The proximity effect, the UV background, and the statistics of the Ly alpha lines at high resolution

We present results from high-resolution (R similar or equal to 28,000) spectra of six high-redshift QSOs taken at the ESO New Technology Telescope that allow the detailed study of the Ly alpha population in the redshift interval z = 2.8-4.1. The typical Doppler parameters found for the Ly alpha 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 less than or equal to b less than or equal to 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 Ly alpha sample (more than 1100 lines with log N-HI greater than or equal to 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 Ly alpha statistics and of the UV background (UVB). 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 log N-HI similar or equal to 14, with a slope beta(s) = 1.8 for higher column densities and a flatter slope beta(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 Ly alpha clouds with log N-HI greater than or similar to 14 and radii R similar or equal to 200 kpc are found to be similar, which suggests a possible common association with galaxies.