GALAXY INTERACTIONS AND THE ORIGIN OF RADIO-SOURCES IN HIGH-DENSITY GROUPS

The influence of interactions among galaxies in high-density environments is studied by a radio survey of galaxies of different types in 66 compact groups from Hickson's catalogue. A total of 302 galaxies (79 E, 95 S0, 128 S) were included in the survey, of which 71 were detected as radio sources. It is found that radio-loud E & S0 galaxies are predominantly the optically brightest galaxies in a group, while the detected spirals are uniformly distributed among the three brightest members of a group. The median projected separation of radio-loud E & S0 galaxies is about 15 kpc while that of the radio-quiet sample is about 24 kpc. The radio detectability of a spiral, however. is not correlated with the distance of its neighbour. A similar distinction between E & S0 and S is also found for the distribution of a tidal interaction parameter. It is known that the velocity dispersion of S-dominant groups is nearly half that of E-dominant groups. Since many of the spirals in these groups show obvious optical signs of interaction, the differences in the distributions of nearest neighbours and the tidal parameters are interpreted by a model in which the incidence and duration of activity related to radio radiation in galaxy groups are dependent on the time-scale of tidal interactions in a particular group. These results have important consequences for the detailed numerical simulation of interactions involving elliptical and spiral galaxies.