Effect of Internal Flows on Sunyaev‐Zeldovich Measurements of Cluster Peculiar Velocities

Galaxy clusters are potentially powerful probes of the large-scale velocityfield in the Universe because their peculiar velocity can be estimated directlyvia the kinematic Sunyaev-Zeldovich effect (kSZ). Using high-resolutioncosmological simulations of an evolving cluster of galaxies, we evaluate howwell the average velocity obtained via a kSZ measurement reflects the actualcluster peculiar velocity. We find that the internal velocities in theintracluster gas are comparable to the overall cluster peculiar velocity, 20 to30% of the sound speed even when a cluster is relatively relaxed. Nevertheless,the velocity averaged over the kSZ map inside a circular aperture matched tothe cluster virial region provides an unbiased estimate of a cluster's radialpeculiar velocity with a dispersion of 50 to 100 km/s, depending on the line ofsight and dynamical state of the cluster. This dispersion puts a lower limit onthe accuracy with which cluster peculiar velocity can be measured. Although thedispersion of the average is relatively small, the velocity distribution isbroad; regions of low signal must be treated with care to avoid bias. Wediscuss the extent to which systematic errors might be modelled, and theresulting limitations on using galaxy clusters as cosmological velocitytracers.