Abstract
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 β1 (1015 cm−2 < NHI < 1017.2 cm−2) and β2 (1017.2 cm−2 < NHI < 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 αν 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 αν (softer ionising quasar spectrum) and would vanish only with values of αν which are not supported by existing observations. Conclusions. We provide arguments to retain αν = −2, a value already extreme with respect to those measured with HST/COS. Further fitting of power law index β1 and β2 leads to a higher density by a factor of 1.7 (β1 = −1.7, β2 = −1.5), possibly 1.5 (β1 = −1.7, β2 = −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.
Highlights
Decades of quasar absorption line studies have provided a wealth of information on the column density and redshift distributions of the neutral hydrogen pockets surviving in the nearly completely ionised intergalactic medium (IGM)
In order to support our view that the red colours of quasars may indicate the presence of neutral hydrogen absorption we have looked at the distribution of GALEX UV colour vs. emission redshift in existing samples with spectroscopically identified Lyman limit systems (LLS) at low redshift
We emphasise that the number density in ordinate is the fraction of absorbers in the range 1017.2 cm−2 < NHI < 1019 cm−2 normalised by the density of LLSs over the same range of HI column density obtained from the evolution law of Ribaudo et al (2011) integrated to the appropriate redshift
Summary
Decades of quasar absorption line studies have provided a wealth of information on the column density and redshift distributions of the neutral hydrogen pockets surviving in the nearly completely ionised intergalactic medium (IGM). These distribution functions have become basic ingredients for the evaluation of the IGM opacity to ionising radiation, a crucial link between the emissivity of the sources of ionising photons and the intensity of the ionising ultraviolet background radiation O’Meara et al (2013) have analysed the incidence of LLS in 71 quasars observed with the low dispersion modes of HST/ACS and HST/WFC3; they report 19 LLSs (τ > 1) at z < 2 but with an average redshift of about 1.8 and the bulk of statistical power at 2 < z < 2.5
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