ESA published this original story on July 11, 2024. Edits by EarthSky.
An international team of astronomers has used more than 500 images from the Hubble Space Telescope spanning two decades to detect seven fast-moving stars in the innermost region of Omega Centauri. Omega Centauri is the largest and brightest globular cluster in the sky. These stars provide strong evidence for the presence of an intermediate-mass black hole.
The journal Nature published the peer-reviewed study on July 10, 2024.
Intermediate-mass black holes are a long-sought ‘missing link’ in black hole evolution. Astronomers have found only a few other intermediate-mass black hole candidates to date. Most known black holes are either extremely massive, like the supermassive black holes that lie at the cores of large galaxies. Or else they’re relatively lightweight, with a mass less than 100 times that of the sun. Black holes are one of the most extreme environments known. And so they’re a testing ground for the laws of physics and our understanding of how the universe works.
Astronomers have many questions. If intermediate-mass black holes exist, how common are they? Does a supermassive black hole grow from an intermediate-mass black hole? How do intermediate-mass black holes themselves form? Are dense star clusters their favored home?
An intermediate-mass black hole in Omega Centauri
Omega Centauri is visible from Earth with the unaided eye. It’s one of the favorite celestial objects for stargazers in the Southern Hemisphere. The cluster is 17,700 light-years away, lying just above the plane of the Milky Way. Yet observers in dark rural areas see it appear almost as large as the full moon.
The exact classification of Omega Centauri has evolved through time, as our ability to study it has improved. Ptolemy’s catalog first listed Omega Centauri nearly 2,000 years ago as a single star. Edmond Halley reported it as a nebula in 1677. And in the 1830s, the English astronomer John Herschel was the first to recognize it as a globular cluster.
Globular clusters typically consist of up to one million old stars tightly bound together by gravity. They lie both in the outskirts and central regions of many galaxies, including our own. Omega Centauri has several characteristics that distinguish it from other globular clusters. It rotates faster than a run-of-the-mill globular cluster, and its shape is highly flattened. Moreover, Omega Centauri is about 10 times as massive as other big globular clusters, almost as massive as a small galaxy.
Omega Centauri consists of roughly 10 million gravitationally bound stars. An international team has now created an enormous catalog of the motions of these stars. They’ve measured the velocities for 1.4 million stars by studying over 500 Hubble images of the cluster. The intention of most of these observations was to calibrate Hubble’s instruments rather than for scientific use. But they turned out to be an ideal database for the team’s research efforts. The extensive catalog is the largest catalog of motions for any star cluster to date.
This video, provided by the Max-Planck-Institut für Astronomie shows the 7 fast stars in Omega Centauri ‘that should not be there. A team of astronomers using images from the Hubble Space Telescope spanning two decades detected 7 fast-moving stars in the innermost region of Omega Centauri, the largest and brightest globular cluster in the sky. These stars provide evidence for a long-predicted intermediate-mass black hole.
7 fast stars
Maximilian Häberle of the Max Planck Institute for Astronomy in Germany led this investigation. Häberle said:
We discovered seven stars that should not be there. They are moving so fast that they should escape the cluster and never come back. The most likely explanation is that a very massive object is gravitationally pulling on these stars and keeping them close to the center. The only object that can be so massive is a black hole, with a mass at least 8,200 times that of our sun.
Several studies have suggested the presence of an intermediate-mass black hole in Omega Centauri. However, other studies proposed that the mass could be due to a central cluster of stellar-mass black holes. And the studies had suggested the lack of fast-moving stars above the necessary escape velocity made an intermediate-mass black hole less likely in comparison.
Team lead Nadine Neumayer, also of the Max Planck Institute for Astronomy, initiated the study with Anil Seth of the University of Utah in the United States. Neumayer said:
This discovery is the most direct evidence so far of an intermediate-mass black hole in Omega Centauri. This is exciting because there are only very few other black holes known with a similar mass. The black hole in Omega Centauri may be the best example of an intermediate-mass black hole in our cosmic neighborhood.
If confirmed – at its distance of 17,700 light-years – the candidate black hole resides closer to Earth than the 4.3 million solar mass black hole in the center of the Milky Way, which is 26,000 light-years away. Besides the galactic center, it would also be the only known case of a number of stars closely bound to a massive black hole.
The location of the intermediate-mass black hole
Future studies of the black hole
The science team now hopes to characterize the black hole. While astronomers believe it measures at least 8,200 solar masses, they don’t know its exact mass and its precise position. The team also intends to study the orbits of the fast-moving stars, which requires additional measurements of the respective line-of-sight velocities. The team now has time with the NASA/ESA/CSA James Webb Space Telescope to do just that. They also have other pending proposals to use other observatories.
Omega Centauri was also a recent feature of a new data release from ESA’s Gaia mission, which contained over 500,000 stars. Team member Mattia Libralato of the National Institute for Astrophysics in Italy (INAF), and previously of AURA for the European Space Agency during the time of this study, said:
Even after 30 years, the Hubble Space Telescope with its imaging instruments is still one of the best tools for high-precision astrometry in crowded stellar fields, regions where Hubble can provide added sensitivity from ESA’s Gaia mission observations. Our results showcase Hubble’s high resolution and sensitivity that are giving us exciting new scientific insights and will give a new boost to the topic of intermediate-mass black holes in globular clusters.
Bottom line: A new study finds strong evidence for an intermediate-mass black hole in the Omega Centauri globular cluster. This type of black hole has been elusive to astronomers.
Source: Fast-moving stars around an intermediate-mass black hole in Omega Centauri
Via ESA