The heavy-element redshifts are thought to be due to intervening galaxies. The lines of evidence in favor of this hypothesis are as follows.
The frequency of absorption systems of a given type may be used to infer the required mean cross-section for absorbers of different types, assuming that the Schechter (1976) function describes the distribution over galaxy luminosities and that the radius of a galaxy is related to its luminosity in the same manner as the Holmberg radius, namely R L5/12. With these assumptions the effective radius, R*, of an L* galaxy comes out to be 75 kpc for the Lyman limit systems, 50 kpc for the MgII systems, 90 kpc for the CIV systems and 480 kpc for the Ly- clouds, supposing that they originate in galaxies. These numbers, which were given to me by Tytler (1986, private communication), are evaluated for a mean redshift of z = 2.5, for H0 = 100 km s-1 Mpc-1 and include only spiral galaxies. As is well known, these effective cross-sections are much larger than the optical sizes of galaxies at the present epoch. Also, according to Tytler (1987a) there are 6 times as many Lyman limit absorbers with N(HI) 3.0 x 1018 cm-2 at z = 2 than are seen locally in 21 cm measurements. In a similar vein, Wolfe (1988) has deduced that there are five times as many damped Ly- absorbers (presumed to be galactic disks) with N(HI) > 2.0 x 1020 cm-2 at z = 2.6 than there are locally per unit co-moving volume. These observations may constitute evidence for substantial evolution of galactic disks in relatively recent epochs.
A critical question concerning the heavy-element redshifts is the origin of the velocity structure. Three types of possible origin have been proposed: