Shapes of Distant Clusters : the Connection to Blue Galaxy Fractions

Based on ROSAT PSPC pointed observations, we have determined the aggregate X-ray shapes of 10 distant (z = 0. Four of the clusters have global X-ray ellipticities ∼ > 0.2, as measured on a scale of diameter ∼ 3h −1 50 Mpc. These strongly elongated clusters tend to show substantial amounts of substructure, indicating that they are dynamically young systems. Most interestingly, the global X-ray ellipticities of the clusters correlate well with their blue galaxy fractions; the correlation coefficient is 0.75 with a 90% confidence range of 0.44-0.92. This correlation suggests that blue cluster galaxies originate in the process of cluster formation, and that the blue galaxy proportion of a cluster decreases as the intracluster medium relaxes onto equipotential surfaces.