The Gaseous Extent of Galaxies and The Origin of QSO Absorption Line Systems

We present results of an ongoing program to study the gaseous extent of galaxies and the origin of QSO absorption line systems. For \lya absorption systems, we find that absorption equivalent width depends strongly on galaxy impact parameter and galaxy $B$-band luminosity, and that the gaseous extent of individual galaxies scales with galaxy $B$-band luminosity as $r\propto L_B^{0.40\pm0.09}$. Applying the results to galaxies in the Hubble Deep Field to calculate the predicted number density of \lya absorption lines as a function of redshift and comparing it with observations, we find that at least 50% and perhaps as much as 100% of \lya absorption systems with $W\apg0.32$ \AA can be explained by the extended gaseous envelops of normal galaxies. The anti-correlation analysis has also been performed on \civ absorption line systems, and the results show that the ionized gas cross section scales with galaxy $B$-band luminosity as $r\propto L_B^{0.76\pm0.26}$.