Johns Hopkins Gazette: July 24, 1995

Hopkins Telescope Sharpens Focus On Center Of Galaxy

Emil Venere
Homewood News and Information

     A cosmic coincidence is helping astronomers refine their
knowledge of and test theories about the peculiar structure at
the center of an exceptionally bright galaxy, called an active

     Astronomers, using the Hopkins Ultraviolet Telescope, 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. HUT was one of three ultraviolet telescopes on
the orbiting observatory.

     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.

     The galaxy's 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, an associate research professor
in the Department of Physics and Astronomy.

     Kriss, HUT's project scientist, presented a paper on his
findings during a meeting of the American Astronomical Society in

     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, he

     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 ringlike accumulation of gas clouds
and the torus that surrounds it. The rushing gas and radiation
are forced into the shape of two cones as they exit opposite
sides of the galaxy.

     But up until now, astronomers have been 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 the
shaping mechanism. 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.

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