Analysis of Ly absorption lines in the vicinity of QSOs

We have compiled, from the literature, a sample of Ly ${\alpha}$ forest linesin the spectra of 69 QSOs, all observed with a resolution between 60 to 100 kms$^{-1}$. The sample is studied for proximity effect. We have tried to accountfor the effect of blending which is inherent in the intermediate resolutionsample, by calculating the column density distribution, using an effectivevelocity dispersion parameter, from the observed equivalent width distribution.The use of this column density distribution in the proximity effect analysisreduces the background intensity values by a factor of 2 to 3 compared to thevalues obtained by using the column density distribution obtained from highresolution observations. Evidence is presented for a weak correlation betweenthe effective velocity dispersion parameter and equivalent width. Such acorrelation if present can increase the background values by a factor of up to1.5. Considerations of proximity in the spectra of 16 QSOs, from our sample,exhibiting damped Ly ${\alpha}$ lines gives a background intensity which is 3times smaller than the values obtained from the whole sample, confirming thepresence of dust in the damped Ly $\alpha$ systems. Lines close to the QSOs areshown to be marginally stronger and broader compared to lines away from theQSOs. The Ly $\alpha$ lines with absorption redshift larger than emissionredshift are shown to be uncorrelated with QSO luminosity, radio loudness oroptical spectral index. These lines occur more frequently at high redshifts.Their presence is correlated weakly with the presence of associated metal linesystems. The possibilities that the QSO emission redshift is considerablyhigher and that either the Ly $\alpha$ clouds or the QSOs have peculiarvelocities are considered. It is argued that a combination of both theseComment: 33 pages, revtex, 8 figures available on request at anand@iucaa.ernet.in. (MNRAS in press