Tidal triggering of active disc galaxies by rich clusters

Summary

Seyferts with companions

Galaxy activity is correlated with companions (Keel et al. 1985, van der Hulst et al. 1986). Using matched samples of Seyferts and controls, Dahari (1984) searched for companions, measuring the galaxy-companion separations and their sizes. He measure the tidal perturbation strength by a parameter P = (companion mass)/(separation)³ in units of the galaxy mass and radius. Dahari found that more Seyferts (37%) have companions than do normal spirals (21%), and that Seyferts with companions are perturbed more strongly. Selection effects cause companions of higher redshift Seyferts to be missed and Byrd et al. (1987) estimate that 75% to 90% of Dahari's Seyferts have companions.

Byrd et al. (1986) tested the correlation using computer models of tidally perturbed spiral galaxies. Observations require a gas mass inflow rate of > 0.5 M_⊙ yr₁ for Seyfert activity. We used a self-gravitating 60 000 particle disc and inert “halo” perturbed by a companion on a parabolic orbit. Tidal perturbation of the disc throws gas clouds into nucleus-crossing orbits to fuel activity. The experiments demonstrated that the inflow rate exceeded the required value at perturbation levels matching those where Dahari finds many more Seyferts than normals. We therefore conclude that observed companions of Seyferts do have tidal fields sufficient to trigger activity.

Seyferts in rich clusters

If individual gravitational encounters are responsible for activity, the incidence of activity should correlate with the enounter rate. Gavazzi & Jaffe (1987) argue that individual encounters should be less important in rich clusters than in groups.