Carnegie Institution

Carnegie Institution

News Release

Contact Luis Ho at the Carnegie Observatories, 626-304-0248, e-mail lho@ociw.edu; or Nancy Davis, Carnegie Observatories External Affairs, 626-304-0270, e-mail ndavis@ociw.edu

New class of black holes discovered

Pasadena, California. A 30-year hunt for a new class of black holes has finally paid off in two separate searches. For some time scientists have known about supermassive black holes at the centers of galaxies, and small, stellar-size black holes, which result when stars about 10 times the mass of the Sun collapse after exploding as supernovae. Now, in two different studies, researchers have found two intermediate-mass black holes—one that is 4,000 times the mass of the Sun in our Milky Way Galaxy, and another at 20,000 solar masses found in the neighboring galaxy Andromeda. “It is very exciting to finally find compelling evidence that nature knows how to make these strange beasts,” says Luis Ho of the Carnegie Observatories in Pasadena, California, codiscoverer of the larger object.

Both of these mid-size black holes are located at the core of globular clusters, systems with up to a few million very old stars that orbit galactic centers. This is the first time black holes more massive than 10 solar masses have been detected in globular clusters. In both cases, the scientists used the Hubble Space Telescope (HST) for their observations.

Supermassive black holes in the centers of galaxies weigh between one million to several billion times the mass of the Sun and are the driving force behind “active galaxies” and quasars, objects so energetic they can be seen to the edge of the universe. The stellar-size black holes are thought to be responsible for gamma-ray bursts and certain types of X-ray binaries. The 20,000-solar-mass object was found in the globular cluster G1. At about 10 million Suns, it is one of the most massive globular clusters known, and is located 2.2 million light-years from Earth in Andromeda. It was found by Karl Gebhardt of the University of Texas at Austin, Michael Rich of the University of California at Los Angeles, and Luis Ho at the Carnegie Observatories. The researchers report their results in the October 10 issue of Astrophysical Journal Letters. “People have speculated on the existence of nonstellar black holes in globular clusters for a long time,” says Ho. “Now we’ve found the evidence.”

Black holes cannot be seen. They are typically discovered by the energy emitted in the form of X-rays as gas swirling down the hole heats up to tremendously hot temperatures. These new black holes, however, do not emit X-rays because very little gas is available in old globular clusters. Instead, the scientists found them by examining the rapid motions of stars swarming around the clusters’ cores, as they are drawn by the intense gravitational pull of a central dark object. The unseen object is presumed to be a black hole because of its small size and large mass. Ho explains: “To measure the gravitational influence of the black hole, we have to get very close to the center—the closer the better. That’s why we had to use HST. Its high resolving power is absolutely crucial for this experiment.”

The scientists hope that the new type of black hole may be able to shed light on other questions. According to Rich, “Not only will we learn about the formation of the black holes, but these new data from Hubble help us connect globular clusters to galaxies, providing information on one of the most important unsolved problems in astronomy today: How galaxy structure forms in the universe.”

Research in recent years has revealed that the formation of supermassive black holes must be very intimately connected with the formation and evolution of galaxies. One such clue comes from the discovery that the masses of black holes in galactic nuclei, mostly measured with HST, represent a roughly fixed fraction of the total mass of the galaxy. “It is not much, only about half a percent,” states Ho. “But the astonishing fact is that the fraction is constant from galaxy to galaxy, suggesting that there is some very fundamental link between the central black hole and the galaxy in which it lives. There is no shortage of ideas of how this might come about, but there is yet no consensus on which is correct.” It is even more astonishing that the two new globular-cluster black holes, hundreds of thousands times lighter than the most massive black holes known, fall exactly on the same trend.

Perhaps these new objects represent an adolescent step in black hole formation, eventually growing into the massive galactic black holes. “There are two main theories of black hole formation,” says Gebhardt. “You could either make the black hole all at once when the galaxy is forming by dumping a lot of material in the middle, or you could start with a seed black hole that subsequently grows over time. The observational evidence now points to the idea that you start out with a small seed black hole.”

These issues will be hotly debated in Pasadena in late October (20-25), where over 150 of the world’s top astronomers and astrophysicists will attend an international symposium entitled “Coevolution of Black Holes and Galaxies.” This symposium, the first of a series of four, was organized by Ho in celebration of the centennial of the Carnegie Institution and its Observatories, headquartered in Pasadena. The work of Luis Ho is supported by the Carnegie Institution and by NASA.

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The Carnegie Institution (www.CarnegieInstitution.org) has been a pioneering force in basic scientific research since 1902. It is a private, nonprofit organization with six research departments in the U.S.: Plant Biology, Global Ecology, Embryology, Geophysical Laboratory, Department of Terrestrial Magnetism, and The Observatories.