Spectacular Star Cluster May Host Black Hole Missing Link (4/9/2008)

Figure 1. This artist's concept shows an exaggerated-size version of the intermediate-mass black hole that may exist at the center of Omega Centauri. It has the orbital lines of nearby stars drawn in for reference only. Close to the black hole, star motions are faster than those farther away. Such differential velocities are one telltale signature of a black hole's existence. Most of the cluster's stars are older, cooler ones, with a scattering of hotter, bluer stars mixed in. - Credit: Illustration by Lynette Cook for Gemini Observatory

The well-known naked-eye star cluster Omega Centauri may be home to an elusive intermediate-mass black hole. Observations made using Gemini Observatory and Hubble Space Telescope provide convincing evidence that such black holes do exist and could even lead to an understanding of how they might evolve into larger supermassive black holes like the ones found at the cores of many galaxies.

To deduce the existence of the Omega Centauri black hole, astronomers Eva Noyola (Max Planck Institute for Astrophysics) and Karl Gebhardt (University of Texas, Austin), relied on the combined power of ground-based and orbiting instruments. Using spectra obtained by the Gemini Multi-object Spectrograph (GMOS) at Gemini South in Chile and archive images produced by the Advanced Camera for Surveys on Hubble Space Telescope, they measured the motions and brightness of stars at the heart of this massive cluster in a two-pronged approach that indicated the existence of something very massive hidden among the cluster's stars.

"Finding a black hole at the heart of Omega Centauri could have profound implications for the past history of the cluster itself," said Noyola. "Intermediate-mass black holes like this could be the seeds of full-sized supermassive black holes. We may be on the verge of uncovering one possible mechanism for the formation of intermediate-mass black holes."

Noyola's observations, made as part of her Ph.D. thesis research under Gebhardt's direction at the University of Texas, show that there is non-luminous matter at the center of Omega Centauri that is on the order of 40,000 times the mass of the Sun. "If it is a black hole, it's larger than a stellar black hole but not as large as the supermassive variety," she said.

Since supermassive black holes are well known to exist in the cores of galaxies, and stellar-mass black holes are found scattered throughout galaxies, astronomers have long sought to find conditions where black holes with masses between these two extremes could form and evolve. "If one was to find a "minuscule galaxy", that would be a good place to look for an intermediate-size black hole," said Noyola

According to Noyola and Gehbhardt, these kinds of black holes could turn out to be "baby" supermassive black holes. "They may be rare and exist only in former dwarf galaxies that were stripped of their outer stars," said Gebhardt. "They could also be more common than we expect, existing at the centers of globular clusters as well. If this is true, then they could provide numerous seeds necessary to grow supermassive black holes in the centers of larger galaxies."

To search out for the existence of a black hole in Omega Centauri, Noyola and Gebhardt used GMOS to take spectra of stars in the very center of the cluster. Those measurements gave radial velocity information for stars in and around the location of the suspected black hole. "The advantage of using this particular instrument is that we can get velocity information in 700 mini-regions in our field of view," said Gebhardt. "It allows us to perform more sophisticated analysis than with ordinary spectrographs."

Noyola pointed out that the results showed a considerable rise in stellar velocities between a region near the center and the very center of the cluster. "We then used an observation of the same region made by the Advanced Camera for Surveys to help us estimate how many stars are in the central region and their masses."

Noyola and Gebhardt calculated the expected radial velocities of the visible stars making the assumption that there was no extra matter there. Then they compared their calculations to the GMOS measurement. From that detailed analysis, they found 40,000 solar masses of non-luminous matter at the center of Omega Centauri. That object is very likely Noyola's intermediate-mass black hole. Its strong gravitational influence is causing nearby stars to move substantially faster than stars farther away from the core. The spectra also hint that this black hole is not in an aggressive, matter-eating stage, like others found in the hearts of galaxies. "This is one of the quietest black holes found to date" said Noyola. "We see no evidence for accretion of matter in our spectra."

Note: This story has been adapted from a news release issued by the Gemini Observatory

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