Signatures and bias assessment of rotation in galaxy cluster members

TL;DR

This study examines the presence of systematic rotation in galaxy cluster members, assessing how low galaxy counts affect rotation measurements, and finds significant rotation in some relaxed clusters, highlighting biases in small samples.

Contribution

It introduces a method to measure rotation in galaxy clusters and evaluates the impact of low-number statistics on these measurements, providing insights into cluster dynamics.

Findings

Eight clusters show significant rotation compared to velocity dispersion.

Relaxed, cool-core clusters tend to exhibit higher rotation.

Small sample sizes can lead to overestimation of rotation velocities.

Abstract

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. 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 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 included in the analysis with a bootstrapping technique. 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 ($v_{rot}/\sigma\geq 0.15$). Interestingly, three of them (A1650, A2029 and A2199) are confirmed to be cool-core, relaxed clusters with no evidence of recent mergers, as suggested by X-ray observational data. We also find a general tendency to overestimate the value of $v_{rot}$ when the number of galaxies with measured velocities is reduced, for which we put forward an analytical justification.