If the damped Ly- systems are the progenitors of galactic disks, then our findings restrict scenarios suggested for disk formation. For example if the disk-like structure indicated in the case of PKS0458-02 is a generic feature of the damped systems, then their detection out to z = 2.8 indicates that galactic disks have formed by t (0.9 - 2.1) h^{-1} Gyr. This pushes the epoch of disk formation back to the time when globular clusters first formed, and consequently argues against a significant age gap (Mihalas and Binney 1981) between the disk and the halo. Furthermore, the early formation of disks restricts models in which disks form slowly, on a Hubble time scale (Gunn 1982b).
If, in addition, the radii of the high-z disks are large compared to R_{Ho}, then a natural sequence of events is one in which the protogalaxy first collapses to a giant disk which subsequently contracts in the plane. Schiano, Wolfe and Chang (1990) recently ran a series of hydrodynamical calculations to check the plausibility of this scheme. The protogalaxy was simulated by axially symmetric spheroids of gas and dark matter in which the ratio of masses, M_{dm} / M_{gas} 10. The gas and the dark matter obtain their spin from tidal torques induced by neighboring galaxies (Peebles 1969; Fall and Efstathiou 1980). Initially both spheroids are hydrostatically supported by the random motions of isothermal density distributions which fall off like r^{-2} beyond the core radius, r_{c}. The gas collapses because it cools on a time-scale short compared to the free-fall time. Figure 3 shows the density contours in a meridional slice of the spheroid at early stages of collapse. The important point is that collapse to a large disk is possible by z 3 in most cosmological models.
Figure 3. Logarithmic density contours (cgs units) and velocity fields in the r-z plane for a model with r_{c} = 3 kpc. Elapsed times are (a) 0.30, (b) 0.49, (c) 0.70, (d) 1.03 Gyr. |
In the second phase of the calculation we examined the contraction of the centrifugally supported disk that formed out of the initial collapse shown in Figure 3. We considered a mechanism which is a dynamical consequence of the infall of mass shed by halo stars (Ostriker and Thuan 1975). When the infall increases the mass of the disk by a factor, f, the radius of the disk must contract by the same factor: this occurs because the radius, R, of the disk times M(R), the mass within R, is an adiabatic invariant. Our numerical calculations showed that the disk did contract by a factor f in about 1 Gyr, although the contraction was accompanied by axisymmetric instabilities. The rapid shrinkage of the disk implies that the detection rate of damped Ly- systems at z 1.5 should be much less than the rate reported in our survey, even after cosmological effects are taken into account.
This work was supported by NSF grant AST 84144-14.