The Evolving Shape of Galaxy Clusters

We present the first measurement of the evolution of the apparent projected shape of galaxy clusters from 0.2<~ z<~2. We measure the ellipticities (ɛ_cl) of homogeneously selected galaxy clusters over this wide redshift range. We confirm the predictions of N-body simulations that clusters are more elongated at higher redshift, finding the mean projected ellipticity changes linearly from 0.36+/-0.01 to 0.25+/-0.01 over that range. The fraction of relaxed clusters (defined as having ɛ _cl <0.2) is 9+5_-3% at z~1.8, steadily increasing to 42+7_-6% by z~0.3. Because more spherical clusters have a higher degree of virialization, our result shows significant evolution in the degree of cluster virialization over cosmic time.