GALEX colours of quasars and intergalactic medium opacity at low redshift

Aims. The distribution of neutral hydrogen in the intergalactic medium (IGM) is currently explored at low redshift by means of UV spectroscopy of quasars. We propose here an alternative approach based on UV colours of quasars as observed from GALEX surveys. We built a NUV-selected sample of 9033 quasars with (FUV-NUV) colours. The imprint of HI absorption in the observed colours is suggested qualitatively by their distribution as a function of quasar redshift. Methods. Because broad band fluxes lack spectral resolution and are sensitive to a large range of HI column densities a Monte Carlo simulation of IGM opacity is required for quantitative analysis. It was performed with absorbers randomly distributed along redshift and column density distributions. The column density distribution was assumed to be a broken power law with index beta(1) (10(15) cm(-2) < N-HI < 10(17.2) cm(-2)) and beta(2) (10(17.2) cm(-2) < N-HI < 1019 cm(-2)). For convenience the redshift distribution is taken proportional to the redshift evolution law of the number density of Lyman limit systems (LLS) per unit redshift as determined by existing spectroscopic surveys. The simulation is run with different assumptions on the spectral index alpha(nu) of the quasar ionising flux. Results. The fits between the simulated and observed distribution of colours require an LLS redshift density larger than that derived from spectroscopic counting. This result is robust in spite of difficulties in determining the colour dispersion other than that due to neutral hydrogen absorption. This difference decreases with decreasing alpha(nu) (softer ionising quasar spectrum) and would vanish only with values of alpha(nu) which are not supported by existing observations. Conclusions. We provide arguments to retain alpha(nu) = -2, a value already extreme with respect to those measured with HST/COS. Further fitting of power law index beta(1) and beta(2) leads to a higher density by a factor of 1.7 (beta(1) = -1.7, beta(2) = -1.5), possibly 1.5 (beta(1) = -1.7, beta(2) = -1.7). Beyond the result in terms of density the analysis of UV colours of quasars reveals a tension between the current description of IGM opacity at low z and the published average ionising spectrum of quasars.