10/07/2024
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An international team of astronomers has used more than 500 images from the NASA/ESA Hubble Space Telescope spanning two decades to detect seven fast-moving stars in the inner 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 a long-sought “missing link” in black hole evolution. So far, only a few other IMBH candidates have been found. Most known black holes are either extremely massive, such as the supermassive black holes found in the centers 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 known to man, and as such they are a proving ground for the laws of physics and our understanding of how the universe works. If IMBHs exist, how common are they? Does a supermassive black hole grow from an IMBH? How do IMBHs themselves form? Are dense star clusters their preferred home?
Omega Centauri is visible to the naked eye from Earth and is a favorite of stargazers in the Southern Hemisphere. Although the cluster is 17,000 light-years away, just above the plane of the Milky Way, it appears almost as large as the full Moon when viewed from a dark rural area. The exact classification of Omega Centauri has evolved over time as our ability to study it has improved. It was first noted in Ptolemy’s Catalogue as a single star almost 2,000 years ago. Edmond Halley reported it as a nebula in 1677, and in the 1830s, English astronomer John Herschel was the first to recognize it as a globular cluster.
Globular clusters typically consist of up to a million old stars that are tightly bound to each other by gravity, and are found in both the outer and central regions of many galaxies, including our own. Omega Centauri has several features that set it apart from other globular clusters: it rotates more rapidly than a typical 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 is made up of about 10 million gravitationally bound stars. An international team has now created a massive catalog of the motions of these stars, measuring the velocities of 1.4 million stars by studying more than 500 Hubble images of the cluster. 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 comprehensive catalog, which is the largest catalog of motions for a star cluster to date, will be made publicly available (more information is available here).
“We have discovered seven stars that should not be there,” explained Maximilian Häberle of the Max Planck Institute for Astronomy in Germany, who led the research. “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 pulling on these stars and keeping them close to the center. The only object that could be that massive is a black hole, with a mass of 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 suggested that the mass could be contributed by a central cluster of stellar-mass black holes, and had suggested that the lack of fast-moving stars above the necessary escape velocity made an IMBH less likely by comparison.
Black hole candidate in Omega Centauri
“This discovery is the most direct evidence of an IMBH in Omega Centauri so far,” added team leader Nadine Neumayer, also of the Max Planck Institute for Astronomy, who initiated the study. “This is exciting because very few other black holes of comparable mass are known. The black hole in Omega Centauri is perhaps the best example of an IMBH in our cosmic neighborhood.”
If confirmed, the black hole candidate would be 17,000 light-years 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. Other than the galactic center, it would also be the only known case of a pair of stars being closely associated with a massive black hole.
The science team now hopes to characterize the black hole. Although it is thought to be at least 8,200 times the mass of the sun, its exact mass and precise position are not fully known. The team also plans to study the orbits of the fast-moving stars, which will require additional measurements of their respective line-of-sight velocities. The team has been granted time on the NASA/ESA/CSA James Webb Space Telescope to do so, and has other proposals in the works to use other observatories.
Omega Centauri was also a recent feature of a new data release from ESA’s Gaia mission, which included over 500,000 stars. “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 star fields, regions where Hubble can provide additional sensitivity to ESA’s Gaia mission observations,” shared team member Mattia Libralato of the National Institute for Astrophysics in Italy (INAF), and previously of AURA for the European Space Agency at the time of this study. “Our results demonstrate Hubble’s high resolution and sensitivity, which give us exciting new scientific insights and will give new impetus to the topic of IMBHs in globular clusters.”