X-ray shapes of distant clusters and blue galaxy fractions

Based on ROSAT PSPC pointed obervations, we determine the aggregate X-ray shapes of 10 distant (z=0.17-0.54) rich clusters: A2397, A222, A520, A1689, A223B, A1758, A2218, A2111, A2125 and CL 0016 + 16. Four of the clusters have global X-ray ellipticities ≳0.2, as measured on a scale of diameter ∼3 h₅₀^-1 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 with their blue galaxy fractions; the correlation coefficient is 0.75 with a 90 per cent confidence range of 0.44-0.92. This correlation, though with limited significance, 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 on to equipotential surfaces.