The dynamical status of Stephan's Quintet

Multiwavelength data for Stephan's Quintet (SQ) are consistent with the following model for this compact galaxy group. (1) Discordant-redshift NGC 7320 is an unrelated foreground galaxy. (2) In the past SQ was an accordant-redshift quartet involving NGC 7317, 7318A, 7319, and 7320C. NGC 7320C collided (probably not for the first time) with the group a few times 10(8) yr ago and stripped the interstellar medium from NGC 7319. (3) In the present SQ is again an accordant quartet involving NGC 7317, 7318A, B, and 7319. NGC 7318B is now entering the group at high velocity for the first time, giving rise to a shock zone. If most compact groups are like SQ, then they are frequently visited by infalling neighbors that perturb the group and themselves. SQ represents strong evidence for secondary infall in a small group environment. Tidal stripping reduces the mass of the infalling galaxies, thereby increasing the timescale for their orbital decay. There is little evidence that these high-velocity ''intruders'' are rapidly captured and/or merge with the system. Instead, they are the mechanism that sustains compact groups against collapse. Efficient gas stripping may account for the low star formation rate observed in compact groups, and infall of residual gas into galactic nuclei may also foster the onset of active galactic nucleus activity.