SDSS footprint represented with black dots. In yellow, we show the galaxies with spectroscopic redshift. In green, we overplotted the region analysed by Chen et al. (2016). Finally, the area delimited by the red thick solid lines is the one used in this work and the corresponding α_J2000 and δ_J2000 values are highlighted in red in the axes. We only rendered a random subsample of points for each colour. This was done for display purposes only.