10/07/2024
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An international team of astronomers has used more than 500 images from NASA/ESA’s Hubble Space Telescope spanning two decades to discover seven fast-moving stars in the innermost region of Omega Centauri, the largest and most massive globular cluster. bright 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. Only a few other IMBH candidates have been found to date. The best-known black holes are either extremely massive, like the supermassive black holes that lie in the cores of large galaxies, or relatively light, with a mass less than 100 times that of the Sun. Black holes are one of the most extreme environments that humans are aware of, and thus they are a testing 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 are IMBHs themselves formed? Are dense star clusters their preferred 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, lying 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 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 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 a million old stars tightly bound together by gravity and are found in both the periphery 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 flowing globular cluster and its shape is very flattened. Furthermore, Omega Centauri is about 10 times more massive than other large globular clusters, almost as massive as a small galaxy.
Omega Centauri consists of approximately 10 million stars that are gravitationally bound together. An international team has now created a large catalog of the motions of these stars, measuring the velocities for 1.4 million stars by studying over 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 vast catalogue, which is the largest catalog of motions for any star cluster to date, will be made openly available (more information is 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 are moving so fast that they must escape the group 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 could 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 proposed 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 so far of 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 with a similar mass. The black hole in Omega Centauri may be the best example of an IMBH in our cosmic neighborhood.”
If confirmed, at a distance of 17,000 light-years, the candidate black hole would lie 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. Apart from the galactic center, it would also be the only known case of a number of stars tightly bound to a massive black hole.
The science team now hopes to characterize the black hole. While it is believed to measure at least 8,200 solar masses, its exact mass and exact position are not fully known. The team also aims to study the orbits of fast-moving stars, which requires additional measurements of the corresponding line-of-sight velocities. The team has been given time with the NASA/ESA/CSA James Webb Space Telescope to do this, and there are also 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. “Even after 30 years, the Hubble Space Telescope with its imaging instruments is still one of the best tools for high-precision astrometry in star-filled fields, regions where Hubble can provide increased sensitivity from Gaia mission observations. of ESA,” said 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. “Our results show Hubble’s high resolution and sensitivity are giving us exciting new scientific insights and will give new impetus to the topic of IMBHs in globular clusters.”