Many dynamical avenues are available to drive tidal evolution in cluster galaxies. The most obvious is the cluster potential itself, particularly for galaxies whose orbit takes them close to the cluster center (e.g., Henriksen & Byrd 1996). More recently, the importance of repeated, fast collisions in stripping cluster galaxies has been emphasized by Moore et al. (1996, 1998). However, because of the large velocity dispersion within galaxy clusters, conventional wisdom held that strong interactions and mergers between cluster galaxies were rare (Ostriker 1980).
More recently, a greater understanding of the nature of hierarchical clustering is changing this view. While slow encounters are rare for an individual galaxy falling into a well-established environment (e.g., Ghigna et al. 1998), many galaxies are accreted onto clusters from within the small group environment. Clusters show ample evidence for substructure in X-rays, galaxy populations, and velocity structure (see, e.g., reviews by Buote 2002; Girardi & Biviano 2002). Interactions within infalling groups can be strong - witness, for example, the classic interacting pair "the Mice" (NGC 4676), found in the outskirts of the Coma cluster. Clearly strong interactions can and do occur during the evolution of clusters, either early as the cluster forms, or late as groups are accreted from the field.
The effects of the cluster potential on the evolution of tidal debris during a slow encounter can be dramatic. To illustrate this, Fig 3 shows the evolution of the same merger shown in Fig 1, except this time occurring in a Coma-like cluster potential. The orbit of the galaxy pair in the cluster carries it within 0.5 Mpc of the cluster core, with an orbital period of ~ 3.5 Gyr. As the galaxies merge, the very loosely bound material forming the tidal tails is now subjected to the large scale tidal field of the cluster, and is very efficiently stripped out of the galactic potential altogether.
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Figure 3. Evolution of an equal-mass merger, identical to that in Fig. 1, but occurring as the system orbits through a Coma-like cluster potential (see text). Note the rapid stripping of the tidal tails early in the simulation; the tidal debris seen here is more extended and diffuse than in the field merger, and late infall is shut off due to tidal stripping by the cluster potential. |
An extremely important facet of this kind of encounter is the enhanced efficiency of the tidal stripping. This is shown in Figure 4, which shows the fraction of material stripped to large radius (r > 35 kpc, or approximately 5 Re in the simulation) in the field and cluster versions of the simulations, as well as in a single disk galaxy on the same cluster orbit. The combination of the local and cluster tides causes significant stripping - encounters of galaxies in small infalling groups effectively "prime the pump" for the cluster tides to do their work. Indeed, the individual disk galaxy is hardly tidally stripped at all, suggesting that estimates of tidal stripping based on the tidal radius of individual galaxies falling into a cluster potential may significantly underestimate the effect.
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Figure 4. Material stripped to large radius (r > 35 kpc) for the isolated merger, the cluster merger, and a isolated spiral orbiting in the cluster potential. Cluster peri passages are shown as dashed lines. Note that most of the tidal material in the isolated merger remains bound to the remnant, while it is unbound in the cluster merger. |
The combined effects of galactic and cluster tides not only raise the efficiency of tidal stripping, they also result in particularly deep stripping. That is, the stronger galactic tides can strip material out from deep in the galaxies' potential well, which is then vulnerable to the gentler but long-lived cluster tides that liberate it entirely. As a result, the stripped material will be relatively high in metallicity, coming from the inner parts of the disk, and has a mean metallicity of [Fe/H] = -0.25, with a significant spread. This has important consequences for studies of the intracluster light (ICL), particularly in terms of searches for individual intracluster stars which are sensitive to the metallicity of the population (e.g., Durrell et al. 2002).
In terms of galactic recycling, the cluster has the effect of essentially shutting down various recycling paths. The ability for tidal tails to grow large tidal dwarfs may be extremely limited, as the cluster tides rapidly disperse the tidal material. The hot intracluster medium may also act to heat the tidal gas, making it difficult to form stars. If any dwarfs or, on smaller scales, star clusters do form in the tidal debris, they will be rapidly stripped from their hosts, perhaps contributing to the populations of cluster dwarfs or intracluster globular clusters.
The cluster will also shut down reaccretion from the tidal tails spawned during a merger. The combination of cluster tides and ram pressure stripping from a hot intracluster medium will "sweep clean" the tidal debris and any low density gas that might remain in the remnants. For example, the diffuse HI disk in the merger remnant Centaurus A (Nicholson et al. 1992) is unlikely to survive any passage through the hot ICM of a dense cluster. Models for forming S0 galaxies from mergers of galaxies followed by reformation or survival of a gaseous disk (e.g., Bekki 1998, or see the discussion in Schweizer 1998) seem difficult to envision in the dense cluster environment. However, the S0 classification is a very diverse one, and the mechanism which gives rise to disky cluster S0's may well be quite different than the merger mechanisms hypothesized to give rise to bulge-dominated S0's in the field environment.