We continue our discussion of the physical properties of individual damped systems. Some very interesting results have been derived with techniques other than optical spectroscopy, and these are considered first.
Observations of the polarized radio continua of a sample of 116 QSOs reveals the presence of Faraday rotation in 20% of the objects. Furthermore the incidence of Faraday rotation is linked statistically with the occurrence of metal-line absorption (Kronberg and Perry 1982; Welter, Perry and Kronberg 1984). The implication is that the foreground gas is a magneto-ionic medium with a significant rotation measure; i.e., a B field with a significant component along the line-of-sight B|| is associated with partially ionized gas. In many cases the detected gas is intrinsic to the QSO. Since one expects one or more intervening CIV clouds per line-of-sight (Young, Sargent and Boksenberg 1982), it is safe to assume that less than 20% of these clouds exhibit Faraday rotation. On the other hand 4 out of the 5 QSOs in the sample with 21 cm absorption exhibit statistically significant Faraday rotation. Although we are dealing with small number statistics, the implication is that B fields are a generic feature of the damped Ly- population. Table 1 shows the results. The Kronberg and Perry (1984) formalism has been used to calculate xe B|| at the absorption redshift, z, from RM, the rotation measure corrected to exclude the Galactic contribution: xe B|| is the fractional ionization of hydrogen times the density weighted B||. Since xe is unlikely to exceed 0.1 in gas with a Lyman-limit optical depth of 104 we conclude that B|| 10 µGauss in gas with z 2. The existence of significant B fields in a galactic disk with z 2 may be difficult to reconcile with the dynamo theory for the formation of magnetic fields. In the dynamo theory, an infinitesimal seed field is amplified to µGauss strength after a few rotation periods of the Galactic disk (Parker 1979). The problem is that the rotation period of a giant disk at z 2 is comparable to its age, and as a result there may not be enough time for significant amplification.
|3C 196||0.437||145||(0.86-3.5)x(TS/100 °K)||14.0-0.33|
|3C 286||0.692||3||3x(TS/100 °K)||0.11|
Radio investigations have also been valuable in providing detailed information on the kinematics and spatial structure of damped systems with 21 cm lines. Since these topics are discussed by Briggs (1988) in this volume, they will not be repeated here. It is important to stress, however, that the VLBI investigation of the z = 2.040 21 cm line in PKS0458-02 reveals (a) that the absorber extends more than (8h-1) kpc transverse to the line-of-sight (h = H0 / 100 km s-1 Mpc-1), and (b) that the self-gravity and low velocity dispersion of the HI imply a hydrostatic scale-height of less than 500 pc. That the transverse dimension greatly exceeds the line-of-sight dimension is direct evidence for a disk-like structure with a radius of curvature large compared to the radii of dwarf galaxies.
We now consider the density parameter. The global mass density of the damped systems is obtained by averaging < N(HI) > over the space-time volume occupied by these absorbers. A mean molecular weight of µ = 1.4 is adopted to convert from number density to mass density. This mass density is then divided by the critical density corresponding to < z >, the average absorption redshift, and the result is redshifted to the present to obtain the density parameter, damp. The results, which depend on q0 and h, are summarized in Table 2. damp is relatively well determined, because the ratio of total H to detected H0 is of order unity. By contrast the density parameter of the Ly- forest clouds is highly uncertain, because H/H0 is unknown. In most cases damp dominates the mass contributed by model Ly- forest clouds (see Tytler 1988). damp takes on added significance when it is compared with lum-disk, the density parameter due to luminous matter (stars) in the disks of spiral galaxies. Combining the luminosity density of galaxies, LB = 3 x 108 hL Mpc-3 (Gunn 1982) with a (M / L)lum-disk 2 we find that lum-disk 2 x 10-3 h-1. The rough agreement between the mass per unit comoving volume of the damped Ly- absorbers at high redshifts and the stellar disks of spiral galaxies at the present epoch might be a coincidence. More likely it indicates that we have detected the same baryons at different stages of galactic evolution. As a result it is possible that we have detected galactic disks soon after the collapse of the protogalaxy, but before most of the gas was converted into stars.