2.3. The Metallicity of DLAs
Even before the advent of 8-10m class telescopes,
it was realised that the metal and dust content of DLAs
could be investigated effectively by targeting
a pair of (fortuitously) closely spaced multiplets, Zn II
2025, 2062 and
Cr II 2056, 2062, 2066
(Meyer, Welty, & York
1989;
Pettini, Boksenberg, &
Hunstead 1990).
The key points here are that while Zn is essentially undepleted
in local diffuse interstellar clouds, Cr is mostly locked up in dust grains
(Savage & Sembach
1996).
Consequently,
the ratio N(Zn II) / N(H I) observed in DLAs, when
compared with the solar abundance of Zn, yields
a direct measure of the degree of metal enrichment
(in H I regions Zn is predominantly singly ionised,
and the ratios Zn I / Zn II and Zn III / Zn II are
both << 1). On the other hand, a deficit - if one is found - of
N(Cr II) / N(Zn II) compared to the solar
relative abundances of these two elements would measure the
extent to which refractory elements have condensed
into solid form in the interstellar media traced by DLAs
and, by inference, be an indication of the presence of
dust in these early galaxies (Cr is also singly ionised in H I gas).
From the point of view of chemical evolution, both Zn and Cr closely trace Fe in Galactic stars with metallicities [Fe/H] between 0 (i.e. solar) and -2 (1/100 of solar; Sneden, Gratton, & Crocker 1991, McWilliam et al. 1995). Additional advantages are the convenient rest wavelengths of the Zn II and Cr II transitions, which at z = 2 - 3 (where DLAs are most numerous in current samples) are redshifted into a easily observed portion of the optical spectrum, and the inherently weak nature of these lines which ensures that they are nearly always on the linear part of the curve of growth, where column densities can be derived with confidence from the measured equivalent widths (e.g. Bechtold 2002).
All in all, a small portion of the red spectrum of a QSO
with a damped Ly
system
has the potential of providing
some important chemical clues on the nature of these absorbers
and the evolutionary status of the population of galaxies
they trace. One of the first detections of Zn and Cr in a DLA
is reproduced in Figure 6. However, data of
sufficiently high signal-to-noise ratio to detect the weak absorption
lines of interest (or to place interesting upper limits
on their equivalent widths) typically require nearly
one night of observation per QSO with a 4m class telescope.
Consequently, it was necessary to wait until the mid-1990s before
a sufficiently large sample of Zn and Cr measurements in DLAs
could be assembled.
 |
Figure 6. Portion of the Palomar spectrum
of the bright QSO PHL 957 recorded by
Pettini et al. (1990),
encompassing the region of the Zn II and Cr II absorption
lines in the zabs = 2.3091 DLA.
The vertical tick marks indicate the positions of the lines as follows.
Line 1: Zn II
|
Figure 7 shows the current data set from the survey by our group (e.g. Pettini et al. 1997b; 1999). There are several interesting conclusions which can be drawn from these results.
 |
Figure 7. Plot of the abundance of Zn against redshift for the full sample of 41 DLAs from the surveys by Pettini and collaborators. Abundances are measured on a log scale relative to the solar value shown by the broken line at [Zn/H] = 0.0; thus a point at [Zn/H] = -1.0 corresponds to a metallicity of 1/10 of solar. Upper limits, corresponding to non-detections of the Zn II lines, are indicated by downward-pointing arrows. Upward-pointing arrows denote lower limits in two cases where the Zn II lines are sufficiently strong that saturation may be important. |
(a) Damped Ly systems
are generally metal poor at all redshifts sampled. Evidently DLAs arise
in galaxies at early stages of chemical evolution,
since nearly all of the points in Figure 7 lie below
the line of solar abundance.
The statistic of relevance here is the
the column density-weighted mean abundance of Zn
 |
(2.4) |
where
 |
(2.5) |
and the summations in eq. (2.5) are over the n DLA systems in a given sample. In this way, by counting all the Zn atoms per unit cross-section (cm-2) and dividing by the total column density of neutral hydrogen we find that DLAs have a typical metallicity of only ~ 1/13 of solar ([<Zn / HDLA>] = - 1.13).
(b) There appears to be a large range in the values of metallicity reached by different galaxies at the same redshift. Values of [Zn/H] in Figure 7 span nearly two orders of magnitude, pointing to a protracted `epoch of galaxy formation' and to the fact that chemical enrichment probably proceeded at different rates in different DLAs. The wide dispersion in metallicity goes hand in hand with the diverse morphology of the DLA galaxies which have been imaged at z < 1, as discussed earlier (Section 2.1).
When the metallicity distribution of damped
Ly systems
is compared with those of different stellar populations of the Milky Way,
we find that is broader and peaks at
lower metallicities than those of either thin or thick disk
stars (Figure 8). At the time when our Galaxy's
metal enrichment was at levels
typical of DLAs, its kinematics were closer to those of the
halo and bulge than a rotationally supported disk.
This finding is at odds with the
proposal that most DLAs are large disks with rotation velocities in
excess of 200 km s-1, put forward by
Prochaska & Wolfe
(1998).
 |
Figure 8. Metallicity distributions,
normalised to unity, of DLAs at z
|
(c) There is little evidence from the data in Figure 7 for any redshift evolution in the metallicity of DLAs. This question has also been addressed using the abundance of Fe which, thanks to its rich absorption spectrum, can be followed to higher redshifts and lower metallicities than Zn (Prochaska, Gawiser, & Wolfe 2001). Once allowance is made for the fraction of Fe in dust grains, Vladilo (2002b) finds a gradient of -0.32 in the linear regression of [Fe/H] vs. zabs. However, while Figure 7 suggests that the chances of finding a DLA with [Zn/H] > - 1.0 are possibly greater at z < 1, the column density-weighted metallicities - which measure the density of metals per comoving volume - are consistent with no evolution over the range of redshifts probed so far, irrespectively of whether Zn or Fe are considered (see Figure 9). Evidently, the census of metals at all redshifts is dominated by high column density systems of low metallicity.
 |
Figure 9. Column density-weighted metallicities of DLAs in different redshift intervals, from the surveys by Pettini et al. (1999) for Zn, and Prochaska & Wolfe (2002) for Fe. The lower abundance of Fe relative to Zn probably reflects the presence of moderate amounts of dust in most DLAs (Vladilo 2002b). |
The lack of evolution in both the neutral gas and metal content of DLAs was unexpected and calls into question the notion that these absorbers are unbiased tracers of these quantities on a global scale. On the other hand, the paucity of data at redshifts z < 1, that is over a time interval of more than half of the age of the universe (Table 1), makes it difficult to draw firm conclusions and it may yet be possible to reconcile existing measurements with models of cosmic chemical evolution (Pei, Fall, & Hauser 1999; Kulkarni & Fall 2002).
2 - 3
and of stars belonging to the disk
(