4.2 Evidence for Tidal Stripping in Clusters
Given the existence of cD galaxies which have grown in size as a result of the cluster environment, we might expect other galaxies in the cluster to have lost material in order to feed the cDs. Unlike many of the mechanisms discussed in Section 2, it is clear that strong tidal effects are present in at least some galaxies. For example, Borne (1988a, 1988b, l988c) finds that he can nicely model both the azimuthal distortions in the luminosity distribution, and the kinematic data, for several interacting pairs of elliptical galaxies in terms of simple tidal encounters. The question is how often and how much material is lost in these interactions, not whether they occur.
Tidal stripping by galaxy-galaxy collisions (Gallagher and Ostriker 1972, Richstone 1976), or the mean tidal field of the cluster (Merritt 1984), have both been suggested on theoretical grounds. Observational support comes from the extensive study by Strom and Strom (1978a, 1978b, 1978c). They studied 600 elliptical and S0 galaxies in six clusters using photographic plates. They find that the radii of the elliptical galaxies (measured at the 26th mag arcsec-2 isophote in the R band) in the dense spiral-poor clusters are about 30% smaller at a given MV than in the spiral-rich clusters. In addition, galaxies within 0.5 Mpc (1.0 Mpc in the case of A 2199) of the core appear to be 15% smaller than outer galaxies in the same cluster. Even more striking is that for the cluster with the largest cD, Abell 2199, the difference between the inner and outer galaxies is nearly 60% when Re is used for the size measurements. Lugger (1989) also finds evidence that the luminosity functions within 0.5 Mpc of three dense clusters are deficient in bright galaxies.
One concern with the Strom's study has always been the use of photographic photometry for the measurements. However, Schombert (1986) recently showed his photometry was in good agreement with the Strom's results. He also found that faint ellipticals in his sample had outer cutoffs in their profiles. Since most of these faint ellipticals were near the centers of clusters this may indicate that they have been tidally truncated. Several other studies including Bothun and Schombert (1988), and Lauer (1988), have also found evidence for tidal truncations. More recent studies that supported the Strom's results have been Peterson et al. (1979), and Giuricin et al. (1985 and 1988). However, Vader (1986) found that Virgo ellipticals were smaller than Coma ellipticals (using the RC2 which may contain systematic biases), and Giuricin et al. (1989) found no effect using the new CCD photometry from Burstein et al. (1987).
Kormendy and Djorgovski (1989) have argued that while strong tidal interactions can shrink a galaxy, weak encounters may heat galaxies up and cause a more distended outer envelope. This is based on both photometric observations of ellipticals with companions (Kormendy 1977), and N-body experiments by Aguilar and White (1986). Schombert (1988) and deCarvalho and daCosta (1988) were not able to confirm Kormendy's result.