Searching for cluster substructure using APM and ROSAT data

We present a detailed study of the morphological features of 22 rich galaxy clusters. Our sample is constructed from a cross-correlation of optical (A bell+APM) data with X-ray (0.1-2.4 keV) ROSAT pointed observations. We syst ematically compare cluster images and morphological parameters in an attemp t to reliably identify possible substructure in both optical and the X-ray images. To this end, we compute various moments of the optical and X-ray su rface-brightness distribution such as the ellipticities, centre-of-mass shi fts and ellipsoidal orientations. We assess the significance of our results using Monte Carlo simulations. We find significant correlations between th e optical and X-ray morphological parameters, indicating that in both parts of the spectrum it is possible to identify correctly the dynamical state o f a cluster. Most of our clusters (17/22) have a good one-to-one correspond ence between the optical and the X-ray images, and about 10 appear to have strong indications of substructure. This corresponds to a minimum percentag e of order similar to 45 per cent, which is in very good accordance with ot her similar analyses. Finally, five out of 22 systems (similar to 22 per ce nt) seem to have distinct subclumps in the optical which are not verified i n the X-ray images, and thus are suspect of being due to optical projection effects. These results will serve as a useful guide in interpreting subseq uent analyses of large optical cluster catalogues.