Analysis of Ly $α$ absorption lines in the vicinity of QSOs

Abstract

We have compiled, from the literature, a sample of Ly $α$ forest lines in the spectra of 69 QSOs, all observed with a resolution between 60 to 100 km s$^{-1}$. The sample is studied for proximity effect. We have tried to account for the effect of blending which is inherent in the intermediate resolution sample, by calculating the column density distribution, using an effective velocity dispersion parameter, from the observed equivalent width distribution. The use of this column density distribution in the proximity effect analysis reduces the background intensity values by a factor of 2 to 3 compared to the values obtained by using the column density distribution obtained from high resolution observations. Evidence is presented for a weak correlation between the effective velocity dispersion parameter and equivalent width. Such a correlation if present can increase the background values by a factor of up to 1.5. Considerations of proximity in the spectra of 16 QSOs, from our sample, exhibiting damped Ly $α$ lines gives a background intensity which is 3 times smaller than the values obtained from the whole sample, confirming the presence of dust in the damped Ly $α$ systems. Lines close to the QSOs are shown to be marginally stronger and broader compared to lines away from the QSOs. The Ly $α$ lines with absorption redshift larger than emission redshift are shown to be uncorrelated with QSO luminosity, radio loudness or optical spectral index. These lines occur more frequently at high redshifts. Their presence is correlated weakly with the presence of associated metal line systems. The possibilities that the QSO emission redshift is considerably higher and that either the Ly $α$ clouds or the QSOs have peculiar velocities are considered. It is argued that a combination of both these