Black hole hidden in centre of Omega Centauri

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Washington, April 2 : Astronomers have used the NASA/ESA Hubble Space Telescope and the Gemini Observatory to discover a black hole hidden in the centre of the Omega Centauri.

The discovery of this black hole was a result of an attempt by astronomers to solve an important mystery surrounding Omega Centauri.

Omega Centauri is visible from Earth with the naked eye, and is 17 000 light-years away, located just above the plane of the Milky Way.

But, exactly how Omega Centauri should be classified has always been a contentious topic.

In the 1830s, the English astronomer John Herschel was the first to recognise it as a globular cluster. Now, more than a century later, a new result has suggested that Omega Centauri is not a globular cluster at all, but a dwarf galaxy stripped of its outer stars.

Globular clusters consist of up to one million old stars tightly bound by gravity and are found in the outskirts 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, its shape is highly flattened and it consists of several generations of stars - more typical globulars usually consist of just one generation of old stars.

Moreover, Omega Centauri is about 10 times as massive as other big globular clusters, almost as massive as a small galaxy.

These peculiarities have led astronomers to suggest that Omega Centauri may not be a globular cluster at all, but a dwarf galaxy stripped of its outer stars by an earlier encounter with the Milky Way.

"Finding a black hole at the heart of Omega Centauri could have profound implications for our understanding of its past interaction with the Milky Way," said astronomer Eva Noyola of the Max-Planck Institute for Extraterrestrial Physics in Garching, Germany, and leader of the team that made the discovery.

For finding the black hole, Eva Noyola and her colleagues measured the motions and brightnesses of the stars at the centre of Omega Centauri.

The measured velocities of the stars in the centre are related to the total mass of the cluster and were far higher than expected from the mass deduced from the number and type of stars seen.

So, there had to be something extraordinarily massive (and invisible) at the centre of the cluster responsible for the fast-swirling dance of stars - almost certainly a black hole with a mass of 40,000 solar masses.

Images obtained with the Advanced Camera for Surveys onboard the NASA/ESA Hubble Space Telescope and data obtained by the GMOS spectrograph on the Gemini South telescope in Chile show that Omega Centauri appears to harbour an elusive intermediate-mass black hole in its centre.


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