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We have seen that the study of galaxy collisions is a relatively young, but rapidly maturing field. Thus, it is understandable that most progress to date has been in understanding the most spectacular collisions, ULIRG/major mergers, and the nature of some of the closest systems which can be studied in detail. Most of the latter occur in quite small groups like our own local group. However, studies of collisions in other environments date back to Spitzer & Baade (1951), and their number has been increasing recently.

3.1. Cluster Bustle

At the opposite end of the spectrum from the local group environment is that of massive, dense clusters of galaxies. It will suffice for our purposes to briefly note the different processes in this environment relative to that of local groups. These differences include: high speed collisions, galaxy 'harassment,', ram pressure stripping, 'strangulation,' and induced slow collisions (see Figure 8).

Figure 8

Figure 8. Arp (1966) atlas image of Arp 120 (NGC 4438), a Virgo cluster system in collision, with a starburst galactic wind, and likely also experiencing environmental effects. See Boselli et al. (2005) and Kenney & Yale (2002) for details.

Spitzer & Baade (1951) first suggested that high velocity collisions in galaxy clusters might have little effect on the stellar components, but could blast away the overlapping parts of gas disks. This is because the typical random galaxy velocities in clusters of up to several thousand km/s, are not only highly supersonic for the intercluster gas, but are in excess of normal disk escape velocities. Generally, we expect a moderation of tidal gravitational effects, but in some cases a drastic increase in hydrodynamic effects.

High-speed collisions may have weak gravitational effects, but encounters are much more common in the cluster environment. Thus, in the aggregate, tidal effects are not negligible in clusters. The cumulative effect of many weak (high-speed or distant) galaxy-galaxy interactions in clusters, as well as perturbations from the cluster potential, and possibly from intermediate scale sub-structure is called 'harassment' (see Moore et al. (1996), Moore et al. (1999)). In recent high resolution numerical studies of the growth of moderate clusters, Gnedin (2003a), Gnedin (2003b) has shown how this process can secularly erode galaxy halos, thicken moderate mass stellar disks and truncate SF, and destroy small disks.

Ram pressure stripping (RPS) by the intra-cluster medium can have somewhat similar effects on disks. RPS is an interesting subject, with a number of recent developments, and worthy of a review of its own. Thus, it is beyond the scope of this review, except for a few comments. First of all, long time residents of dense galaxy clusters were probably stripped long ago, so RPS is most relevant to gas-rich galaxies falling into the intra-cluster medium for the first time. X-ray satellites have provided much evidence that the infall of galaxy groups into clusters and cluster-cluster mergers are still common events Mushotsky (2004b). Once the intra-group medium is stripped in such interactions, individual disk galaxies are vulnerable to RPS. RPS of spheroidal galaxies has been studied for 30 yrs., but in the last 10 yrs. a small literature on stripping of disk galaxies has blossomed.

The outermost parts of gas disks are stripped promptly, and slower interactions continue for some time. Slow viscous interactions at the edge of disks moving face-on into the intra-cluster medium has recently been studied in detail by Roediger & Hensler (2005). The three-dimensional dynamics in tilted cases have been modeled in several recent papers (Abadi, Moore & Bower (1999), Vollmer et al. (2000), Vollmer et al. (2001), Schulz & Struck (2001)). Schulz and Struck, in particular, pointed out that if the gas disk is not promptly stripped, it can nonetheless be displaced relative to the stellar disk and the halo center. The displaced gas disk experiences tidal compression (perpendicular to the disk plane) and asymmetric torques in the tilted case, which generate spiral waves. The waves transfer some gas and much angular momentum outwards, where it is stripped. The remaining gas, with less angular momentum, compresses radially, which "anneals" it against further stripping. The various compressions should induce SF. The tidal forces and induced SF are much like those in galaxy collisions. Thus, after the stripped material is gone, it could be difficult to discern whether excess SF is the result of RPS, a minor merger, or harassment. In fact, since these processes could work simultaneously, the question may be academic.

Vollmer and Schulz and Struck emphasized another aspect of stripping: some of the removed material can later fall back onto the disk. This can occur either when the galaxy moves into regions of lower intra-cluster medium densities, where the levitating pressure is reduced, or when gas clouds move into the disk 'shadow' where the pressure is also reduced. In either case we would expect effects akin to those of mass transfer in galaxy collisions.

