In 1877, the French astronomer E. M. Stephan, observing visually with the 40cm refracting telescope of the Observatory of Marseilles, discovered a small, dense group of galaxies. This remarkable system, now known as Stephan's Quintet (aka Hickson 92, Arp 319, & VV 228), consists of five galaxies which appear so close together that their images overlap. Many of the galaxies are extraordinary in themselves, showing signs of violent interaction, including highly distorted shapes, and long filaments of stars and gas extending far from the body of the galaxy. This system is the prototype of a class of small clusters of galaxies commonly known as "compact groups of galaxies".
In 1961 Geoffrey and Margaret Burbidge obtained spectra of the galaxies in Stefan's Quintet. Their observations revealed that all but one of the galaxies are receding from the earth at about the same velocity. The discordant galaxy is receding less rapidly.
Two possible explanations were advanced for this phenomenon. The first, called the Projection Hypothesis, argues that the discordant galaxy does not belong to the group, but is simply a foreground or background galaxy that happens to lie along the line of sight to the group. (The Hubble Law states that, because of the expansion of the universe, the velocity at which a galaxy is receding is proportional to the distance to that galaxy.) The difference in velocity between the discordant galaxy and the other members of the group is, then, just a result of the difference in distances.
The alternative explanation, preferred by a few astronomers, is that the discordant galaxies are true members of their groups, but have a different velocity either because they are moving very rapidly, or because of some as-yet unidentified phenomenon. At present there are no observations which conclusively rule out one or the other explanation.
In the case of Stephan's Quintet, the discordant galaxy is NGC 7320. Its lumpy appearance is consistent with the idea that it is much closer than the other members. Also, its 'red shift' is about the same as that of the nearby spiral galaxy NGC 7331, and its neighbors. Most astronomers believe that NGC 7320 is a foreground object that is actually a part of the NGC 7331 group.
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Classification: Compact Galaxy Group |
Classification: Elliptical Galaxy |
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Classification: Elliptical Galaxy - peculiar |
Classification: Barred Spiral Galaxy - peculiar |
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Classification: Barred Spiral Galaxy - peculiar - Seyfert II |
Classification: Spiral Galaxy |
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George Normandin, KAS
February 15th, 1999