Dynamical History of Ly \alpha clouds

The clustering properties of Ly ${\alpha}$ lines are analysed using an intermediate resolution ($\sim 1\AA$) spectra of 67 QSOs compiled from the literature. The pair velocity correlation function indicates a weak excess in the velocity intervals up to ${\rm \Delta v\;=\;600\; km\;s^{-1}}$. The z-integrated probability distribution of interline spacings also confirms the excess in the low velocity intervals. The dependence of pair velocity correlation on redshift and equivalent width are investigated. The cross-correlation properties of Ly $\alpha$ clouds with metal line redshifts are analysed. We do not find any tendency of Ly $\alpha$ lines to cluster around metal line systems. Depedence of the cross-correlation on redshift and equivalent widths of Ly $\alpha$ clouds are investigated. Various implications of the results are discussed.