  The redshift-space distortion (RSD) of galaxies surrounding massive clusters
is emerging as a promising testbed for theories of modified gravity.
Conventional applications of this method rely upon the assumption that the
velocity field in the cluster environment is uniquely determined by the cluster
mass profile. Yet, real dark matter halos in N-body simulations are known to
violate the assumption that virial mass determines the configuration space
distribution, an effect known as assembly bias. In this Letter, I show that
assembly bias in simulated dark matter halos also manifests in velocity space.
In the 1-10 Mpc environment surrounding a cluster, high-concentration "tracer"
halos exhibit a 10-20% larger pairwise-velocity dispersion profile relative to
low-concentration tracer halos of the same mass. This difference is comparable
to the size of the RSD signal predicted by f(R) models designed to account for
the cosmic acceleration. I use the age matching technique to study how
color-selection effects may influence the cluster RSD signal, finding a ~10%
effect due to redder satellites preferentially occupying higher mass halos, and
a ~5% effect due to assembly-biased colors of centrals. In order to use cluster
RSD measurements to robustly constrain modified gravity, we likely will need to
develop empirical galaxy formation models more sophisticated than any in the
current literature.
