Example of a galaxy spectrum simulated in the Horizon-AGN simulation. The galaxy lies at z = 3.60 and has a magnitude of m_rest, UV = 25.05 mag. Its synthetic spectrum is shown in blue. Red dashes indicate the positions of the Lyα, Lyβ, and Lyγ lines that originate in the galaxy. A spectrum with the added Lyα forest (even though we do not use it in the further analysis, we also plot the Lyβ forest etc.) is shown in black. Gaussian noise is added to this spectrum (see text for details), and the resulting spectrum is shown in orange in the zoomed plot. The noise corresponds to S/N = 5 and a resolution of R = 5000 (an exposure of ∼1.5 h at a fiducial resolution of MOSAIC, see Sect. 3).