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Orbiting Hubble eyes active galaxy's disk.

The Hubble Space Telescope has captured the sharpest image ever obtained of a dusty disk of material encircling a galaxy's unusually violent heart.

Based on studies from the ground, astronomers speculate that the disk may fuel a suspected black hole at the galaxy's center. But even if the black hole connection doesn't prove correct, says Walter J. Jaffe of the State University of Leiden in the Netherlands, the Hubble image provides the closest look yet at the center of a type of galaxy -- called an active galactic nucleus (AGN) -- that has a luminous, energetic core.

The new image depicts a doughnut-shaped disk surrounding the core of the giant elliptical galaxy NGC 4261. Hubble's picture of the disk, whose outer edge measures 300 light-years across, is one of several taken by the telescope in an effort to probe the fireworks at the heart of a variety of AGNs. Jaffe, who analyzed the image with colleagues including Holland C. Ford of the Space Telescope Science Institute and Johns Hopkins University in Baltimore, announced the findings at a press briefing last week.

Jaffe and his team decided to include NGC 4261 in their Hubble survey of AGNs because of intriguing ground-based observations -- notably a pair of radio jets some 88,000 light-years long streaming out from the galaxy's core. Such activity suggests that a superheavy object -- possibly a black hole 10 million times the mass of the sun -- spews out energy from a tiny region at the galaxy's center. And although visible-light studies had only imaged the galaxy's core as a fuzzy blob, they did indicate that dust patches surround the center -- even though elliptical galaxies contain little dust.

Jaffe says he agrees that the disk of dust imaged by Hubble "doesn't prove any more than we've already known from the ground, that there's a black hole there." But, he adds, it suggests "we're seeing the fuel tank, we're seeing the carburetor... that are driving the central engine [black hole]." Compelling evidence of a black hole, he notes, awaits measurements of the orbital velocity of material near the galaxy's center -- a study Hubble can't perform until a late 1993 repair mission corrects its flawed optics.

John L. Tonry of the Massachusetts Institute of Technology says it remains uncertain whether the visible disk, lying tens of light-years from the galaxy's center, can indeed fuel the suspected black hole. Jaffe calculates it would take about a million years for material in the disk to get sucked into the hole.

The disk's size has surprised some researchers. Anne L. Kinney of the Space Telescope Science Institute suggests that large, cooler disks may circle AGNs that have relatively less luminous cores, such as NGC 4261, while smaller, hotter disks may orbit the cores of brighter, more quasar-like AGNs.
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Title Annotation:Hubble Space Telescope
Author:Cowen, Ron
Publication:Science News
Date:Nov 28, 1992
Words:461
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