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

¹ Tartu Observatory, 61602 Tõravere, Estonia
e-mail: maret@aai.ee
² Tuorla Observatory, University of Turku, Väisäläntie 20, Piikkiö, Finland
³ National Institute of Chemical Physics and Biophysics, 10143 Tallinn, Estonia
⁴ Institute of Physics, Tartu University, Tähe 4, 51010 Tartu, Estonia
⁵ Estonian Academy of Sciences, 10130 Tallinn, Estonia
⁶ ICRANet, Piazza della Repubblica 10, 65122 Pescara, Italy
⁷ Observatori Astronòmic, Universitat de València, Apartat de Correus 22085, 46071 València, Spain

Received: 21 December 2011
Accepted: 19 February 2012

Abstract

Context. The study of the signatures of multimodality in groups and clusters of galaxies, an environment for most of the galaxies in the Universe, gives us information about the dynamical state of clusters and about merging processes, which affect the formation and evolution of galaxies, groups and clusters, and larger structures – superclusters of galaxies and the whole cosmic web.

Aims. We search for the presence of substructure, a non-Gaussian, asymmetrical velocity distribution of galaxies, and large peculiar velocities of the main galaxies in clusters with at least 50 member galaxies, drawn from the SDSS DR8.

Methods. We employ a number of 3D, 2D, and 1D tests to analyse the distribution of galaxies in clusters: 3D normal mixture modelling, the Dressler-Shectman test, the Anderson-Darling and Shapiro-Wilk tests, as well as the Anscombe-Glynn and the D’Agostino tests. We find the peculiar velocities of the main galaxies, and use principal component analysis to characterise our results.

Results. More than 80% of the clusters in our sample have substructure according to 3D normal mixture modelling, and the Dressler-Shectman (DS) test shows substructure in about 70% of the clusters. The median value of the peculiar velocities of the main galaxies in clusters is 206 km s^-1 (41% of the rms velocity). The velocities of galaxies in more than 20% of the clusters show significant non-Gaussianity. While multidimensional normal mixture modelling is more sensitive than the DS test in resolving substructure in the sky distribution of cluster galaxies, the DS test determines better substructure expressed as tails in the velocity distribution of galaxies (possible line-of-sight mergers). Richer, larger, and more luminous clusters have larger amount of substructure and larger (compared to the rms velocity) peculiar velocities of the main galaxies. Principal component analysis of both the substructure indicators and the physical parametres of clusters shows that galaxy clusters are complicated objects, the properties of which cannot be explained with a small number of parametres or delimited by one single test.

Conclusions. The presence of substructure, the non-Gaussian velocity distributions, as well as the large peculiar velocities of the main galaxies, shows that most of the clusters in our sample are dynamically young.