Astronomers Find Suspected Medium-Sized Black Hole in Omega Centauri

April 2, 2008

A well-known star cluster that glitters with the light of millions of stars may have a mysterious dark object tugging at its core, according to researchers at The University of Texas at Austin.

Astronomer Karl Gebhardt has teamed up with recent Ph.D. graduate Eva Noyola to find evidence for a medium-size black hole at the core of Omega Centauri, one of the largest and most massive globular star clusters orbiting the Milky Way galaxy. The finding will appear in the April 10 issue of The Astrophysical Journal.

Gebhardt and team leader Noyola discovered the black hole with NASA’s Hubble Space Telescope and Gemini Observatory on Cerro Pachon in Chile.

Globular star clusters are ball-shaped, gravitationally bound swarms of typically up to a million stars. More than 200 exist in the Milky Way galaxy. The ancient cluster Omega Centauri is 17,000 light-years from Earth.

The black hole in Omega Centauri is estimated to be about 40,000 times the mass of the Sun, falling in between the masses of supermassive black holes at the hearts of galaxies like the Milky Way and stellar-mass black holes that result when the most massive stars explode as supernovae.

“This result shows that there is a continuous range of masses for black holes, from supermassive, to intermediate, to small, stellar types,” Noyola said.

She and Gebhardt completed their data and analysis of Omega Centauri while Noyala was working on her Ph.D. in Austin under Gebhardt’s supervision. Noyola is now with the Max Planck Institute for Extraterrestrial Physics in Garching, Germany.

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

Astronomers have debated the existence of moderately sized black holes because they have not found strong evidence for them, and there is no widely accepted mechanism for how they could form. They have ample evidence that small black holes of a few solar masses are produced when giant stars die. There is similar evidence that supermassive black holes weighing the equivalent of millions to billions of solar masses sit at the heart of many galaxies, including the Milky Way.

“Before this observation, we had only one example of an intermediate-mass black hole in the globular cluster G1, in the nearby Andromeda galaxy,” Gebhardt said. “This study suggests that moderately sized black holes may be common residents in globular clusters.”

Noyola and Gebhardt used Hubble and Gemini to gather evidence for the black hole. Hubble’s Advanced Camera for Surveys showed how the stars are bunching up near the center of Omega Centauri, as seen in the gradual increase in starlight near the center.

Measuring the speed of the stars swirling near the cluster’s center with the Gemini Observatory, the astronomers found that the stars closer to the core are moving faster than the stars farther away. The measurement implies that some unseen matter at the core is tugging on stars near it.

By comparing these results with standard models, the astronomers determined that the most likely cause of this accelerating stellar traffic jam is the gravitational pull of a massive, dense object, the astronomers said. They also used models to calculate the black hole’s mass.

Although the presence of an intermediate-mass black hole is the most likely reason for the stellar speedway near the cluster’s center, the astronomers said they have not ruled out a couple of other possible causes.

Noyola and Gebhardt will use the European Southern Observatory’s Very Large Telescope in Paranal, Chile to conduct follow-up observations of the velocity of the stars near the cluster’s center to confirm the discovery.