Abundances at high redshift can now be measured in a variety of ways
involving emission and absorption lines, thanks to large telescopes and
sensitive light detectors. Once the high redshifts
of quasars were recognized, the potential to use these
remarkable objects as probes of the early universe was clear. Derivation of
abundances from the emission lines of AGN is difficult, however. The width of
the broad lines impedes measurement of weak lines, and the high electron
density in the broad line region (BLR) and the large optical depths of some
of the lines makes analysis difficult. This is true in particular for the
measurement of the electron temperature, necessary to calculate the line
emissivity.
Shields (1976)
noted that relative abundances of C, N,
and O could be derived with less sensitivity to the uncertainty in the
electron temperature. The N/O and N/C ratio might in turn be an indicator of
the overall metallicity, given the secondary nature of nitrogen production. He
found high N/C in two QSOs and suggested a parallel to high nitrogen
abundances in nearby normal galactic nuclei and AGN.
Hamann and Ferland
(1993)
studied QSO abundances as a function of redshift by
bringing together chemical evolution models and photoionization models. They
concluded that most luminous, high redshift QSOs (z
2 to 4) have
abundances higher than solar. Hamann & Ferland also noted that iron
abundances in QSOs at high redshift might constrain cosmological models.
Abundances are also measured in absorbing gas clouds on the line of sight to
a QSO. This includes the high column density "damped
Ly
" systems (DLAs)
and the Ly forest. An example
of the study of heavy element abundances
in DLAs is the work of
Pettini et al. (1999).
They find [Zn/H]
-1.2 in the redshift range
z = 1 to 3, with rather little systematic
dependence on redshift. The relative abundances of the heavy elements are
consistent with a mild degree of depletion onto grains, and [Si/Zn] is
essentially solar. (The ratio Si/Zn is a useful surrogate for O/Fe.) Thus,
these absorbers do not appear to share the [O/Fe] enhancement of metal poor
stars in the Galactic halo. Evidently, even at redshifts of 2 or so,
past star formation in the DLA systems had proceeded gradually enough that
iron production kept pace with oxygen and other SN II products.
Ly forest clouds
often show C IV
1548, 1550 absorption lines when
observed with sufficient resolution.
Songaila & Cowie
(1996)
measured O VI
as well and found that ionization ratios and line widths pointed to
photoionization rather than collisional ionization. Mean abundances ratios
appear to be [C/H] -1.5 and
[O/C] [Si/C]
+0.5,
with a spread of order a factor 3 in C/H
(Songaila & Cowie
1996;
Davé et al. 1998;
Ellison et al. 2000).
These authors note that the high
O/C resembles old halo stars; but as discussed above, O/C is also high in
metal poor H II regions.
Abundances in DLAs resemble those measured in
GEHRS in the more metal poor dwarf irregulars and in the outermost disks of
spirals, that is, [O/H] -1.5.
Silk, Wyse, & Shields
(1987)
suggested that this widespread minimum may result from reaccretion of enriched
gas lost from an early population of dwarf galaxies. The more metal poor
stars in the Galactic halo might then represent the cannibalized remains of
the primordial dwarf galaxies. The very metal poor clouds of the
Ly forest must then largely
have escaped the reaccretion process, at least
at the epoch at which they are observed.
Observations of abundances in ionized gas in galaxies at high redshifts are
also becoming available.
Kobulnicky & Zaritsky
(1999)
observed a sample of
emission-line galaxies at redshifts z = 0.1 to 0.5 and found them to be only
marginally more metal poor than the metallicity-luminosity relation observed
for low redshift galaxies. In contrast,
Kobulnicky & Koo
(2000)
obtained near infrared observations of two
Ly emitting galaxies at
z = 2.3 and 2.9,
finding 12 + log O/H = 8.2 to 8.8. From these and other data, they find that
emission-line galaxies at these redshifts have abundances substantially below
the present day metallicity-luminosity relation. They note that these low
abundances resemble those of metal rich globular clusters and raise the
possibility that these objects may resemble the formation of galaxies like the
Milky Way at z
3. The emission-line
galaxies at high redshift have abundances
higher than the DLAs, indicating that they represent different environments.