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NGC 5128, Centaurus A, a radio galaxy

Date created: 1987-08-06

Tags: galaxy, spiral, elliptical, dust, supernova

The galaxy in Centaurus, now known as NGC 5128 was first discovered by James Dunlop at the Parramatta Observatory near Sydney in 1826. NGC 5128 is also a powerful radio source known as Centaurus A. It is a most unusual object, an elliptical galaxy crossed by a dust lane. Elliptical galaxies are usually almost featureless and almost dust-free, while galaxies with pronounced dust lanes are usually spirals. The elliptical-like, uniformly bright portion is composed of several thousand million stars, most of them old and yellowish. The prominent dust lane obscures and reddens the light of stars behind it, though some younger, blue stars can be seen at the edges of the dust cloud, indicating recent star formation, which is not expected in an elliptical galaxy.

One of the nearer galaxies, NGC 5128 is about 10 million light years away, and host to the most powerful nearby radio source. This remarkable galaxy is also a copious source of X- and gamma rays as well as visible and infrared radiation. These are characteristics of an 'active' galaxy, one where the massive black hole present in most large galaxies is actively accreting material from its surroundings. This intense activity is probably the result of the merger of a dusty spiral with an elliptical galaxy, an explanation that accounts for this galaxy's optical appearance.

The curious green star in the dust lane appears because the blue- and green-light plates were taken when supernova 1986g had just appeared and was bright, while the red-light plate was taken over a year later, after the supernova had faded. Finally, NGC 5128 has a huge, faint halo of stars seen in this very deep image made using special photographic techniques.

Photograph made from plates taken in March, 1979 (G, R) and March 1980 (B). Top left is NE. Image width is about 16 arcmin.

Credit: David Malin

© Australian Astronomical Observatory