central black hole, which would be proof of its existence.
Maximilian Häberle led the search, creating a catalog of star movements in Omega Centauri by studying over 500 images from the Hubble Space Telescope. These images, originally used for calibration, turned out to be perfect for the research. Häberle found seven fast-moving stars in the center of Omega Centauri, indicating the presence of a central black hole with a mass of at least 8,200 suns.
This discovery confirms that Omega Centauri contains an intermediate-mass black hole, the closest known example at about 18,000 light-years from Earth. This finding resolves a long-standing debate and provides the best candidate for an intermediate-mass black hole.
Neumayer, Häberle, and their team plan to study Omega Centauri’s center in more detail using the James Webb Space Telescope and future instruments. Their goal is to understand how stars orbit around the black hole, a project for future generations of astronomers.
