The Caldwell Objects

Harvard College Observatory's 24-inch Bruce
refractor, and a 10-hour exposure taken with
Harvard's 60-inch reflector in South Africa in
1934, led John S. Paraskevopoulos to conclude
that NGC 5128 could not be an external galaxy.
Indeed, Edwin Hubble himself detected gaseous
emission lines emanating from the object, and for
this reason he classified it in 1922 as a local
nebulosity. But 10 years later Harlow Shapley
and Adelaide Ames listed NGC 5128 as an
irregular galaxy in their now-famous catalog.
Shapley explained his reasoning in his 1947 work,
Galaxies. NGC 5128, he said, is "a 'pathologic'
specimen — one of the external galaxies with [a]
peculiar spectrum. Any fully successful theory of
galactic structure must take into account such
abnormal forms." By then even Hubble had
conceded that NGC 5128 was truly abnormal.
Two years later, in 1949, it was suggested
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that NGC 5128 was identical with the strong
radio source known as Centaurus A, which blasts
out 1,000 times as much radio energy as our
Milky Way emits. The galaxy's strongest radio
emission originated in its dust lane, while a
weaker source discovered in 1954 radiated from a
nearly circular area, 2° in diameter, centered on
the galaxy's core. Then in 1958, during a Paris
Symposium on Radio Astronomy, C. A. Shain
announced that this unusual radio galaxy had an
X-ray jet and large radio lobes shooting out in
opposite directions at large angles to the dust
lane. Walter Baade and Rudolph Minkowski
explained all these outstanding discoveries by
proposing that NGC 5128 was the violent
aftermath of a collision between an elliptical
galaxy and an edge-on spiral galaxy.
That theory fell in and out of favor over the
years, with some arguing that the galaxy is
The Caldwell Objects 307