The Shapes of Galaxy Clusters

We have reanalyzed a data set of 99 low redshift ($z < 0.1$) Abell clusters and determined their shapes. For this, three different measures are used. We use Monte-Carlo simulations to investigate the errors in the methods. The corrected distribution of cluster ellipticities shows a peak at $ε\sim 0.4$ and extends to $ε\sim 0.8$. The results are self-consistent, i.e., with the corrected distribution over projected cluster shapes we can reconstruct the observed distribution and the observed relation between the number of galaxies in a cluster and its ellipticity. It is shown that the richer clusters are intrinsically more nearly spherical than the poorer ones. The corrected distribution of cluster shapes is more consistent with a prolate population than with an oblate population. We compare the corrected true distribution of (projected) ellipticities with predictions from N-body simulations. For this, we use a catalogue of 75 N-body simulated clusters (van Kampen 1994) which assume a CDM spectrum with $Ω= 1.0$. The simulations include a recipe for galaxy formation and merging. 'Observing' these simulated clusters produces an ellipticity distribution that extends to much higher $ε$ and that has too few nearly spherical clusters. Preliminary results of simulations of the formation of clusters in an $Ω= 0.2$ universe suggest that, on average, clusters are more nearly spherical in this case.

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