Hubble shows Milky Way's neighbor feasting Black hole at center of nearby galaxy appears to be gulping small galaxy

May 15, 1998|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

WASHINGTON -- Feeding a black hole the size of our solar system, it turns out, is as simple as tossing it an occasional galaxy.

Yesterday, scientists from the Space Telescope Science Institute in Baltimore released spectacular photos of a vast galaxy called Centaurus A, snapped as it swallows the scraps of a much smaller galaxy that first blundered into its path 500 million to 1 billion years ago.

The Hubble Space Telescope's infrared camera detected what looks like remnants of the smaller galaxy deep inside the larger galaxy, spiraling into the gullet of a giant black hole, like suds down a tub drain.

"There is very strong evidence from the Hubble Space Telescope, and other telescopes, that there are monsters lurking at the center of many galaxies," said David S. Leckrone, senior project scientist for Hubble at NASA headquarters at Goddard Space Flight Center. The new photos "allow astronomers, for the first time, to peer into the monster's cave."

A black hole is an object so massive that nothing that gets near it can escape, not even light.

Solving the mystery of what happens close to a black hole has been one of the space agency's top goals for Hubble. Its new infrared camera can see through cold dust and gas to the heated structures that surround black holes at the center of certain "active" galaxies -- a sort that spews torrents of X-rays and radio waves into space.

The discoveries, by astronomers Ethan J. Schreier of the Space Telescope Science Institute and Alessandro Marconi of the Astrophysical Observatory of Arcetri in Florence, Italy, are to appear in the June 1 edition of Astrophysical Journal Letters.

Centaurus A is an eerie, glowing blob composed of hundreds of billions of stars. It is five to 10 times the size of the Milky Way.

On a clear night, it is visible to the naked eye from Earth's Southern Hemisphere. A mere 10 million light-years from our Milky Way galaxy, it is the closest active galaxy to Earth.

(A light-year is the distance traveled by light in one year, about 5.9 trillion miles.)

In 1847, pioneering astronomer Sir John Herschel described it as a glowing elliptical nebula "appearing to be cut asunder and separated by a broad obscure band."

Ground telescopes have since revealed that the "band" is a dark lane of dust and gas that circles the galaxy like a beauty queen's sash.

Images of Centaurus A are so striking that one was used as the background for the credits at the end of television's old "Twilight Zone" program, said Bruce Margon, an astronomy professor at the University of Washington in Seattle. "You just look at it and it's this evil caldron that says, 'Don't step here,' " he said.

Elliptical galaxies are typically old and settled, with little gas and dust remaining amid their stars. The dust across Centaurus A suggests that it was in a relatively recent collision with a dusty, spiral galaxy like the Milky Way.

"It's the spiral galaxy that fell in that supplied all the dust we see," Schreier said.

Blue stars at the edges of the dust lane in the Hubble photos, he said, are hot young stars ignited by the compression of dust and gas during the collision. Only tens of millions of years old, they are evidence that the galactic wreck was recent.

But Hubble found more evidence deep inside the galaxy.

X-ray and radio telescopes have revealed that Centaurus A is bracketed by a pair of jets, like fire hoses, shooting intense plumes of radiation in opposite directions. Such jets are common to many active galaxies, and are probably powered by huge black holes at their core.

Centaurus A's black hole -- as broad as our solar system and with a mass equivalent to a billion suns -- can't be seen. But with Hubble's Near Infrared Camera, Schreier and Marconi were able to peer through the dust into the galaxy's center, focusing on the structure near the core as small as seven light-years across. That's less than the distance from our sun to the nearest star.

There they discovered a small, bright nucleus surrounded by a glowing disk of matter -- probably the stars and other matter from the spiral galaxy swirling in toward the black hole.

In theory, the disk should be perpendicular to the radio jets pouring from the poles of the spinning black hole. Instead, the matter is swirling in at an oblique angle.

Astronomers aren't sure what's going on. But they suspect that as the captured matter spirals nearer to the black hole, it is being yanked out of the disk they can see, into a smaller one they can't see. And that smaller, inner disk probably is perpendicular to the jets.

More study may provide an explanation, Margon said. "It's going to keep people busy for quite a while."

Pub Date: 5/15/98

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