Evolution of Substructure in Galaxy Clusters as Observed in X-Rays
  Clusters of galaxies are the largest gravitationally-bound objects in the Universe, having diameters on order of Mpc. Our work asked whether their shapes (morphologies) change over time as the Universe ages. We observed a sample of 165 galaxy clusters, at 0.1 < z < 1.3. A variety of measures were used to quantify the shapes of galaxy clusters. Archive observations from the Chandra X-Ray Observatory were used. Morphology evolution was probed at two different distances from clusters' centers -- 300 kpc and 500 kpc -- for comparison. In almost all cases, we were able to rule out that clusters retain their morphology over the history of the Universe, which is in agreement with our current picture of large-scale structure formation. In addition, we found that ellipticities, as a means of quantifying morphologies, are of limited use in studies such as this work. 
Brian C. Hart