Johns Hopkins Gazette: July 10, 1995

Space 'Hurricane' Blowing At 450,000 MPH
Heavy Elements Found In Wind From Distant Galaxy

Emil Venere
Homewood News and Information

     Astronomers have discovered a galaxy-size "hurricane"
blowing gas at speeds of 450,000 miles per hour--providing one
possible explanation for a cosmic mystery.

     Chemical elements such as carbon, nitrogen, oxygen and iron
are produced inside stars and dispersed in space when stars
explode. These "heavy elements" are deposited in regions near
stellar explosions; how then, could they be found in the space
between galaxies, where there are few, if any, stars?

     The "superwind," speeding out from the center of a galaxy 86
million light-years away, is offering a clue.

     "This tells you that, at some moment in the evolution of the
galaxy, you can have an event that blows out away from the galaxy
a significant amount of enriched gas," said Zlatan Tsvetanov, an
associate research scientist in the Department of Physics and
Astronomy. "This would be a powerful way to enrich the gas with
heavy elements, at least in the relatively immediate surroundings
of the galaxy."

     The ionized gas is streaming out from the center of a galaxy
in the southern sky called NGC 2992. It is speeding away in the
shape of two cones, or bowls, one from either side of the
galaxy's center, extending more than 20 kiloparsecs, or 62,000
light-years, from the galaxy's core.   

     "The gas blows away from the galaxy, in each direction, just
like a very strong wind," Tsvetanov said. "That's why I called it
a hurricane."

     The wind probably originates somewhere in the center of the
galaxy, but astronomers aren't sure what the driving force is.
One possibility is a powerful starburst--the simultaneous birth
of many stars--which releases a tremendous amount of energy.
Another possibility is that NGC 2992 might be a Seyfert galaxy, a
type of galaxy that has an exceptionally bright, compact center
producing as much light as an entire normal galaxy of stars,
suggesting the presence of a massive black hole in the nucleus. A
black hole could produce the enormous energy required to drive
the wind, Tsvetanov said.

     The Hopkins astronomer, along with Professor Michael Dopita
and graduate student Mark Allen, both from Australian National
University, wrote a scientific paper on the research, which was
presented June 14 during a meeting of the American Astronomical
Society in Pittsburgh.

     NGC 2992 is about the size of the Milky Way. It contains a
huge mass and produces a powerful gravitational attraction. That
means a huge amount of energy would be needed to drive the gas
away from the galaxy at such high speeds seen in the superwind.
     Astronomers think NGC 2992 is a spiral galaxy, like the
Milky Way. But they can't be sure, because the galaxy does not
face the Earth; astronomers see it edgewise, which makes it
possible to observe the two cones of superwind extending from
each side of the galaxy's disk.

     The astronomers used spectrographs and cameras on three
telescopes to observe the galaxy: the Anglo-Australian telescope
and the ANU 2.3-meter Advanced Technology Telescope, both at
Siding Springs, Australia, and the New Technology Telescope at
the European Southern Observatory in La Silla, Chile. The
superwind's gas is highly ionized--its atoms have been stripped
of many of their electrons by powerful radiation. Astronomers
viewed the galaxy through two filters, enabling them to isolate
gas in particular ionization states at numerous positions across
regions of gas.

    "We basically mapped the region with spectroscopy," Tsvetanov
said. By making precise spectrographic measurements of the
radiation emitted by the gas, the scientists were able to
calculate how fast the gas is moving. The wind is speeding at 150
to 200 kilometers per second (335,000 to 450,0000 mph) in either
direction from the galaxy's center.

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