INFALL-RELATED OBSERVATIONAL EFFECTS AROUND CLUSTERS - LARGE-SCALE STRUCTURE, TRANSVERSE MOTIONS, AND REDSHIFT-SPACE ARTIFACTS

We explore the effects of large-scale flows on the redshift-space structure of the infall regions of galaxy clusters. It is shown that the compressed feature resulting from infall, the ''infall ring'' (which shows up in redshift plots as a crossbar bisecting the cluster's ''finger of god''), can reveal relative transverse motion between the observer and the center of the infall. The crossbar tilts away from the direction of the cluster's relative motion. A simulated spherically symmetric galaxy cluster is presented in which the effect is explored, and the two nearby clusters Virgo and Fornax are compared against the model. Substantial tilt is found in the presumed infall rings of both Virgo and Fornax. The directions of the tilts are not consistent with a transverse motion of the local standard of rest. They are qualitatively consistent with a tidal shear model like the Great Attractor model-although the shear we would estimate is much larger-or possibly with some rotational motions within the clusters.