8.3 Galaxies in Clusters

The halos of galaxies in clusters could have been modified by the dense and perilous environment where close encounters of galaxies may be frequent. Studying such halos may therefore yield valuable information about the formation and evolution of clusters. Dynamical considerations suggest that all of the DM in clusters is unlikely to be associated with individual galaxies. If this were the case, dynamical friction would have led to the most massive galaxies being dragged into the cluster center. Some mass segregation of this kind is observed, but the degree suggests that most DM is distributed smoothly in many clusters (White 1977). The DM may have been associated with the galaxies originally, in which case tidal distruption of DM halos could produce a smoother DM distribution by the present epoch (Richstone 1976). West and Richstone (1988) have presented N-body simulations which show that the removal of DM halos from galaxies may be a natural consequence of cluster formation.

Some evidence for stripped halos around spirals in clusters is provided by Whitmore, Forbes and Rubin (1988). They found that galaxies close to the center of clusters typically had flat or falling curves, whereas those in the outer regions of galaxy clusters have flat or rising curves. This is the sort of effect one would expect if the cluster environment leads to halo stripping (see also Whitmore 1990).

The correlation between rotation curve shape and distance from the cluster center is less apparent in other samples (Distefano et al. 1990; Amram at al. 1992a, b). Whitmore, Gilmore and Jones (1992) suggest this discrepancy might arise because these samples contain a large number of galaxies at distances beyond 1 Mpc from the cluster center. If the physical conditions responsible for truncating halos are more effective within 1 Mpc, as is physically plausible, these samples would be expected to show weaker correlations. Whitmore et al. (1992) also note that using galaxies with extended halos for these studies (i.e., extended rotation curves) could also bias the results, since the survival of extensive gas disks probably only occurs in galaxies that have not undergone much tidal stripping.

Bird, Dickey and Salpeter (1993) and Bird (1993) have studied the DM distribution in the Hercules cluster of galaxies. Using HI observations of spirals in this cluster and by applying statistical mass estimators they derive M/L 300h for the cluster as a whole. These authors also have HI linewidths for some of the spirals in the cluster. Bird et al. (1993) conclude that the halos of the galaxies, if isothermal, need to extend to about 15 times the distance of the measured rotation curves to account for all the DM in Hercules. This corresponds to halo radii around 300 kpc. Such halos would be larger than those inferred around isolated galaxies. Moreover, as Bird et al. (1993) point out, 300 kpc is uncomfortably close to the typical separation between galaxies in Hercules.

These kinds of studies have the potential for constraining the dark halo radius of cluster galaxies, but are currently somewhat limited by the complex problem of deriving reliable mass estimates for the clusters themselves. This problem is beyong the scope of the present review (see, however, papers in the proceedings edited by Fabian [1992], as well as the literature cited at the beginning of this Section).