The black hearts of the galaxies
A new analysis of X-rays from the brightly lit center of a galaxy supports the notion that an extremely massive black hole is hiding there. The investigation provides the clearest statements so far about the bizarre environment at the edge of the singularity; from them one can infer the existence of other superheavy black holes in other galaxies. The Hubble Space Telescope has previously found evidence of massive black holes at the centers of some galaxies: stars moving as if caught in a powerful suction. Two years ago, scientists used X-ray telescopes on Japan's ASCA satellite to get a more direct look at the scene. The satellite received radiation from iron atoms moving at an amazing one-fifth the speed of light. They came from a flat disk of gas at the center of galaxy MCG-6-30-15. There appears to be an enormous concentration of mass in the center of the disk. However, in order to be able to decide whether this is a dense star cluster or a black hole, the researchers had to know the extent of the gas accumulation. Their size depended heavily on the assumptions made about the nature of the cloud.
The theorist Benjamin Bromley from the Harvard-Smithsonian Center for Astrophysics and his colleagues have now been able to dispel these uncertainties. They examined the spectrum of the iron atoms of MCG-6-30-15 using a different analysis method, which allowed the distance of the atoms from the center of the disk to be calculated directly, independent of the shape and other properties of the gas disk. They concluded that the rapidly rotating inner region of the disk can be at most 2.6 times the size of a black hole with the mass of ten million suns. Because of this good agreement, Bromley says, "We really need to discard all models other than the superheavy black hole."
Astrophysicist Shuang Zhang of the NASA Marshall Space Flight Center agrees. What's more, he adds, the X-rays offer an amazing first glimpse of the black hole's possible spin -- Bromley's team estimated its speed to be around a quarter of what Einstein's general theory of relativity allows. "Black holes only have three properties: mass, spin and charge," says Zhang. "It's very exciting to pinpoint two of the three for a supermassive black hole."