We now turn to properties of the individual absorbers. Accurate spectra
have been
obtained at the MMT and CTIO 4-m in order to (a) test the damping
hypothesis for
the Ly-
profile of each `disk
candidate', (b) confirm the redshift with the wavelengths
of narrow metal lines, and (c) derive physical parameters for the
absorbing gas. A
full description of the observations and analysis is given in
Turnshek et al.
(1989) and
Wolfe et al.
(1988).
The essential results are summarized in what follows.
All of the spectra blueward of
Ly- emission have been obtained
with the MMT
at a resolution of 
= 1 - 2 Å.
Figure 1 is a sample of spectra in which redshifts
determined from the metal lines have been used to shift the wavelength
scale to the restframe of each damped
Ly- absorber. In QSOs with two
damped systems, a
restframe spectrum is shown for both redshifts. In every case
Ly- takes on the
characteristic shape of a profile with damping wings. Quantitative tests
show that
most of the damped fits to the observed profiles are quite good. The wide range
in W(Ly-) illustrated in
Figure 1 indicates that the damped systems exhibit wide
variations in N(HI). Figure 1 also shows that in
cases where the metal lines are predicted to occur redward of
Ly- emission, CII
1334 is detected. When low-ion
metal lines such as CII are predicted to occur blueward of
Ly- emission, confusion
with the Ly- forest lines is
inevitable. To determine redshifts in those cases, we
obtained the FeII
2367-2600 multiplets and the MgII doublet with red spectra
acquired with the CTIO 4-m.
|
Figure 1. Restframe
Ly- |
Comparison between the metal-line and
Ly- profiles on a velocity scale is
particularly valuable, as it provides a sensitive test of the damping
hypothesis, even in
cases where the signal-to-noise ratio of the data is not optimum. In
every case the metal-line profiles resemble the example in
Figure 2, in which a narrow, unresolved
(v
150 km s-1) metal
line is located at the center of a symmetric
Ly- feature
which appears to have v
3000 km s-1. The
broadening mechanisms for the two
transitions are obviously different. The CII line is broadened by
Doppler motions, while quantum mechanical effects dominate the
Ly- profile. This is in contrast to
CIV-selected systems with wide
Ly- lines. In those cases the
metal lines break up
into multiple narrow components (a) which span velocity intervals not
much smaller
than that covered by Ly- , and
(b) which are not symetrically displaced with
respect to the center of Ly-
(Bechtold, Green and
York 1987;
Bechtold 1987).
In this
instance both Ly- and the
metal-lines are Doppler broadened, with
Ly- assuming
a larger width due to an increase in saturation.
|
Figure 2. Comparison between z = 2.8 and
damped Ly- |
The N(HI) inferred from the fits of Voigt profiles are better determined
than in
most absorption systems, because the fits are independent of velocity
dispersion. The
errors are dominated by inaccuracies in setting the continuum, and are
unlikely to
exceed 50% . Above 2 x 1020 cm-2, where the
distribution of N(HI) is complete, we
find that the mean column density, < N(HI) > = 1021
cm-2. While this is typical for
the HI disks of spirals at R 
RHo, it greatly exceeds the upper limit of 1018
cm-2
set at R 2.5 x RHo, for
a complete sample of spiral galaxies
(Briggs et al.
1980).
If the spirals have the same comoving number density as the damped systems, the
latter are greater than the former in HI content as well as size.
Because the damped Ly- systems
are selected primarily on the basis of their
HI properties, the resulting distribution of metal-line properties is
unbiased with
respect to metal-line selection criteria. Comparison between the
detected W(CIV) and
W(CII), for example, should provide a fair sample of the actual metal-line
properties of the damped systems. We find that for the vast majority of damped
systems, W(CII) W(CIV). This
distinguishes the damped systems from most
CIV-selected clouds in which W(CIV)
W(CII)
(Wolfe 1983;
Danly, Blades and
Norman 1987).
Rather the damped systems resemble the MgII-selected clouds in this respect
(Lanzetta, Turnshek
and Wolfe 1987).
This is an interesting result in view of the
recent identification of MgII absorption systems with the halos of
galaxies at z 0.8
(Bergeron 1988).
The large W(CII) that are found for some damped systems may
then indicate the presence of a turbulent halo associated with the
quiescent disk.
In principle the damped systems are ideal for abundance determinations. Unlike
most QSO absorption systems the ionization state of the abundant
elements is known;
i.e., H0 / H 1,
C+ / C 1,
etc. However, a spectral resolution of
v
10 km s-1 is
required to separate the quiescent disk component from the turbulent
halo component.
Failure to do this leads one to underestimate the ionic column densities
deduced
from lines that are invariably saturated. A detailed description of the
dilemma is given in
Briggs et al. (1983).
t the same time we (Lanzetta, Turnshek and Wolfe)
are attempting to search for heavy elements locked up in grains. A program of
absolute spectrophotometry was initiated in order to compare reddening in QSOs
located behind damped systems with a control sample that shows no sign
of damped
Ly- absorption. The absence of
observed reddening leads to a preliminary constraint
on the dust-to-gas ratio given by (D/G)damped
0.5 x (D/G)Galaxy (see
also Pei and Fall 1987).
This limit is consistent with the strict upper limits placed on the presence
of H2 in one of the damped systems in our sample
(Black, Chaffee and
Foltz 1987).