The Proximity Effect, the UV Background and the Statistics of the Lyman-Alpha Lines at High Resolution

We present results from high resolution (R\simeq 28000) spectra of six high- redshift QSOs taken at the ESO NTT 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\div30 km/s, corresponding to temperatures T>24000K, with a fraction of the order 15% in the range 10<b<20km/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 a dditional 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 logN_{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 Ly-alpha$ 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 logN_{HI}\simeq14, 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 \pm1 \times10^{-22} erg/cm^2/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 and Lyman Lymit systems and Ly-alpha clouds with log N_{HI}>= 14 and radii R\simeq 200 kpc are found to be similar, suggesting a possible common association with galaxies.