Medium Black Hole May Be Hiding in Omega Centauri

Omega Centauri is one of Milky Way's most massive globular star cluster, located about 17,000 light years away from Earth and containing several millions of stars. Evidence suggests that there may be a massive object in its core, with tremendous gravitational pull, possibly a medium-sized black hole with a mass estimated at about 40,000 solar masses.

Usually, globular star clusters are held together only by the collective gravitational pull exerted by the millions of stars inside them. There are possibly more than 200 globular star clusters in all of our galaxy. Max Planck Institute for Extraterrestrial Physics astronomer, Eva Noyola says that: "This result shows that there is a continuous range of masses for black holes, from supermassive, to intermediate, to small, stellar types."

"This finding also is important because the theory of formation for supermassive black holes requires seed black holes that are exactly in the mass range of the one we found. Such seeds have not been identified so far. If these types of intermediate-mass black holes happen to be common in stars clusters, then they can provide numerous seeds for the formation of the supermassive black holes," she added.

Intermediate black holes

There is little evidence that intermediate-sized black holes even exist, as none have ever been found. Additionally, their formation process is not very well understood; it is known that low mass black holes may take shape out of the massive stars in their final stages of life, and there is evidence that supermassive black holes are found in galactic nuclei, such as that of the Milky Way, however medium-sized black holes remain mostly elusive.

"Before this observation, we had only one example of an intermediate-mass black hole in the globular cluster G1, in the nearby Andromeda Galaxy," said astronomer Karl Gebhartd from the University of Texas, part of the team that made the discovery.

Measurements made with the Hubble Space Telescope and Gemini Observatory revealed that the stars in the near vicinity of the core of Omega Centauri move at speeds much higher than those expected, thus there must be some kind of unseen object that is interacting gravitationally with them.

The only object which could create such a massive gravitational disturbance must be a black hole, however there may be a couple of other causes which determine such a configuration.

Alternatives to black hole

Another possible explanation for the speed excess would be the presence of a large collection of dead stars in the core, such as white dwarfs and neutron stars. Secondly, the star in the center of Omega Centauri may actually have a highly elliptical orbit, which could appear to us as they are moving really fast.

"For both alternative scenarios it is very hard to get stars to behave that way, either the burned-out stars really bunched up in the center or many stars with very elongated orbits. The normal evolution of a star cluster like Omega Centauri should not end up with stars behaving in those ways. But even if we assume that either scenario did happen somehow, both configurations are expected to be very short lived. A clump of burned-out stars, for example, are expected to move farther away from the center quickly. The stars with elongated orbits are expected to become circular very quickly," said Noyola.

Galactic core

The Omega Centauri globular star cluster is extremely massive for its type of object. That's why it is believed that it is most likely the core of a former dwarf galaxy which may have collided with our galaxy at some point in time. Usually, there is a direct relation between the black hole in the galactic nuclei and the mass of the host galaxy, and with a mass 10 million times that of the Sun, Omega Centauri matches exactly the size of its black hole.