Fig. 2.— A log µ -log R plot of the surface brightness. The green line is determined by the slope of the inner two points and has a slope of ∼ 1. The black curve shows the fit found in Fig. 1. The central magnitude µ$_{0}$ is estimated from the slope of the inner points in Fig. 1 to be 17.6 mag arcsec − $^{2}$. 36