Strangulation is a weaker cousin of RPS. It is the process of removing the potential feedstock of disk SF, gas in the galactic halo, usually via RPS Larson, Tinsley, & Caldwell (1980). The feedstock could include gas blown out of the disk by supernovae or stellar or galactic winds, it could include gas tidally removed from companions, or primordial gas falling into the halo for the first time. This process is likely to be most important in the young universe, when there is still much gas outside of galaxy disks, but it also hampers gas recycling from dying populations in cluster galaxies.

3.2. Cluster Slow Dance

A final process that may be very important in cluster environments is induced slow encounters in infalling groups. Recent studies of the Butchler-Oemler effect (see review of Pimbblet (2003)), which is an excess of blue galaxies in clusters at redshifts of less than 1, provide evidence that many of the blue galaxies are mergers or interacting (also see Zabludoff et al. (1996), Hashimoto & Oemler (2000), Ellingson et al. (2001), Goto (2005)). It seems unlikely that high speed interactions could be responsible for this effect. Mihos (2004) has emphasized that large-scale cluster formation models show that slow interactions continue to occur even in large clusters, and are quite common during cluster formation. He also notes that slow encounters can occur in groups or small clusters with modest velocity dispersions falling into large clusters (also see Poggianti et al. (2004)).

This is an interesting phenomenon that has not been much investigated. Mihos notes that a number of different processes may be involved and it may be impossible to disentangle them. For example, tidal forces from the cluster potential could perturb orbits in the infalling group inducing interactions, and intracluster medium annealing could induce increased SFRs. Personally, I suspect these are secondary processes.

At least for galaxies that fall through the cluster core the primary process may well be gravitational shocking, which depends directly on the cluster potential rather than indirectly or on the derivative of the potential (tidal forces). Consider the relatively simple example of a galaxy group containing about 30-100 galaxies, falling into a large dense cluster. Cold dark matter structure formation models predict a common density profile across the range of structures from galaxy halos to the dark halos of large clusters, and observational tracers (e.g., intracluster starlight, see Feldmeier et al.(2002) and references therein) show good agreement with profile functions derived from these simulations, like the popular NFW profile Navarro, Frenk, & White (1997). Then it is reasonable to assume our example group and cluster have similar density profiles, though not necessarily with the central cusp of the NFW profile.

The observations also suggest that the central density decreases slowly with mass in dark matter halos. We might, for example, model our large cluster after Abell 2029, whose mass profile was studied in detail by Lewis, Buote, & Stocke (2003). They find a mass of about 9.2 × 1013 / h70 Modot contained within a radius of 260 / h70 kpc, yielding a central density of 0.0052 Modot / pc3. As an example of a group, on the other hand, we can take a 'poor' group like those studied by Zabludoff & Mulchaey (1998). These groups have virial masses of about 7 × 1013 / h70 Modot within radii of about 300 / h70 Mpc. The authors estimate that 80-90% of the virial mass is in the group halo, so the mean group halo density is about 0.0022 Modot / pc3. These numbers are meant to be representative, not precise. Group parameters, e.g., in compact versus poor groups, could easily range over factors of a few. Nonetheless, the message is clear that passing through the cluster core would substantially increase the instantaneous group halo mass.

Moreover, the time to pass through the core is of the same order as the group dynamical time. We can estimate the former by dividing the core radius above by a typical cluster velocity of about 2000 km/s; the result is about 300 Myr. The free fall time at the edge of our example group is about 200 Myr. Therefore there is time for group galaxies to be pulled into a much denser and compact configuration. For roughly comparable group and cluster halo core densities, galaxies could be pulled in to roughly half their previous distance from the core, increasing the galaxy density by nearly an order of magnitude and their collisions by about a factor of 100 (density squared).

Like stars in clusters that pass through the galactic disk, when the group leaves the cluster core, and gravity is reduced, the galaxies will fly outward. (This is also like collisional ring galaxies.) However, collisions between galaxy halos are stickier than those between cluster stars. During the compression period, along near parallel but converging trajectories on the way down, encounters are likely, and there is time for dynamical friction to dissipate relative orbital energy. The result would frequently be a fairly slow interaction and eventual merger.

Considering all the various ways that clusters accelerate galaxy evolution, one can view life for average galaxies in small local groups as more or less a holding pattern, or at least a matter of slow maturation. Evolution doesn't begin in earnest until they fall in a larger group or cluster; the classic tale of youth leaving the farm for the big city.

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