Optimally extracted SB map of the Lyα-CEN. The quasar position is marked by a black dot. The thick black contour corresponds to the two-dimensional projection of the boundary of the CEN from the 3D-mask. It indicates an SB level of about 10⁻¹⁸ erg s⁻¹ cm⁻² arcsec⁻². The thin black contours indicate the S/N levels 3, 5, 15, 25, 35, and 45 extracted from the S/N map. The wavelength layer at the Lyα profile peak is used to report the background fluctuations in the image. On the top-right corner, the circle indicates the size of the FWHM of the instrument PSF. The green dashed lines delimit two regions of the Lyα-CEN (i.e. north and south from the quasar position) used for the extraction of the radial profile (see Fig. 3). The low-S/N region close to the quasar with SB values around zero could be the result of either random fluctuations or real features of the distribution of the CGM gas emission.