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We search for the presence of substructure, a non-Gaussian, asymmetricalvelocity distribution of galaxies, and large peculiar velocities of the maingalaxies in galaxy clusters with at least 50 member galaxies, drawn from theSDSS DR8. We employ a number of 3D, 2D, and 1D tests to analyse thedistribution of galaxies in clusters: 3D normal mixture modelling, theDressler-Shectman test, the Anderson-Darling and Shapiro-Wilk tests and others.We find the peculiar velocities of the main galaxies, and use principalcomponent analysis to characterise our results. More than 80% of the clustersin our sample have substructure according to 3D normal mixture modelling, theDressler-Shectman (DS) test shows substructure in about 70% of the clusters.The median value of the peculiar velocities of the main galaxies in clusters is206 km/s (41% of the rms velocity). The velocities of galaxies in more than 20%of the clusters show significant non-Gaussianity. While multidimensional normalmixture modelling is more sensitive than the DS test in resolving substructurein the sky distribution of cluster galaxies, the DS test determines bettersubstructure expressed as tails in the velocity distribution of galaxies.Richer, larger, and more luminous clusters have larger amount of substructureand larger (compared to the rms velocity) peculiar velocities of the maingalaxies. Principal component analysis of both the substructure indicators andthe physical parameters of clusters shows that galaxy clusters are complicatedobjects, the properties of which cannot be explained with a small number ofparameters or delimited by one single test. The presence of substructure, thenon-Gaussian velocity distributions, as well as the large peculiar velocitiesof the main galaxies, shows that most of the clusters in our sample aredynamically young.

Multimodality in galaxy clusters from SDSS DR8: substructure and velocity distribution