Hubble Discovers Possible Intermediate-Mass Black Hole in Omega Centauri

A group of international astronomers have analyzed over 500 images from the NASA/ESA Hubble Space Telescope over two decades. From this analysis, they identified seven rapidly moving stars in the core of Omega Centauri, the largest and brightest globular cluster visible from Earth. The movement of these stars suggests the presence of an intermediate-mass black hole (IMBH).

IMBHs are often referred to as the ‘missing link’ in black hole evolution. They are elusive and quite rare compared to the well-documented supermassive black holes found at the center of galaxies or the smaller black holes of less than 100 solar masses. The existence of IMBHs raises questions about their prevalence, formation, and possible contribution to the growth of supermassive black holes. They are often found in dense star clusters like Omega Centauri.

Visible to the naked eye, Omega Centauri is a favourite among southern hemisphere stargazers. It is located 17,000 light-years away, just above the plane of the Milky Way. Interestingly, its classification has changed over time. Initially identified as a single star by Ptolemy nearly 2000 years ago, it was later classified as a nebula by Edmond Halley in 1677, and then a globular cluster by John Herschel in the 1830s.

Omega Centauri is unique due to its rapid rotation, highly flattened shape, and massive size which is nearly equivalent to a small galaxy. It contains around 10 million stars. The international team of astronomers measured the velocities of 1.4 million of these stars to create an extensive catalogue of stellar motions. This catalogue, the largest of its kind, was made possible by Hubble’s high resolution and sensitivity.

The team discovered seven stars in Omega Centauri that were moving at such a rapid pace that they should have escaped the cluster and never returned. However, they were being held close to the centre by the gravitational pull of a very massive object. This object is believed to be a black hole with a mass at least 8200 times that of our Sun.

There have been several studies suggesting the presence of an IMBH in Omega Centauri. However, some studies proposed that the mass could be attributed to a central cluster of stellar-mass black holes. The discovery of the fast-moving stars provides the most direct evidence so far of an IMBH in Omega Centauri.

If confirmed, the candidate black hole would be closer to Earth than the 4.3 million solar mass black hole at the center of the Milky Way, located 26,000 light-years away. The team is planning further studies to accurately determine the mass and position of the black hole and to study the orbits of the fast-moving stars.

Omega Centauri was also featured in a recent data release from ESA’s Gaia mission, which included over 500,000 stars. The team member Mattia Libralato of the National Institute for Astrophysics in Italy highlighted the importance of Hubble’s high resolution and sensitivity in providing new scientific insights and boosting the study of IMBHs in globular clusters.

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