March 3, 1998
Analyzing an image taken by NASA's Hubble Space Telescope, Devereux discovered what appears to be a dark, dusty disk surrounding the unusually bright nucleus of the galaxy known as NGC 1350. He expects further observations of the disk, including measurements of its rotational speed, will confirm the existence of a black hole at the center of the galaxy.
"These nuclear disks are quite rare," Devereux said. "Only three others have been discovered. They are important because measuring how fast the disk is rotating allows you to set constraints on the existence of a black hole at the nucleus. The speed of the disk betrays the mass of the black hole."
A black hole is an object so massive yet compact that not even light can escape its gravitational pull. "Black holes have been speculated about for years, but only with the advent of the Hubble Space Telescope have we been able to make the quantitative analyses to confirm their existence," Devereux said.
The orbiting telescope, which sees the universe without the distortion caused by Earth's atmosphere, is providing astronomers with images of unprecedented clarity. Devereux said he and his collaborators will submit a proposal to NASA for observing time to use the Hubble telescope to get clearer images of the disk in NGC 1350 and a spectrum to measure its rotational speed.
A spectrum can be used to calculate the speed of astronomical objects because light waves from objects moving away from the observer shift toward the red end of the spectrum, while waves from objects moving toward the observer shift toward the blue end. The faster the movement, the greater the redshift or blueshift.
In the case of a rotating disk, light from the side spinning away from Earth would be redshifted while light from the side spinning toward us would be blueshifted.
Devereux said the disk in NGC 1350 is about 200 light-years in diameter, meaning it would take light 200 years to get across it. It probably consists of gas and dust, he said.
Devereux has posted a digital image of the disk on his World Wide Web site.
The gravitational field at the center of the galaxy is what causes the disk to spin. If the disk is spinning faster than the combined mass of the visible stars in the galaxy could cause, that would be evidence of an unseen mass -- a black hole.
That has been the case with the three other disks analyzed to date, Devereux said. Devereux discovered one of those disks, too, in the galaxy known as M81.
NGC 1350, a spiral galaxy visible from the southern hemisphere, is relatively close to Earth by astronomical standards. But at 54 million light-years, "we are talking about vast distances," Devereux said. By comparison, the closest star to our solar system is four light-years away.
He said NGC 1350 is interesting because its nucleus is unusually bright and appears to be elongated in both directions perpendicular to the disk surrounding it. The elongations suggest the presence of jets -- structures that are believed to be associated with black holes, he said.
"What we have now is the circumstantial evidence of a black hole," Devereux said. Further observations may provide the conclusive evidence.
The Hubble Space Telescope image shows what appears to be a dark, dusty disk surrounding the bright nucleus in a nearby galaxy. Such nuclear disks are rare, but very important because measurement of the disk's rotational speed allows a useful constraint to be placed on the existence of a "Massive Black Hole" in the nucleus. The disk is 200 lightyears in diameter and is the fourth such nuclear disk to be discovered with the Hubble Space Telescope. Image courtesy Nick Devereux, Salman Hameed & NASA.