5.1. A Global View of Metal Enrichment in the Universe Two Billion Years after the Big Bang
We could briefly summarise everything we have covered so far as follows.
(a) The intergalactic medium at z = 3 does not consist entirely of pristine material. At least the regions we have been able to probe so far show traces of metals at levels between 1/1000 and 1/100 of solar metallicity. Matter in these regions, however, is still at a higher density than the mean density of the universe at that epoch, and it is unclear at present whether a pre-galactic episode of star formation (often referred to as Population III stars), is required to explain the large scale distribution of elements in the IGM.
(b) Damped Ly
systems
have abundances similar to those of Population II stars in our Galaxy.
Perhaps they represent an early stage in the formation
of spiral galaxies, before most of the gas had been
converted into stars. It is also clear that a wide
variety of galaxy morphologies, including low surface brightness
galaxies, share the common characteristic of providing
a large cross-section on the sky at high surface densities of H I.
(c) Finally, Lyman break galaxies strongly resemble
what we call Population I stars in the
Milky Way. They are the sites of vigorous star formation
which (i) has produced a relatively high level of
chemical enrichment at early epochs, (ii) has built up stellar masses of
1010
M
in a sizeable fraction of the population,
and (iii) drives large-scale outflows of gas, metals and dust
into the intergalactic medium.
All of these characteristics point to LBGs as the
progenitors of today's spiral bulges and elliptical
galaxies, observed during the most active phase in their lifetimes.
The connection between LBGs and DLAs is currently the subject of
considerable discussion, as astronomers try and piece together
these different pieces of the puzzle describing the universe
at z = 3. Table 2 summarises some of the
relevant properties. Lyman break galaxies have systematically higher
star formation rates and metallicities, and their interstellar media
have been stirred to higher velocities, than is the case in most
DLAs. These seemingly contrasting properties can perhaps be reconciled
if the two classes of objects are in fact drawn from the same luminosity
function of galaxies at z = 3.
Since they are selected from magnitude limited samples,
the LBGs are preferentially bright galaxies - the data
in Table 2 refer to galaxies brighter than
L*
which corresponds to
= 24.5
(Adelberger & Steidel
2000).
If, on the other hand, the H I absorption cross-section decreases
only slowly with galaxy luminosity, as is the case at lower redshifts
(Steidel, Dickinson, &
Persson 1994),
the DLA counts would naturally be dominated by the far more numerous
galaxies at the steep
( = - 1.6) faint end of
the luminosity function.
| Property | LBGs | DLAs |
| SFR
(M
yr-1) |
~ 50 | < 10 |
| Z
(Z) |
~ 1/3 | ~ 1/20 |
 v (km
s-1) |
~ 500 | 
200 |
Such a picture finds theoretical support in the results of hydrodynamical
simulations and semi-analytic models of galaxy formation (e.g.
Nagamine et al. 2001;
Mo, Mao, & White
1999).
In the coming years it will be tested by deeper and more extensive
searches for DLA galaxies, by comparing the clustering
of LBGs and DLAs
(Adelberger et al. 2002, in preparation),
and by more reliable measurements of the star formation activity
associated with DLAs. This last project is best tackled in the
near-infrared, by targeting the
H emission line
with integral field - rather than slit - spectroscopy,
as provided for example by the Cambridge Infrared
Panoramic Survey Spectrograph (CIRPASS - see
Figure 31).
 |
Figure 31. (Reproduced from
Bunker et al. 2001).
Left: An H |
When we combine the available abundance determinations
for Lyman break galaxies, damped
Ly systems and
the Ly forest with those
for the inner regions of active galactic nuclei
(from analyses of the broad emission lines and
outflowing gas in broad absorption line QSOs - see
Hamann & Ferland
1999),
a `snapshot' of metal enrichment in the universe at
z 
3
emerges (Figure 32).
The x-axis in the figure gives the typical
linear dimensions of the structures to which
the abundance measurements refer, from the 10-100 pc
broad emission line region of QSOs,
to the kpc scales of LBGs revealed by HST imaging
(Giavalisco, Steidel,
& Macchetto 1996),
to the 10 kpc
typical radii of DLAs deduced from their number density per unit redshift
(Steidel 1993),
to the 100 kpc dimensions of condensations in the
Ly forest
with N(H I)
1014 cm-2 indicated
by the comparison of the absorption along adjacent sightlines
in the real universe (e.g.
Bechtold et al. 1994)
and in the simulations (e.g.
Hernquist et al. 1996).
 |
Figure 32. Summary of our current knowledge of
abundances at high redshift. The `metallicity' is plotted
on the y-axis on a log scale
relative to the solar reference; the latter is shown as
the broken horizontal line at 0.0 and corresponds to approximately
2% of the baryons being incorporated in elements heavier than helium.
The x-axis shows the typical linear dimensions
of the strucures to which the abundance measurements refer, from the
central regions of active galactic nuclei on scales of 10-100 pc to
the intergalactic medium traced by the
Ly |
These different physical scales in turn reflect the depths
of the underlying potential wells and therefore the overdensities
of matter in these structures relative to the mean density
of the universe. Even from such an approximate sketch as
Figure 32,
it seems clear that it is this overdensity parameter
which determines the degree of metal enrichment
achieved at any particular cosmic epoch.
Thus, even at the relatively early times which correspond
to z = 3, the gas in the deepest potential
wells where AGN are found had already undergone considerable
processing and reached solar or super-solar abundances. At the
other end of the scale, condensations in the
Ly forest which
correspond to mild overdensities contained only traces of metals
with metallicity
Z 1/100 - 1/1000
Z. The
dependence of Z on the environment appears to be stronger than
any age-metallicity relation - old does not necessarily mean
metal-poor, not only for our own Galaxy but also on a global
scale. This empirical conclusion can be understood in a general way
within the framework of hierarchical structure formation in cold
dark matter models (e.g.
Cen & Ostriker 1999;
Nagamine et al. 2001).
1 from the Hubble
Deep Field B-band. The star-forming H II regions are prominent in
the rest-frame UV. CIRPASS will accurately determine the true star
formation rates, since (1) the compact knots of star formation are
well-matched to the fibre size, reducing the sky background and
increasing the sensitivity; (2) the large area surveyed by the integral
field unit covers most of a spiral disk and (3) the
H