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Huge black hole may lurk in nearby galaxy.

Huge black hole may lurk in nearby galaxy

Three years ago, a trio of astronomers began observing a celestial object about 300 million light-years from Earth, the likely product of a collision between two galaxies. They had hoped to study the puzzling source of its brilliant, infrared light. Instead, they stumbled onto a far weightier enigma: evidence hinting at the possible presence of the most massive black hole ever postulated to reside within a galaxy. Black holes are dense, compact objects believed to exist but never definitively detected.

Jonathan Bland-Hawthorn of Rice University in Houston and his two co-workers maintain that the simplest explanation for their observations appears to be a black hole as massive as all the visible stars in the Milky Way, yet compressed into a region just one-ten-thousandth our galaxy's volume. But they agree with other researchers that an unusual feature in the relatively nearby galaxy, named NGC 6240, may have more mundane explanations.

Indeed, Francois Schweizer of the Carnegie Institution of Washington (D.C.) says, "I think there's only a one in 10 chance there's a black hole there."

The astronomers embarked on their odyssey using a special instrument, a Fabry-Perot interferometer. Acting like a highly selective filter, it uses the wave-like properties of light and a variable gap between two polished mirrors to pick out whatever visible-light wavelength the astronomers choose to view. Attached to the 2.2-meter University of Hawaii telescope on Mauna Kea, it enabled Bland-Hawthorn and his colleagues to simultaneously chart the velocity of hydrogengas atoms throughout much of the telescope's field of view, allowing the first visible-light map of a predominantly infrared-emitting galaxy.

In the April 10 ASTROPHYSICAL JOURNAL LETTERS, the scientists report evidence for two rotating disks of gas in this galaxy. One disk orbits around two light-emitting centers at a speed governed by the mass of the ordinary stars and gas within it. In contrast, measurements from another region some 19,000 light-years away show evidence for a second disk with unusual properties.

The team did not directly view the second disk, but deduced its existence from velocity measurements indicating the presence of a rotating body of gas. From its outer to its inner edge, the gaseous disk increases its rotational speed by more than 400 kilometers per second, the researchers found. They also noted a rise in luminosity toward its center, an indication that more hydrogen gas clusters there.

After a colleague confirmed their results last year, the researchers used elementary physics to deduce the gravitational tug of an extremely massive, dark and compact object -- between 40 billion and 200 billion solar masses -- hidden in the region enclosed by the disk.

Cramming severl trillion brown-dwarf stars or neutron stars into the tiny region enclosed by the disk would also explain the findings, Bland-Hawthorn says. But he suspects such a concentration of stellar material would not survive without collapsing into a black hole.

William C. Keel has also extensively studied NGC 6240. An astronomer at the University of Alabama in Tuscaloosa, he notes that the character of this galaxy -- believed to be in the final throes of forming from the merger of two others -- must be carefully considered in interpreting the current work. The black-hole scenario depends on the assumption that Bland-Hawthorn's team really detected a rotating disk of gas, he explains. "If they're being fooled, if the motion in this merging system [merely looks like a disk], then this is just a case of inappropriate interpretation."

Schweizer says several possible discrepancies point to another explanation. Noting that the researchers admit in their article that parts of the second disk have a relative drop in velocity that can't be explained by simple, planet-like motions, he questions their ability to deduce the presence of a black hole. Instead, Schweizer suggests that material blown out radially from a common center might account for the velocities measured by the researchers. To date, neither Bland-Hawthorn nor co-worker Andrew S. Wilson from the University of Maryland in College Park have ruled out such a possibility.

Bland-Hawthorn told SCIENCE NEWS his team hopes to study NGC 6240 with the X-ray satellite ROSAT. Together with higher-resolution observations they made from Mauna Kea last month, these new data might resolve the galactic mystery by the end of the year.
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Author:Cowen, Ron
Publication:Science News
Date:Apr 20, 1991
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