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tokens: is particularly true for the Ethos-4 model, where the DAOs are nearly absent as early as z = 20 (given the finite resolution of our numerical set-up). 4.2 The Lyman-α forest at a glance In the top row of Fig. 4 we present the transmitted flux fraction, F , measured along random lines of sight at z = 5 (left) and z = 4.2 (right) for the CDM and sDAO models. The spectra have been created following the procedure outlined in Section 3.2 , after rescaling the mean transmission in each simulation box to the observed transmitted flux. In each case, the lines of sight have been chosen so as to probe the same spatial region in the two simulation volumes. The lower sub-panels show the difference in the transmitted flux, Δ F = F CDM − F sDAO . This figure highlights the fact that the same line of sight probes different intervening structures in the IGM of the two simulations. In particular, it is clear that, just as in WDM, absorption lines are in general deeper for a random line of sight in the CDM simulation than for the sDAO model, signifying the presence of a more clumpy IGM. This difference is a direct consequence of the cut-off in the initial power spectrum. Figure 4. Top row : synthetic mock spectra extracted from our simulations at z = 5 (left) and z = 4.2 (right). Each panel shows the transmitted flux fraction, F , in velocity space for a specific randomly selected line of sight through the simulation volume at the corresponding redshift. The lines of sight are chosen such that the same spatial region is probed in the CDM (black) and sDAO (red) simulation volumes. Clearly, more intervening structure can be inferred from the CDM spectra as evidenced by the deeper transmission lines than in sDAO . The lower panels show the relative difference in transmitted flux, i.e. Δ F = F CDM − F sDAO . Middle row : The same lines of sight shown in the top row but before we rescale the mean flux to the observed values. Bottom row : The unscaled flux PDF at these redshifts. These panels show the ‘true’ difference in the transmitted flux between the CDM and sDAO models, i.e. by removing any artefacts that may be brought in by the different amount of rescaling required for the two DM models. The sDAO and CDM models have been rescaled by different amounts when matching the simulated spectra to the observed mean transmitted flux. To disentangle the implications of rescaling from the different line shapes due to the modified cosmology at a given UV background, the middle row of Fig. 4 shows the unscaled line-of-sight spectra at z = 5 and z = 4.2. Clearly, there are residual differences between the two models even in the unscaled case. Quantitatively, these effects are seen more clearly in the bottom row of Fig. 4 , which shows the probability distribution function (PDF) of the unscaled transmitted flux for the two models at these redshifts. Considering just CDM to begin with, it is noticeable that as time proceeds, there is a cut-off in the flux PDF at higher values