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In 1847 Sir John Herschel, working at Cape Town, first reported observations
of Centaurus A, now also known as NGC 5128. He described it as two semi-ovals
of elliptically formed nebula appearing to be cut asunder and separated by a broad
obscure band parallel to the larger axis of the nebula.
For the next hundred years astronomers paid very little attention to it as yet another
nebulous object. In the 20th century many of these nebulous objects were identified
as galaxies of stars. Some astronomers doubted whether these "galaxies" were
actually beyond the boundaries of the Milky Galaxy. Radio astronomy gave the
final answer that these objects were indeed galaxies exterior to the Milky Way.
Using the 80-foot radio antenna at Dover Heights, Australia, G Stanley and Bruce
Slee were the first to identify Centaurus A as a powerful radio galaxy.
In 1954, using the Mt Palomar 5-metre-telescope, Walter Baade and Rudolph
Minkowski confirmed that NGC 5128 is indeed a galaxy. They proposed that the
peculiar structure is the result of a merger between two galaxies, a giant elliptical
and a small spiral, the dark dust lane across the object being the spiral galaxy which
is being swallowed up by the giant elliptical galaxy.
Between 1969 and 1971 Stuart Boyer, using a sounding rocket, detected X-rays
emanating from Centaurus A. This was confirmed by the Uhuru satellite.
In 1971 Bill Kunkel and Hale Bradt, at Cerro Telolo in Chile, observed infrared light
originating from a compact source in the galaxy's core. The Netherlands Orbiting
Astronomical Satellite observed changes in the intensity of the X-ray emissions,
of very short periodicity, indicating that the source of the X-rays was confined
to a very small region of space. This suggested that a black hole could be the source
of the X-rays, the black hole being located in the centre of the elliptical galaxy.
In 1975-76, using information from the SIGMA satellite and high-flying balloons,
John Grindlay and R D Hall detected gamma rays coming from the nucleus of the
galaxy. Using the CTIO-TELESCOPE, Victor Blanco discovered a faint jet being
emitted from the galaxy, as well as the existence of blue stars.
Centaurus A had become known as a great enigma.
In the late 1970's John Graham, using the CTIO telescope, found a series of faint
shells of gas in the outer regions of the galaxy. They could have been produced
by collisions of gas from a galaxy merger.
In 1979 Ethan Schreier, using the Einstein Orbiting Observatory, discovered an
X-ray jet emanating from the nucleus and the Very Large Array radio telescope in
New Mexico found that the jet had a radio counterpart.
By 1996 Schreier, using the Hubble Space Telescope's Wide Field and Planetary
Camera 2 ( WFPC2 ) found blue stars in the dust lane, unmistakeable evidence of
star formation taking place. Elliptical galaxies do not have enough gas and dust for
star formation. The gas and dust were provided by the spiral galaxy which had
merged with the elliptical galaxy.
In 1997, K Kellerman, A Zensus and M Cohen, using the Very Large Array, found
that the core of Centaurus A is only 10 light years across, making it the smallest
known extragalactic radio source. They also noted that the energy of the radio
source varied daily and that the mass of this core had to be 100 million suns.
The Hubble Space Telescope has now offered stunning and unpredented close-ups
of the turbulent firestorm of starbirth along the nearly edge-on disc of dust girdling
Centaurus A. At a distance of 10 million light years, Cantaurus A is the nearest
active galaxy. The photographs taken by Hubble show looming storm clouds and
dark filaments of dust mixed with cold hydrogen, silhouetted against the incandescent
yellow-orange glow from hot gas and stars behind. Brilliant clusters of young blue
stars lie along the edge of the dark dust rift. Outside the rift the soft hazy glow of the
red giant and red dwarf stars of the elliptical galaxy pervade. The dusty disc is tilted
nearly edge-on to our line of sight, its inclination being between 10 and 20 degrees.
The dust lane has a rippled "washing board" appearance.
Images in blue, green and red, by Hubble show details as small as seven light years
across. The blue colour is due to the light from extremely hot, newborn stars. The
reddish-yellow colour is due partly to hot gas, partly to older stars in the elliptical
galaxy and partly to scattering of blue light by dust - the same effect as ruddy sunsets
on Earth.
The merging spiral galaxy is providing the fuel to the black hole at ther centre, which
already has the mass of a milliard solar masses ( 10 ~ solar masses ). This mass
indicates that the black hole may be the result of the merging of the original black holes
of the spiral galaxy and the elliptical galaxy.