Black and blue represents the Lyα emission lines in the CEN and quasar spectrum, respectively. These Lyα line spectra are reported in velocity space relative to the z_QSO and they both are normalised to their peak. The white region reports the spectral range of the 3D-mask. The red area marks the peak wavelength of the Lyα line derived through Gaussian modelling including the errors. The yellow area indicates the 1σ uncertainty of the systemic redshift of the quasar (Vietri et al. 2018). The absorption line at ∼1500 km s⁻¹ is a sky feature, which could not be appropriately removed by the algorithm CubePSFSub.