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.