5. ORIGIN OF THE HEAVY-ELEMENT REDSHIFTS

The heavy-element redshifts are thought to be due to intervening galaxies. The lines of evidence in favor of this hypothesis are as follows.

1. The discovery of CaII and NaI absorption lines in the spectra of QSOs close to nearby galaxies on the plane of the sky (Boksenberg and Sargent 1978; see also the review by Blades 1988).
2. The discovery of galaxies at the same redshift as absorption systems observed in the spectra of QSOs with redshifts up to z 0.8. Several impressive cases of such galaxies are detailed in the survey article by Bergeron (1988).
3. The presence of lines of heavy-elements which make it unlikely that intergalactic objects are responsible.
4. The energy arguments referred to in Section 1 which rule out ejection from the QSO.
5. The existence of common absorption systems in the spectra of widely separated QSOs which can be interpreted as being due to galaxies in clusters or superclusters.

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 L^5/12. 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:

1. motions of clouds within galaxies which would be limited to about 200 km s^-1 in order to be gravitationally confined;
2. explosive phenomena in galaxies such as supernova remnants; and
3. the tip of the galaxian correlation function, which should extend out to velocities beyond 1000 km s^-1, well beyond the velocities which would be expected for gravitationally confined clouds. We shall return to this question in Section 7.