News Release

Astronomers, using the Hopkins Ultraviolet Telescope (HUT), were treated to a dazzling display in March when they observed the galaxy, called NGC 4151, about 43 million light years away in the general direction of the constellation Canes Venatici.

Active galaxies have unusually bright centers, possibly because they harbor massive black holes that generate tremendous energy. About 10 percent of all galaxies are active galaxies, but astronomers believe that most galaxies may exhibit similar energetic outbursts at a much lower level.

An active galaxy varies in brightness from time to time, and NGC 4151 happened to be unusually bright as astronomers observed it from the space shuttle Endeavour during the 17-day Astro-2 mission in March.

NGC 4151's unusual brightness enabled scientists to gather exceptionally detailed spectrographic data. By using a spectrograph to analyze ultraviolet light from the galaxy's center, astronomers have drawn a more precise map of NGC 4151's powerful core, which is 10 billion times more luminous than the sun, said Gerard Kriss, HUT's project scientist.

HUT is sensitive to a range of ultraviolet light called the far ultraviolet spectrum, which cannot be studied with other telescopes. It was one of three ultraviolet telescopes on the Astro-2 observatory, operated within the payload bay of Endeavour.

Kriss will present a paper on the work 10 a.m. EDT on Monday, June 12, 1995, during a meeting of the American Astronomical Society in Pittsburgh. The paper's title is "The Far Ultraviolet Spectrum of NGC 4151 as Observed with the Hopkins Ultraviolet Telescope on Astro-2."

Astronomers believe that all active galaxies may have the same basic structure. This "unified model" proposes that powerful radiation is produced near a central, supermassive black hole.

The black hole is surrounded by an opaque torus, or doughnut-shaped ring of material a few light years in diameter -- about the distance from our sun to the nearest star. Inside the torus, even closer to the center, are clouds of gas orbiting the black hole, an object so massive not even light can escape its gravitational attraction.

Hot gas, heated by intense radiation near the center of the galaxy, blows out through the ring-like accumulation of gas clouds and the torus that surrounds it. The wind of blowing gas and the radiation is thus shaped as it exits.

If the galaxy is viewed from the side, astronomers see two illuminated cones of material emanating from each side of its center. But when seen face-on the active galaxy is simply a bright dot, since astronomers are looking down the center of the doughnut hole.

Astronomers are unsure whether the escaping radiation and hot gas are shaped into cones by the torus, or whether dense gas closer to the black hole is responsible.

Now, HUT's precise data about the density and ionization of the gas are helping to refine the model, and astronomers are leaning more toward the latter scenario.

The data have revealed that the innermost gas is very thick yet highly ionized -- its atoms have been stripped of many of their electrons by intense radiation. In order for such dense gas to be so highly ionized, it must be located close to the central engine of radiation, Kriss said.

Hopkins astronomers used HUT to observe NGC 4151 during the Astro-1 mission, in December 1990 on the space shuttle Columbia. But the galaxy was five times brighter during Astro-2, enabling scientists to gather more precise data. Another factor was the telescope itself: improvements to HUT made it about three times more sensitive than its Astro-1 version.

Because most ultraviolet light is filtered out by the ozone layer in the Earth's stratosphere, astronomers had to place the instruments in space. Many stars and other astronomical objects reveal a complex story about their history and composition through the ultraviolet radiation they emit and absorb.

EMBARGOED FOR 10 A.M. EDT MONDAY, JUNE 12, 1995

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