GALEX colours of quasars and intergalactic medium opacity at low
  redshift
  The distribution of neutral hydrogen in the intergalactic medium (IGM) is
currently explored at low-z by means of UV spectroscopy of quasars. We propose
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. Because broad band
fluxes lack spectral resolution and are sensitive to a large range of N_HI 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, 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 redshift
distribution is proportional to the redshift evolution law of the number
density of Lyman limit systems (LLS) per unit redshift as determined by
spectroscopic surveys.The simulation is run with different assumptions on the
spectral index alpha_nu of the quasar ionising flux. 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 HI absorption. 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 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). 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.
