Interaction in Two Contrasting Samples of Galactic Groups

Observations of the two groups of galaxies 0645-741, 0019489 are presented. The first group contains a well-known ring gaiaxy first identified by Lindsay and Shapley (1960) and later studied by Graham (1974). The second group is also referred to as the Phoenix group and consists of NGC 87, 88, 89 and 92 and was recently studied by Rose (1979) and Rose and Graham (1979). Both groups exhibit a comparable degree of compactness and contain similar numbers of galaxies and appear to have undergone tidal interactions; yet, they show markedly different galaxy populations. The former consisting of evolved galaxies, ellipticals and the latter young spirals showing recent star formation. The nature of the tidal mteractions is discussed m the context of the models of Lynds and Toomre (1976) and it is evident that the peculiar ring galaxy in group 0645-741 and the formation of the peculiar extended spiral of the group 0019-489 are consequences of galactic encounters. From the observations it is deduced that the ring is inclined at 31 to the line of sight and has systemic velocity of 6494 km s-1 a rotation velocity of 343 km s-1 and an expansion velocity of 79 km s-1 The ring is 57 kpc in diameter and has a mass of 8.3 x 10^11 M⊙ and is approximately 3.5 x 10^8 years old. These parameters are comparable to those deduced for the Cartwheel by Fosbury and Hawden (1977). It is further pointed out that the ring is sufficiently close to be detected at 21-cm and therefore derive a neutral hydrogen mass from the galaxy. -