Top: fit of Eq. (7) based on the six points denoted with circles. Overplotted with dotted lines is the empirical relation of D_mom(L, Z) determined by Backs et al. (2024a). Bottom: projection of the two-dimensional fit to D_mom(L, M_eff) (Eq. (8)) onto the luminosity axis, shown by black points (see text for further details). In both plots, the square points have been excluded from the fits, red borders indicate troublesome fits, the value for α determined from the fit is provided and the metallicity of both the tracks and the points are colour-coded. Uncertainty regions on the fit have been excluded for clarity. In the top panel they are large and span the entire vertical range of the panel.