Analysis of the possible contribution of dust at 2500 Å. Top: distribution of the difference of the distance moduli (ΔDM, red points; ΔDM = 0 represents the ΛCDM) as a function of redshift. The parameter ΔDM is defined as DM_red − DM, where DM is computed from the quasar data as described in Sect. 9. The DM_red values are estimated by correcting the observed F_UV for an additional intrinsic (and redshift-dependent) extinction. τ is the optical depth at the rest-frame 2500 Å that would be needed to ascribe the discrepancy to an underestimated dust extinction in the UV flux. The black points represent the average (and 1σ uncertainties) of ΔDM in the same narrow redshift intervals used in Figs. 8 and 9. Middle: deviation in σ of the average points with respect to ΛCDM. Bottom: ratio between the cosmographic fit obtained from the data, where F_UV is corrected for additional extinction, and the ΛCDM. The cosmographic fit perfectly matches ΛCDM for the chosen values of dust reddening (see Sect. 11.4).