History

The galaxy NGC 5128 was discovered by Scottish astronomer James Dunlop on 29 April 1826 using the self-built 9-inch reflector at Parramatta observatory near Sydney, Australia. Based on six observation he recorded it as D 482 and described the object as follows: «A very singular double nebula, about 2.5' long and 1' broad, a little unequal: there is a pretty bright small star in the south extremity of the southernmost of the two, resembling a bright nucleus: the northern and rather smaller nebula is faint in the middle, and has the appearance of a condensation of the nebulous matter near each extremity. These two nebulae are completely distinct from each other, and no connection of the nebulous matters between them. There is a very minute star in the dark space between the preceding extremities of the nebula; they are extended in the parallel of the equator nearly.» ^[50]

John Herschel observing from South Africa he recored this object as h 3501 and wrote for sweep 454 (1 June 1834): «A most wonderful object; a nebula very bright; very large; little extended; very gradually much brighter middle; of an elliptic figure, cut away in the middle by a perfectly definite straight cut 40" broad; pos = 120.3; dimensions of the nebula 5' by 4'. The internal edges have a gleaming light like the moonlight touching the outline in a transparency.» On sweep 455 (3 June 1834) he wrote: «[Two nebulae, or two portions of one separated by a division or cut.] The cut is broad and sharp. The two nebulae are very nearly alike. Perhaps the slit is larger towards the north preceding end, where there is a star between them. There is certainly a very feeble trace of nebula, an island as it were, running from this star between the sides of the slit.» ^[11]

In Halton Arp's 1966 Atlas of Peculiar Galaxies, NGC 5128 is listed as Arp 153 in group #153-150 for an example of a disturbed galaxy with interior absorption. ^[199]