The Proximity Effect, the Uv Background and the Statistics of the Lyman-alpha Lines at High Resolution

We present results from high resolution (R≃ 28000) spectra of six highredshift QSOs taken at the ESO NTT telescope that allow the detailed study of the Lyman-α population in the redshift interval z = 2.8− 4.1. The typical Doppler parameters found for the Lyman-α lines lie in the interval b = 20÷30 km s, corresponding to temperatures T > 24000 K, with a fraction of the order 15% in the range 10 ≤ b ≤ 20 km s. These values are still consistent with models of low density, highly ionized clouds. The observed redshift and column density distributions obtained from these spectra and from the observations of 4 additional QSOs taken in the literature allow an accurate estimate of the proximity effect from a relatively large Lymanα sample (more than 1100 lines with logNHI ≥ 13.3) in the redshift interval z = 1.7− 4.1. A Maximum Likelihood analysis has been applied to estimate simultaneously the best fit parameters of the Lyman-α statistics and of the UV background. After correcting for the blanketing of weak lines, we confirm that the column density distribution is best represented by a double power-law with a break at logNHI ≃ 14, with a slope βs = 1.8 for higher column densities and a flatter slope βf = 1.4 below the break. A value JLL = 5 ± 1 × 10 −22 erg cm s Hz sr is derived for the UV background in the redshift interval z = 1.7 − 4.1, consistent with the predicted QSO contribution. No evidence is found for redshift evolution of the UVB in the same redshift interval. The comoving volume density distributions of protogalactic damped systems, Lyman Limit systems and Lyman-α clouds with logNHI ∼> 14 and radii R ≃ 200 kpc are found to be similar, suggesting a possible common association with