The Dynamical Status of Stephan's Quintet

Multiwavelength data for Stephan's Quintet (SQ) are consistent with thefollowing model for this compact galaxy group. (1) Discordant redshift NGC 7320is an unrelated foreground galaxy. (2) In the past SQ was an accordant redshiftquartet involving NGC 7317, 18A, 19 and 20C. NGC 7320C collided (probably notfor the first time) with the group a few times 10$^8$ years ago and strippedthe interstellar matter from NGC 7319. (3) In the present SQ is again anaccordant quartet involving NGC 7317, 18A,B, and 19. NGC 7318B is now enteringthe group at high velocity for the first time, giving rise to a shock zone. Ifmost compact groups are like SQ, then they are frequently visited by infallingn eighbors that perturb the group and themselves. SQ represents strong evidencefor secondary infall in a small group environment. Tidal stripping reduces themass of the infalling galaxies, thereby increasing the timescale for theirorbital decay. There is little evidence that these high velocity ``intruders''are rapidly captured and/or merge with the system. Instead they are themechanism that sustains compact groups against collapse. Efficient gasstripping may account for the low star formation rate observed in compactgroups and infall of residual gas into galactic nuclei may also foster theonset of AGN activity.Comment: 5 pages, 2x3 figures, to be published in ApJ Letter