3.1. Metals in the Ly
Forest
The lack of associated metal lines was originally
one of the defining characteristics of the
Ly
forest and was interpreted as evidence
for a primordial origin of the clouds
(Sargent et al. 1980).
However, this picture was
shown to be an oversimplification by the first
observations - using the HIRES spectrograph on the Keck I
telescope - with sufficient sensitivity to detect the weak
C IV 
1548, 1550 doublet
associated with
Ly clouds with column
densities log N(H I)
14.5
(Cowie et al. 1995;
Tytler et al. 1995).
Typical column density ratios in these
clouds are N(C IV) / N(H I)
10-2 - 10-3,
indicative of a carbon abundance of about 1/300 of the solar value, or
[C/H] - 2.5 in the usual
notation, and with a scatter of perhaps a factor of ~ 3
(Davé et al. 1998).
The question of interest is `Where do these metals come from?'.
Obviously from stars (we do not know of any other way to produce
carbon!), but are these stars located in the vicinity of the
Ly clouds observed -
which after all are still at the high
column density end of the distribution of values of N(H I)
for intergalactic absorption - or are we
seeing a more widespread level of metal enrichment, perhaps
associated with the formation of the first stars which
re-ionised the universe at z > 6
(Songaila & Cowie
2002)?
To answer this question we should like to search for metals
in low density regions of the IGM, away from the overdensities
where galaxies form. Observationally, this is a very difficult
task - the associated absorption lines,
if present at all, would be very weak indeed. Ellison et al.
(1999,
2000)
made some progress towards
probing such regions using extremely long exposures with HIRES of two of the
brightest known high-z QSOs, both gravitationally lensed:
APM 08279+5255 and Q1422+231. The latter set of
data in particular (Figures 13 and
14)
is of exceptionally high quality, reaching a signal-to-noise ratio S/N
300 which translates to
a limiting rest-frame equivalent width limit
W0(3
)
1 mÅ; this in turn
corresponds to a sensitivity to C IV absorbers with column
densities as low as N(C IV)
4 ×
1011 cm-2.
And indeed C IV lines are found at these low levels (see
Figure 16),
showing that metals are present in the lowest column
density Ly clouds
probed, at least down to
N(H I) = 1014 cm-2.
 |
Figure 16. (Reproduced from Ellison et al. 2000). Examples of weak C IV lines identified in the spectrum of Q1422+231; most of these would have remained undetected in spectra of lower signal-to-noise ratios. Green (grey) lines show the profile fits used to deduce the column density of C IV. The weakest C IV systems are indicated with tick marks to guide the eye. |
As can be seen from Figure 17, the number of
weak C IV lines continues to
rise as the signal-to-noise ratio of the spectra increases and any
levelling off in the column density distribution presumably
occurs at N(C IV) < 5 × 1011
cm-2. This limit is one
order of magnitude more sensitive than those reached previously.
In other words, we have yet to find any evidence in the
Ly forest for
regions of the IGM which are truly of
primordial composition or have abundances as low as those of
the most metal-poor stars in the Milky Way halo.
These conclusions are further supported by the recent detection
of O VI
1032, 1038 absorption in
the Ly forest
at z = 2 by
Carswell, Schaye, &
Kim (2002).
In agreement with the results of
Ellison et al. (2000),
these authors found that most
Ly forest clouds with
N(H I)
1014
cm-2 have associated
O VI absorption and that [O/H] is in the range -3 to -2.
Weak O VI lines from regions of lower
Ly optical depth
have not yet been detected directly, but their presence
is inferred from statistical considerations
(Schaye et al. 2000).
 |
Figure 17. (Reproduced from
Ellison 2000).
C IV column density distribution in Q1422+231
at <z> = 3.15;
f(N) is the number of C IV systems per column density
interval and per unit redshift path.
The filled circles are the data;
the straight line shows the best fitting power-law slope
|