Observing with NASA’s Hubble Space Telescope (HST) and the sophisticated giant Earth-based telescope Gemini South, astronomers have found evidence for a moderately massive black hole, residing within a spectacular star cluster. The cluster, known as Omega Centauri, is a huge spherical agglomeration of about 10 million stars. Such spherical gravitationally-bound stellar swarms are known as globular clusters.

There are about 150 globular clusters orbiting our Galaxy, the Milky Way. They are located in the Milky Way’s immense halo of stars. A large globular cluster is over 100 light years across, and contains hundreds of thousands of stars. Omega Centauri is one of the biggest and most massive globular clusters. It lies 17,000 light-years from Earth. The globular clusters of our Galaxy are among the oldest objects in the Universe. Their ages are estimated to be over 12 billion years.

This illustration shows the distribution of the Galaxy’s globular cluster and the position of the globular cluster M4. The Galaxy is viewed edge-on.
Credit: NASA/ESA and A. Feild

A team of astronomers from the Max-Planck Institute for Extraterrestrial Physics in Germany and the University of Texas at Austin have reported their detection of a possible black hole in the core of Omega Centauri. This black hole candidate is classified as an intermediate-mass black hole.

The masses of black holes range from stellar-mass black holes up to supermassive black holes (SMBHs), weighing several billion times the solar mass. Intermediate mass black holes are thousands of times more massive than the Sun. The researchers believe that the discovery of moderately-sized black holes demonstrates the continuity of the mass range of black holes, and provides interesting clues to the theory of the formation of SMBHs.

Stellar-mass black holes are formed when massive stars explode. SMBHs are believed to lurk in the cores of galaxies, including our own Galaxy. They probably power the energetic processes, e.g., jets and explosions, taking place in the cores of some luminous, exotic galaxies, termed active galaxies.

The finding is due to spectroscopic measurements acquired using the Gemini South observatory in Chile which suggest the stars are orbiting Omega Centauri’s central core at higher than predicted velocities.

The study favors a likely explanation for these speedy stars: an intermediate-mass black hole of approximately 40,000 solar masses at the center of Omega Centauri. Its powerful gravitational pull speeds up the motions of the stars near the core.

Hubble images, taken with the Advanced Camera for Surveys (ACS), were crucial for locating the center of the cluster and measuring the amount of starlight at the cluster center.
Team members Eva Noyola and Karl Gebhardt are planning to make follow-up observations using the European Southern Observatory's Very Large Telescope (VLT) in Paranal, Chile, to verify the existence of an intermediate-mass black hole. The Hubble images were taken in June 2002.

Further Reading

Hubble Site
http://hubblesite.org/

Aymen Mohamed Ibrahem
Senior Astronomy Specialist