Optical substructures: A multi-scale analysis of 48 galaxy clusters

We present new developments in the wavelet analysis method in order to better detect substructures in galaxy clusters by using the complete information obtained from optical data, i.e. galaxy positions and redshifts. Among 48 clusters analyzed, whose optical data are taken mainly from literature sources with the addition of some new data, 14% show strong substructures (i.e. they are bimodal or complex). Several clusters exhibit evidence of substructures at small scales. The presence of substructures in unimodal clusters does not affect the estimates of both velocity dispersions and virial masses. Moreover, the galaxy velocity dispersion is generally in good agreement with the X-ray temperature, according to the expectations of the standard isothermal model for galaxy clusters. These facts suggest that unimodal clusters, which are the most frequent cases in the nearby Universe, are not too far from a status of dynamical equilibrium. On the contrary, the estimates of velocity dispersions and masses for some bimodal or complex clusters strongly depend on whether they are treated as single systems or as sums of different clumps. In these cases the X-ray temperature and the velocity dispersion may be very discrepant.