The origin of Ly alpha absorption systems at z > 1: Implications from the Hubble Deep Field

The Hubble Deep Field images have provided us with a unique chance to relat e statistical properties of high-redshift galaxies to statistical propertie s of Ly alpha absorption systems. Combining an empirical measure of the gal axy surface density versus redshift with an empirical measure of the gaseou s extent of galaxies, we predict the number density of Ly alpha absorption systems that originate in extended gaseous envelopes of galaxies versus red shift. We show that at least 50% and as much as 100% of observed Ly alpha a bsorption systems of W greater than or similar to 0.32 Angstrom can be expl ained by extended gaseous envelopes of galaxies. Therefore, we conclude tha t known galaxies of known gaseous extent must produce a significant fractio n and perhaps all of Ly alpha absorption systems over a large redshift rang e.