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Ring around a black hole.

IT'S NO SECRET that supermassive black holes are thought to dwell in the centers of most galaxies, but what might be less well known is that only two such dark monsters have been directly proven to exist. Now, thanks to some blue stars in a very unlikely place, make that three.

The Andromeda Galaxy (M31) has long been known as a likely candidate for harboring a central black hole. In the late 1980s astronomers found stars and gas moving very rapidly in M31's core, suggesting the presence of a large central mass. The nucleus emits the radio waves and X-rays expected from an accretion disk of gas spiraling into a massive black hole. In addition, for almost a decade astronomers have noted mysterious blue light at the core. Using Hubble's Space Telescope Imaging Spectrograph (STIS), astronomers have now traced this blue glow to a flat disk of more than 200 young, hot stars that live only a half light-year from the nucleus.

As reported in the September 20th Astrophysical Journal, Ralf Bender (University Observatory, Munich, Germany, and MPE, Garching), John Kormendy (University of Texas, Austin), and their team measured orbital velocities in the stellar disk by the redshifts and blueshifts observed on opposite sides of the central region. The young stars revolve around the black hole much the way the planets in our solar system revolve around the Sun. But these stars feel a much stronger gravitational pull and orbit with velocities averaging 1,000 kilometers (620 miles) per second. To explain these breakneck speeds, this disk requires a central mass of 140 million Suns--twice as much as previously thought. Such a dark mass in such a tiny volume rules out any known possibility other than a black hole.

Other theoretical possibilities mainly involve a vast number of dark stars such as neutron stars or brown dwarfs that were dumped into the galaxy's center. But creating a dark cluster in M31's nucleus as defined by the blue star ring would be like crowding 140 million solar masses into a space 1 or 2 light-years across. "Such a dark cluster would get into all kinds of trouble," says Kormendy. "Before they died, the dark remnants' progenitors would hit each other. They and their remnants would make supernovae. Brown dwarfs would smash into each other. There would be all sorts of bad news. The simplest explanation is a big black hole." So far, dark clusters have been ruled out in two other black-hole candidate galaxies, M106 in Canes Venatici and our Milky Way.

Meanwhile, astronomers still can't explain how the ring of blue stars could form around the black hole. The massive stars seem to have formed in a burst of activity about 200 million years ago. But current star-formation theories say that this is impossible. For a star to form, a cloud of interstellar gas must collapse. This shouldn't happen with strong tidal forces from the black hole trying to rip any large gas cloud apart.

A study with NASA's Chandra X-ray Observatory has confirmed a similar population of young stars surrounding the Milky Way's central black hole. In a paper submitted to the Monthly Notices of the Royal Astronomical Society, Sergei Nayakshin (University of Leicester, England) and Rashid Sunyaev (Max Planck Institute for Astrophysics, Germany) argue that the Chandra results suggest that the stars formed in a dense gas disk just outside the black hole. The disk presumably had enough gravity to resist the black hole's tidal force, allowing stars to form near the abyss. Says Nayakshin, "In one of the most inhospitable places in our galaxy, stars have prevailed."
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Title Annotation:discovery of blue star ring in black holes
Author:Johnston, Lisa R.
Publication:Sky & Telescope
Geographic Code:1USA
Date:Jan 1, 2006
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