Measured internal stellar velocity dispersion of 15 cluster galaxies as a function of their F160W magnitudes (filled circles). The colors encode the mean signal-to-noise ratio of each measurement, as reported in the color bar to the right. The magenta line is the best-fit σ − mag relation obtained as explained in Sect. 3.1.2, while the light-blue colored band shows the mean scatter around the best fit, Δσ_ap.