The Proximity Effect, the UV Background, and the Statistics of the LY alpha Lines at High Resolution

We present results from high-resolution (R ~ 28,000) spectra of six high-redshift QSOs taken at the ESO New Technology Telescope that allow the detailed study of the Lyα population in the redshift interval z = 2.8-4.1. The typical Doppler parameters found for the Lyα lines lie in the interval b = 20-30 km s^-1^, corresponding to temperatures T > 24,000 K, with a fraction of the order 15% in the range 10 <= b <= 20 km s^-1^. 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 four additional QSOs taken in the literature allow an accurate estimate of the proximity effect from a relatively large Lyα sample (more than 1100 lines with log N_HI_ >= 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 Lyα statistics and of the UV background (UVB). Alter 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 log N_HI_ ~ 14, with a slope β_s_ = 1.8 for higher column densities and a flatter slope β_f_ = 1.4 below the break. A value J_LL_ = 5 +/- 1 x 10^-22^ ergs cm^-2^ s^-1^ Hz^-1^ sr^-1^ is derived for the UVB 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 Lyα clouds with log N_HI_ >~ 14 and radii R ~ 200 kpc are found to be similar, which suggests a possible common association with galaxies.