Measured internal stellar velocity dispersions of 85 cluster galaxies as a function of their magnitudes in the HST/F160W band are shown as filled circles, color-coded according to the mean signal-to-noise ratio of the galaxy spectra. The green line and filled area correspond to best-fit and the associated mean scatter of the σ − m_F160W relation, respectively (see Sect. 3.2). The red and blue areas correspond to the 68% confidence level of the σ − m_F160W relation obtained from the reference LM-model and the WL-model, respectively.