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The distribution of neutral hydrogen in the intergalactic medium (IGM) iscurrently explored at low-z by means of UV spectroscopy of quasars. We proposean alternative approach based on UV colours of quasars as observed from GALEXsurveys. We built a NUV-selected sample of 9033 quasars with (FUV-NUV) colours.The imprint of HI absorption in the observed colours is suggested qualitativelyby their distribution as a function of quasar redshift. Because broad bandfluxes lack spectral resolution and are sensitive to a large range of N_HI aMonte Carlo simulation of IGM opacity is required for quantitative analysis. Itwas performed with absorbers randomly distributed along redshift and columndensity distributions, assumed to be a broken power law with index beta1 (10^15< N_HI <10^17.2 cm^-2) and beta2 (10^17.2 < N_HI <10^19 cm^-2). The redshiftdistribution is proportional to the redshift evolution law of the numberdensity of Lyman limit systems (LLS) per unit redshift as determined byspectroscopic surveys.The simulation is run with different assumptions on thespectral index alpha_nu of the quasar ionising flux. The fits between thesimulated and observed distribution of colours require an LLS redshift densitylarger than that derived from spectroscopic counting. This result is robust inspite of difficulties in determining the colour dispersion other than that dueto HI absorption. We provide arguments to retain alpha_nu = - 2, a valuealready extreme with respect to those measured with HST/COS. Further fitting ofpower law index beta1 and beta2 leads to a higher density by a factor of 1.7(beta1 = -1.7, beta2 = -1.5), possibly 1.5 (beta1 = -1.7, beta2 = -1.7). Beyondthe result in terms of density the analysis of UV colours of quasars reveals atension between the current description of IGM opacity at low z and thepublished average ionising spectrum of quasars.

GALEX colours of quasars and intergalactic medium opacity at low redshift