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Hubble finds an off-center black hole.

Over the past few years, astronomers have gathered compelling evidence that black holes lurk at the heart of several galaxies. However, the latest finding has caught researchers by surprise: The newest unseen monster lies slightly askew.

Instead of residing at the exact center of the elliptical galaxy NGC 4261, the suspected black hole lies slightly to one side, astronomers reported at a Hubble Space Telescope workshop in Paris last week. But how did a black hole more massive than a billion suns move 9 light-years from the center, its presumed birth site?

Still considered hypothetical by many scientists, black holes represent a collapsed state of matter so extreme that not even light can escape their gravitational tug. Evidence over the past decade suggests that these invisible objects power the fireworks at the core of many active galaxies.

Astronomers have suspected for more than 15 years-ever since radio telescopes detected twin, oppositely directed jets of radiowaves streaming out of the galaxy's center-that NGC 4261 harbors a black hole. Last August, Hubble's faint-object spectrograph measured the rotational speed of a disk of gas and dust at the galaxy's core. The high velocity betrays the presence of a massive black hole, report Laura Ferrarese and Holland Ford of Johns Hopkins University in Baltimore and Walter Jaffe of Leiden University in the Netherlands.

Both the disk and the calculated location of the black hole lie slightly apart from the exact center of the galaxy. According to one theory, the location of the disk suggests that an intruder galaxy collided with NGC 4261 long ago. Material from the off-center disk falling onto the black hole may have propelled the hole away from the galaxy's center.

Douglas O. Richstone of the University of Michigan in Ann Arbor believes that the orientation of the disk rather than its location implies a past galactic collision. Because the disk lies perpendicular to the plane of the galaxy, stars in NGC 4261 could not have provided the gas to make the disk, he asserts. Instead, Richstone says, the gas must have come from a colliding galaxy.
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Title Annotation:research collected by the Hubble Space Telescope indicates that galaxy NGC 4261 may have a black hole that is slighty askew
Author:Cowen, Ron
Publication:Science News
Article Type:Brief Article
Date:Dec 16, 1995
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