Top: RV in the system of the cluster as a function of distance X from the optimal rotation axis for models with constant rotation rate (left), constant rotational velocity (centre), and the more realistic rotation curve discussed above (right). The best-fit rotation curves are shown as solid lines, and  ± σ_1D envelopes are represented by dashed lines. Bottom: histogram of amplitudes Ω (left), V_rot (centre) and A (right) from 10 000 Monte Carlo simulations of RV distributions with no rotation. The optimal values for the observed RV configuration are indicated by dashed vertical lines. Confidence levels of 94.6%, 93.6% and 95.6% are found respectively for the best-fit amplitude of each model.