Left: “optimally extracted” Lyα surface brightness map as in Fig. 3 with the overlaid pseudoslits used to extract the spectra shown in the central and right panels. We assign an ID (blue) to each box of the pseudoslits. Center: normalized spectra of the Lyα emission along the pseudoslit shown in the left panel with solid lines. Each spectrum is color-coded following the color of its box on the left (details in Sect. 4). The dashed (dotted-dashed) vertical lines show the systemic (peak of the Lyα) redshifts for QSO1 (blue) and QSO2 (magenta). The respective shaded regions indicate the errors on the redshifts, as estimated by SDSS. The velocity shifts Δv are computed with respect to the systemic redshift of QSO2. We indicate with vertical arrows the flux-weighted centroid for each spectrum. The spectrum for box 4 (with no clear emission) is shown in Appendix D (Fig. D.1). Right: same as for the central panel, but for the second pseudoslit shown in the left panel with dotted lines.