Calibration of the Faber-Jackson relation. The filled circles show the measured stellar velocity dispersions, σ₀, of a subset of 14 bright cluster member galaxies as a function of their total magnitudes in the HST F160W band (colour-coded depending on their averaged spectral ⟨S/N⟩ values) from G25. The black square highlights the σ₀ measurement of the BCG. The solid and dotted black lines correspond to the best-fit σ₀−F160W relations, obtained as described in Sect. 3. The green line and shaded area show the median and the 68% confidence level of the σ₀−F160W relation from our reference strong-lensing model.