As Fig. 8, but including the empirical value obtained for the Galactic globular cluster ω Cen (open circle with error bars), and the empirical relationships by Rizzi et al. (2007) and Jang & Lee (2017b) based on samples of local galaxies (black and red lines, respectively; see text for details). The solid lines denote mean values of the relationships, while the dashed lines denote, respectively, the brighter and lower limits according to the 1σ uncertainty on the slope and zero point of Rizzi et al. (2007) results, and the 1σ uncertainty on the zero point of Jang & Lee (2017b) calibration.