Distribution of the signal-to-noise (S/N) of the scientific spectra in our sample. The position of the slits is shown as red bars, and the colours of the polygon, calculated by Voronoi Tessellation, indicate the S/N according to the scale in the colour bar in the bottom left corner. Slits with white background were excluded from the analysis owing to their low quality (S/N< 10). Black lines show the V-band contours in the range from 20 to 23.5 mag arcsec^-2 in intervals of 0.5 mag arcsec^-2, from Arnaboldi et al. (2012). The geometry of the polygons is shown hereafter in all the other maps in this study.