10/07/2024
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An international team of astronomers used more than 500 images from NASA/ESA’s Hubble Space Telescope over a two-decade period to detect seven fast-moving stars in the innermost region of Omega Centauri, the largest and brightest globular cluster in the sky. These stars provide compelling new evidence for the presence of an intermediate-mass black hole.
Hubble’s view of Omega Centauri
Intermediate-mass black holes (IMBHs) are the long-sought “missing link” in black hole evolution. To date, only a few other IMBH candidates have been found. Most known black holes are either extremely massive, like the supermassive black holes that lie at the cores of large galaxies, or relatively light, with masses less than 100 times that of the Sun. Black holes are one of the most extreme environments that humans are aware of, and are therefore a testing ground for the laws of physics and our understanding of how the universe works. If there are IMBHs, how common are they? Is a supermassive black hole growing from the IMBH? How is the IMBH itself formed? Are dense star clusters their favorite home?
Omega Centauri is visible from Earth with the naked eye and is one of the favorite celestial objects for stargazers in the Southern Hemisphere. Although the cluster is 17,000 light-years away and lies just above the plane of the Milky Way, it appears almost as large as the full Moon when viewed from the dark countryside. The precise classification of Omega Centauri has evolved over time as our ability to study it has improved. It was first listed in Ptolemy’s catalog almost 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 and are found in both the outer and central regions of many galaxies, including our own. Omega Centauri has several properties that distinguish it from other globular clusters: it rotates faster than a normal globular cluster and its shape is very flattened. In addition, Omega Centauri is about 10 times more massive than other large globular clusters, almost as massive as a small galaxy.
Omega Centauri consists of roughly 10 million stars that are gravitationally bound together. An international team has now created a huge catalog of the motions of these stars, measuring velocities for 1.4 million stars by studying more than 500 images of the cluster from Hubble. Most of these observations were intended to calibrate Hubble’s instruments rather than for scientific use, but they proved to be an ideal database for the team’s research efforts. The vast catalog, which is the largest catalog of motions for any star cluster to date, will be open to the public (more information available here).
“We discovered seven stars that shouldn’t be there,” explained Maximilian Häberle of the Max Planck Institute for Astronomy in Germany, who led the investigation. “They move so fast they should escape the cluster and never come back. The most likely explanation is that these stars are gravitationally pulled by a very massive object, holding 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 IMBH in Omega Centauri [1]. However, other studies have suggested that a central cluster of stellar-mass black holes could contribute to the mass, and have suggested that the lack of fast-moving stars above the necessary escape velocity makes IMBHs less likely by comparison.
Black hole candidate in Omega Centauri
“This discovery is the most direct evidence yet for an IMBH in Omega Centauri,” added team leader Nadine Neumayer, also of the Max Planck Institute for Astronomy, who initiated the study. “This is exciting because there are very few other known black holes of similar mass. The black hole in Omega Centauri may be the best example of an IMBH in our cosmic neighborhood.”
If confirmed, at 17,000 light-years away, the candidate black hole is closer to Earth than the 4.3-million-solar-mass black hole at the center of the Milky Way, which is 26,000 light-years away. Aside from the galactic center, this would also be the only known case of a line of stars being tightly bound to a massive black hole.
The science team now hopes to characterize the black hole. Although it is thought to measure at least 8,200 solar masses, its exact mass and its exact position are not fully known. The team also intends to study the orbits of fast-moving stars, which requires additional line-of-sight measurements of the respective velocities. The team was given time with the NASA/ESA/CSA James Webb Space Telescope to do this, and also has other pending proposals to use other observatories.
Omega Centauri was also recently included in new data from ESA’s Gaia mission, which included more than 500,000 stars. “After 30 years, the Hubble Space Telescope and its imagers are still one of the best tools for high-precision astrometry in crowded star fields, regions where Hubble can provide increased sensitivity from ESA Gaia observations,” said team member Mattia Libralato. of the National Institute for Astrophysics in Italy (INAF) and formerly AURA for the European Space Agency at the time of this study. “Our results show the high resolution and sensitivity of HST, which provide us with exciting new scientific insights and give new impetus to the topic of IMBHs in globular clusters.”