Measuring transverse motions for nearby galaxy clusters

Abstract

Measuring the full three-dimensional motions of extragalactic objects in the universe presents a seemingly insurmountable challenge. In this Letter, we investigate the application of a technique to measure tangential motion that has previously only been applied nearby within the Local Group of galaxies, to clusters of galaxies far beyond its borders. We show that mapping the mean line-of-sight motion throughout a galaxy cluster could in principle be used to detect the perspective rotation induced by the projection of the cluster's tangential motion into the line of sight. The signal will be most prominent for clusters of the largest angular extent, most symmetric intrinsic velocity distribution and surveyed with the largest number of pointings possible. We investigate the feasibility of detecting this signal using three different approaches: measuring line-of-sight motions of individual cluster members, taking spectra of intracluster gas, and mapping distortions of the cosmic microwave background radiation. We conclude that future spectroscopic surveys of thousands of members of nearby galaxy clusters hold the most promise of measuring cluster tangential motions using this technique.