Optical Substructures in 48 Galaxy Clusters: New Insights from a Multiscale Analysis

We analyze the presence of substructures in a set of 48 galaxy clusters by using galaxy positions and redshifts. The data are taken from literature sources, with the addition of some new data provided by recent observations of galaxy clusters. We use a multiscale analysis that couples kinematical estimators with the wavelet transform. With respect to previous works, we introduce three new kinematical estimators. These estimators parameterize the departures of the local means and/or local dispersions of the measured radial velocities with respect to their global values for the environment. We classify the analyzed clusters as unimodal, bimodal, and complex systems. We find that about 14% of our clusters are strongly substructured (i.e., they are bimodal or complex) and that about 24% of the remaining unimodal clusters contain substructures at small scales. Thus, in substantial agreement with previous studies, about one-third of clusters show substructures. We find that 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. In contrast, 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 different.