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Stephan's Quintet and the problem of compact groups
The compact group of galaxies now catalogued as Hickson 92 has a proper name : Stephan's Quintet. It was discovered (as a quartet) by Edouard Stephan in 1877, using the Foucault 80-cm reflector at the Marseille Observatory. Stephan was the first director of Marseille Observatory (on its present site) who worked here : the impetus [in the 1860s] for the construction of a new Marseille Observatory and the installation of the Foucault 80-cm telescope having been given by Le Verrier (of Neptune fame). The Foucault 80-cm was the first large telescope with a silver-on-glass mirror, and was designed by Leon Foucault, who applied several inventions in the process, including the famous knife-edge test. It was erected on the present site of the Observatoire de Marseille in 1864, on a hill then outside town (due to some demographic expansion, actually nowadays close to the center). According to independent judgement (Gascoigne, 1996, Quarterly Journal of the Royal Astronomical Society, 37, 101) this telescope was very succesful, and made substantial contributions in a number of fields : 1) the discovery and determination of their position of faint nebulae (including Stephan's Quintet), 2) the first astronomical application of the Fizeau interferometer, made by Stephan, resulting in an upper limit for the diameter of stars of 0.158 arcsec, 3) the pioneering work of Fabry and Perot on the multiple reflection interferometer which bears their name, and included the accurate measurement of the wavelengths of the nebular lines discovered in Orion, and 4) a long series of measurement of double stars. For an extensive and entertaining history of the Foucault telescope, see Tobin (1987, Vistas in Astronomy 30, 153-184). Curiously, it never made the transition to photography. The main telescope structure has recently been rehabilitated and put in a building on the Observatory site used for public education activities; the mirror itself is there on display as well. The astronomical interest in Stephan's Quintet began in the 1960's, when the redshifts of the constituent galaxies were measured. It was found (Burbidge & Burbidge, ApJ 134, 248, 1961) that one of the galaxies, NGC 7320, had a much smaller redshift than the others. This led to much debate, and the Quintet played a prominent role in the so-called redshift controversy (e.g. Field, Arp and Bahcall, 1974, Arp, ApJ 474, 74, 1997). The picture (cf. NOAO) shows the five galaxies : NGC 7317 to the far right, NGC 7318 A and B which seem to be interacting, NGC 7319, a barred spiral with a Seyfert 2 nucleus, and the spiral at the lower left, NGC 7320. The latter one has a redshift similar to the nearby giant spiral NGC 7331. A deep picture shows material apparently coming out of the low redshift galaxy, but which in reality is at high redshift (cf. Arp and Lorre, ApJ 210, 58, 1976 ; Shostak, Allen & Sullivan, A & A 139, 15, 1984) When Paul Hickson made a catalogue of compact groups, Stephan's Quintet was included as Hickson 92. There are other compact groups which have members with discordant redshifts. The astrophysical importance of compact groups was reviewed recently by Hickson in Annual Reviews 35, pp. 357-388, 1997. Modern work on compact groups here |
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