Measuring Transverse Motions for Nearby Galaxy Clusters

TL;DR

This paper explores a novel method to measure the tangential motions of distant galaxy clusters by analyzing line-of-sight velocity patterns, potentially enabling the detection of their full 3D motions.

Contribution

It extends a technique previously used for nearby galaxies to distant clusters, proposing feasible approaches for measuring their transverse motions.

Findings

Mapping line-of-sight velocities can reveal cluster tangential motion.

Spectroscopic surveys of thousands of cluster members are promising.

The method is most effective for large, symmetric clusters with extensive observations.

Abstract

Measuring the full three-dimensional motions of extra-galactic objects in the Universe presents a seemingly insurmountable challenge. In this paper 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 1000's of members of nearby galaxy clusters hold the most promise of measuring cluster tangential motions using this technique.