NGC 5128 - Centaurus A

Centaurus A, also known as NGC 5128, is a peculiar lenticular galaxy in the constellation Centaurus, which appears to be an elliptical galaxy with a huge superimposed dust lane. It is one of the closest radio galaxies, and its active galactic nucleus has been extensively studied.

Discovery and Identity

NGC 5128 was discovered by James Dunlop on August 4, 1826. John Herschel was next to see it, from South Africa in 1834; he cataloged it as h 3501, which became GC 3525 in his General Catalogue of 1864, and NGC 5128 in J. L. E. Dreyer's New General Catalog. Herschel was first to note this galaxy's peculiarity, in 1847.

Halton Arp included NGC 5128 in his 1966 Atlas of Peculiar Galaxies as one of the best examples of a "disturbed" galaxy with dust absorption.

Amateur Observation

Centaurus A appears approximately 4° north of the naked-eye globular cluster Omega Centauri. At magnitude 7.0, this galaxy is the fifth brightest in the sky, making it ideal for observation, although it is only visible from the southern hemisphere and low northern latitudes. Centaurus A has been spotted with the naked eye by expert observers under very good conditions. The bright central bulge and dark dust lane are visible in finderscopes and large binoculars, and additional structure may be seen in larger telescopes.

NGC 5128 is a "lenticular" galaxy, of intermediate type between elliptical and disk (spiral) galaxies. Its main body has all characteristics of a large elliptical, but a pronounced dust belt is superimposed over the center, forming a disk plane around this galaxy.

The only supernova discovered in Centaurus A so far is SN 1986G, a Type Ia event that reached mag 12.5 in May, 1986.

Properties and Evolution

Centaurus A is located about 11 million light-years away, at the center of one of two subgroups within the Centaurus A/M83 Group. Messier 83 (the Southern Pinwheel Galaxy) is at the center of the other subgroup. These two groups are sometimes identified as one, since the galaxies around Centaurus A and the galaxies around M 83 are physically close to each other, and both subgroups appear to be stationary relative to each other. The Centaurus A/M83 Group is part of the Virgo Supercluster.

NGC 5128's strange morphology is the result of a merger between two smaller galaxies. The bulge of Centaurus A is comprised mainly of evolved red stars. Its dusty disk, however, has been the site of more recent star formation; over 100 star formation regions have been identified in the disk. As observed in other such "starburst" galaxies, a collision is responsible for the intense star formation. Scientists using the Spitzer Space Telescope have confirmed that Centaurus A is an elliptical galaxy going through a collision, devouring a spiral.

In the radio part of the spectrum, Centaurus A exhibits two vast regions of radio emission, running along the polar axis of NGC 5128's disk and extending hundreds of light years in both directions. A relativistic jet from what is believed to be a supermassive black hole at the center of the galaxy is responsible for emissions in the X-ray and radio wavelengths. Radio observations of the jet indicate that the inner parts of the jet are moving at about 1/2 the speed of light. X-rays are produced farther out, as the jet collides with surrounding gases, creating highly energetic particles.