Black Hole Discovery Leads to New Field of Study
Astronomers estimate that the black hole's mass is equivalent to 1.2 billion stars like the sun, all compressed into a space the size of a fingernail.
The black hole is surrounded by a disk of dust and gas spinning at speeds of up to 1 million miles per hour. It was discovered by Johns Hopkins graduate student Laura Ferrarese, Hopkins astronomer Holland Ford and astronomer Walter Jaffe, of Leiden University in the Netherlands. The astronomers will present their findings during a news conference on Dec. 4 at the European Space Agency in Paris.
They first spotted the spiraling disk several years ago. But they couldn't be certain that the disk surrounded a black hole until August 1995, when they used Hubble's Faint Object Spectrograph to determine the velocity of the speeding disk. Knowing the velocity enabled scientists to calculate the mass of the object at the disk's center, confirming the presence of a black hole; there is about a thousand times more mass near the disk's center than can be accounted for by the number of stars present, Ferrarese said.
The black hole is in the center of an elliptical galaxy called NGC 4261, in the constellation Virgo. The disk surrounding the black hole has a diameter of about 800 light years -- that's 200 times the distance between the sun and its closest neighbor star.
It's the third confirmed black hole. Only a year ago, the phenomena were considered hypothetical, but the recent discoveries have ushered in a new field of astronomy research -- "black hole demographics," Ford said.
The question is no longer: Do black holes exist?
"We are now asking questions like, 'Does every galaxy have a massive black hole in its center?'" Ford said. "'What are the masses of black holes? How did they get there?'"
The breakthrough came in 1994, when Ford and other astronomers used the Hubble telescope to find a gigantic spiral of ionized gas surrounding an elliptical galaxy, called M87. It is not possible to see a black hole directly. But to the astonishment of astronomers, the black hole in the center of M87 is surrounded by a disk of spiraling gas, something that had not been predicted.
It was the break they needed to confirm the existence of black holes. The other black hole discovery also came in 1994. Astronomers using a radio telescope detected a disk surrounding a black hole with a mass of 40 suns in the center of a spiral galaxy called NGC 4258. Now, with NGC 4261, the focus is rapidly changing from whether black holes exist to studies aimed at solving mysteries surrounding the enigmatic objects.
"These are clearly remarkable creatures," Ford said. "We are talking about 1 billion stars like the sun smashed down to a volume the size of your thumbnail."
It is becoming clear, however, that different black holes have unique signatures and characteristics.
The NGC 4261 black hole, for example, is surrounded by a disk of dust and ionized gas. But the disk surrounding M87's black hole has no dust, and it contains more ionized gas than does NGC 4261's disk. M87's disk also is marked by jets and tendrils of gas spiraling away from its center, which are not present in the NGC 4261 disk.
Astronomers do not yet know why the two disks are different. And they don't understand how the spiral disk of dust developed around the black hole in NGC 4261.
A new spectrograph, scheduled to be installed in the Hubble telescope in 1997, will help astronomers answer such questions, Ford said.
The research was funded by NASA.
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