1999ApJ...512..630N

We reanalyze ROSAT PSPC and HRI data of the cluster of galaxies A2218 in order to investigate its dynamical state. To reveal the cluster's substructure in X-rays, we construct symmetric cluster models by fitting a two-dimensional isothermal β-model, which we subsequently subtract from the original data. Study of the remaining structures and their significance gives new insight into existing substructure, and therefore indications concerning the dynamical state of the cluster, which has not yet been clearly visualized in X-rays. The present analysis enables us not only to qualitatively determine the dynamical state of the clusters, but also to draw conclusions about the significance of existing substructure in giving real numbers. A2218 reveals significant structure after the subtraction, providing clear evidence that it is going through a merger phase. The X-ray morphology is in agreement with the surface mass density map obtained by analysis based on weak lensing. The subgroup falling in the center of A2218 is most likely on its first infall.