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
RHo, 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,
Mdm / Mgas
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, rc. 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 rc = 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.