Finger of light. Diagram shows light emerging from dust falling into a candidate black hole as matter is sucked inward (based on Hubble Space Telescope images). The tall cone perpendicular to the accretion disk is a radio jet.

A Light-Fingered Black Hole

Two astronomers in Germany may have gotten a look at the intimate surroundings of a giant black hole at the center of another galaxy, according to a report in yesterday's issue of The Astrophysical Journal. By peering with the Hubble Space Telescope's Faint Object Camera, the pair spotted a fingerlike structure glowing with ultraviolet light. They interpret it as light from very near the black hole gleaming off a disk of dust and gas swirling around this vast concentration of mass. Other astronomers, however, say the light could be coming from young stars within the dust cloud, not from the black hole.

Evidence for black holes is indirect, such as stars swirling around invisible concentrations of mass, because light cannot escape their powerful gravity. The giant black holes at the centers of galaxies also hide within doughnut-shaped clouds of dust. Along the inner edge of the doughnut, astrophysicists believe, matter flattens into an accretion disk as it spins into the black hole. While studying one galaxy believed to host such a black hole--NGC 6251, which lies 300 million light-years away in the constellation Virgo--Phillippe Crane and Joël Vernet, astronomers at the European Southern Observatory (ESO) in Garching, Germany, stumbled across the strange, fingerlike structure.

This structure is 900 light-years long and powerfully emits ultraviolet (UV) light. "We have never seen anything like that before," says Vernet. He believes it might be light from matter just before it falls into the black hole, scattered by particles in the accretion disk or by a section of the outer dust cloud. The ESO team now plans to measure the velocity of the gas and dust masses swirling around the black hole--data that would make it possible to calculate its mass--by observing Doppler shifts in spectral lines.

However, Andy Fabian of the Institute of Astronomy at Cambridge University in the United Kingdom isn't sure that the UV light is from dust swirling into a black hole. He notes that only 10% of the UV light is polarized--less than expected if it is scattered by dust. Instead, he says, the light might emanate from stars that formed in the accretion disk. "I am not sure that seeing any UV light from a nucleus of a galaxy actually tells us that there is a black hole in the center," he says.