Signatures and bias assessment of rotation in galaxy cluster members

Context. In dynamically relaxed galaxy clusters, the galactic component is typically assumed to have zero or negligible mean motions. Aims. We investigate the possible presence of systematic rotation in the member galaxies of a sample of 17 nearby (z < 0.1), rich (at least 80 identified members) Abell clusters. We also assess the extent to which low-number statistics may influence the recovery of the rotation parameters. Methods. Following the methods often used in the context of globular clusters and of clusters of galaxies, we estimate a representative value of the systematic rotation velocity (vrot) and the position angle of the projected rotation axis for the set of spectroscopically confirmed member galaxies within 1.5 Mpc from the centre of each cluster. We study the robustness of our rotational velocity measurements as a function of the number of galaxies, N, included in the analysis with a bootstrapping technique. Results. Eight clusters with sufficiently abundant and regular data (A1367, A1650, A2029, A2065, A2142, A2199, A2255, and A2670) exhibit a significantly high rotational velocity, when compared to their velocity dispersion (vrot/σ ≥ 0.15). Interestingly, three of them (A1650, A2029, and A2199) are confirmed to be cool-core, relaxed clusters with no evidence of recent mergers, as is suggested by X-ray observational data. We also find a general tendency to overestimate the value of vrot when the number of galaxies with measured velocities is reduced, for which we put forward an analytical justification. This bias mainly affects slowly rotating clusters: we find that clusters with 0.15 ≤ vrot/σ ≤ 0.20 require at least 120 galaxies with measured velocities to limit the percentage error to less than ∼10%, while for rotating clusters with vrot/σ ≈ 0.25, ∼55 kinematic data points are sufficient to achieve the same accuracy.