The dwarf galaxy NGC 5011C (below) is twelve times closer to Earth than the spiral galaxy NGC 5011B (top). Photo: European Southern Observatory ESO
Reading Two galaxies that have been considered cosmic neighbors for two decades are actually more than a hundred million light-years apart. The closer of the two is therefore not a full-grown spiral galaxy, as previously thought, but a dwarf galaxy. That's what Ivo Saviane and Helmut Jerjen found out with the telescope of the European Southern Observatory in La Silla (Chile). The two researchers found the galaxy NGC 5011C strange: according to literature, it should be a member of the Centaurus Cluster, a group of galaxies about 160 million light-years away. Their low brightness suggested that it was a dwarf galaxy, but their size was more akin to a young spiral galaxy. In addition, there seemed to be no reciprocal influence between NGC 5011C and its neighbor NGC 5011B, even though the two appeared to be only 45, 000 light-years apart.

The researchers therefore determined the redshift of both galaxies with the La Silla telescope. They found that NGC 5011C is much closer to Earth than previously thought, in the vicinity of the giant galaxy Centaurus A, which is only 13 million light-years away from Earth. Thus, NGC 5011C is actually a dwarf galaxy containing only about a fraction of the Milky Way's mass. The investigation also showed that the composition of the two supposed neighbors is also very different: NGC 5011B contains significantly more heavy chemical elements than NGC 5011C.

Ivo Saviane (European Southern Observatory) and Helmut Jerjen (Mount Stromlo Observatory, Australia): Astronomy and Astrophysics, Vol. 133, p. 1892 Ute Kehse

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