In the following, we briefly discuss
the relative abundances of some chemical elements for two
DLASs and a sub-DLAS detected in the present spectra.
All the abundances are given relative to the solar values
of Grevesse & Anders (1989) and Grevesse & Noels (1993), in the
notation
(see Table 2).
A summary of the obtained chemical abundances is reported
in Table 3.
H | Hydrogen | 0.00 |
C | Carbon | -3.45 |
N | Nitrogen | -4.03 |
O | Oxygen | -3.13 |
Si | Silicon | -4.45 |
S | Sulphur | -4.79 |
Fe | Iron | -4.49 |
Object | Redshift |
![]() |
log N(H I)b | [Fe/H] | [Si/H] | [N/H] | [S/H] | [N/S] | [C/Fe] | [0/Fe] | |
UM681 | 1.78745 | -150.5 | 18.6 |
![]() |
![]() |
||||||
Q2343+1232 | averagec | 20.35 |
![]() |
-1.1 | -0.7 | ||||||
2.43125 | 0.0 | -0.3 | |||||||||
Q2344+1228 | averagec | 20.4 |
![]() |
-1.85 | -2.75 | ||||||
2.53746 | -35.6 | <0.06 | <0.2 |
a Relative velocities as reported in the
corresponding figures. b Since it is not possibile to disentangle the velocity structure of the H I Lyman- ![]() one or two components at the redshift of the stronger components observed in the neutral and singly ionised absorption lines. c Average value obtained considering the sum of the column densities of all the components of the iron absorption profile. |
The observed H I column density is not high enough to
assure that no ionisation corrections have to be applied
to get the relative chemical abundances (see
e.g. Viegas 1995).
We derive an indicative measure
of the nitrogen and sulphur abundances by using the
Cloudy software package (Ferland 1997) to
build a photoionisation model and estimate the ionisation
corrections. We consider the UV background flux due to
quasars and galaxies (Madau et al. 1999) and try to recover the
observed column densities of the components at
and 1.78765 (
and
-130 km s-1 in Fig. 2). The results do not
change significantly if we adopt an ionizing spectrum due
to a single stellar population of solar metallicity and
0.1 Gyr.
The resulting ionisation corrections are
2.3, 0.2
and 14 to be multiplied for the ratios N I/H I,
S II/H I and N I/S II respectively.
Since the corrections are relatively small in the first two
cases, we compute the corresponding abundance ratios.
N I
is partially
blended (see Fig. 2), thus we estimate the
nitrogen abundance from the two components at lower redshift
(
and -130 km s-1 in Fig. 2),
associated with the
H I component with
H I
.
From this we derive [N/H]
corrected for the ionisation. This value is about
one order of magnitude larger than the higher value
measured in DLASs published in the literature
(Centurión et al. 1998; Lu et al. 1998).
The corrected sulphur abundance ratio for the same
components is [S/H]
which again is
about one order of magnitude larger than what is observed
in DLASs.
These measurements, although slightly uncertain, suggest
that sub-DLAS could have higher metal abundances than
DLASs, as already observed in LLS
(e.g. D'Odorico & Petitjean 2001), and probe a more evolved
chemical stage of high redshift galaxies when gas has
been partly consumed by star formation.
This DLAS, as well as the one seen along the LOS of
Q2344+1228 (see next section), has first been detected by
Sargent (1987).
The two QSOs were observed recently with HIRES+Keck and
the relative chemical abundances of the two systems were
used in statistical samples
(Rauch et al. 1997; Lu et al. 1998; Prochaska & Wolfe 1998, 2001; Prochaska et al. 2001).
The metal absorption complex corresponding to this damped
system is made of two groups of lines.
The major one counts at least 8 components, with the
strongest one at
(v =
0 km s-1 in Fig. 3).
This component is heavily saturated
in C II, O I, Mg II, and Si II. It shows absorption
due to the two triplets of N I,
Å and
Å, and to the S II triplet,
Å.
The Si IV doublet is clearly identified in this complex,
the C IV doublet is outside our spectral range but was
detected by Sargent et al. (1988).
A satellite sub-system is observed at more than 500 km s-1
from the centre of the main one, at
.
It is very weak and shows
transitions due to C II, Mg II, Si II and Si III.
Since we cannot disentangle the velocity structure of the
hydrogen absorption, we assume a single component at the
redshift of the strongest component observed in singly
ionised lines at
.
The total H I column density is
H I
and the error is mainly due to the
uncertainty in the position of the continuum.
The average iron abundance is [Fe/H]
.
While, we obtain [S/H]
and
[N/H]
.
Those estimates are
0.2 and 0.5 dex higher
respectively, than those reported by Lu et al. (1998).
The difference for sulphur is within the uncertainties (at
the
level), while the larger one for nitrogen
could be due to the fact that Lu et al. (1998) used the
saturated N I triplet at
Å.
As previously stated, the main component is badly
saturated for all the observed transitions due to C, O
and Si, so relative abundances for these elements cannot
be determined. On the other hand, we can study the
abundance ratios of S, N and Fe at this redshift,
assuming that ionisation corrections are negligible. This
hypothesis is supported by the large column density
characterising this component and by the absence of Si IV absorption.
We measure column densities:
S II
,
N I
(where we do not
consider the N I triplet at
Å because it is saturated and affected by the wing of the
damped H I Lyman-
line) and
Fe II
.
From which we derive the abundance
ratios: [S/Fe]
,
and [N/S]
.
The latter abundance ratio, which is not
affected by dust, can be compared with the ratios [N/O]
measured for metal poor Galactic stars making the
assumption, [S/O]
as reported by
Centurión et al. (1998).
Our measurement is larger than any other for DLASs
present in the literature (see also Lu et al. 1998) and it
is consistent with values obtained for H II
regions in dwarf galaxies.
The strong Si IV absorption imply that ionisation
corrections could be necessary to determine the abundance
ratios in the other components of the system.
Nothing can be said on the H I column density
corresponding to the single components. This makes hard
the realization of a photoionisation model to determine
the ionisation corrections.
We report the measured column densities of sulphur and
nitrogen (a faint absorption is observed for the
transition N I
)
relative to the
component at
(
km s-1 in Fig. 3),
S II
and
N I
.
![]() |
Figure 4:
Q2344+1228: ionic transitions associated to
the DLAS at
![]() ![]() |
The damped H I Lyman-
absorption line of this system
has been fitted with a single component at the redshift
of the strongest component observed in the neutral and
singly ionised lines of associated heavy elements
(
).
The H I column density is
H I
,
where the error on the column density is due
mainly to the positioning of the continuum level.
In Fig. 4, we show the ionic transitions
observed for the DLAS. The C IV doublet is outside our
wavelength range and it was not observed in the low
resolution spectrum by Sargent et al. (1988).
The iron and silicon column densities can be derived from
non saturated lines to obtain the average values,
[Fe/H]
,
and [Si/H]
.
While, [N/H]
,
which is in
good agreement with the value found by Lu et al. (1998).
The absence of high ionisation lines and the simple
velocity profile of the system allow the assumption that
ionisation corrections are negligible in this case. We
can thus obtain reliable abundance ratios from the column
densities of the transitions observed in the single
components.
N I is observed in the central component, we compute
the abundance ratio [N/Fe]
,
which
is consistent with previous measurements for DLAS.
The central component is unusable to derive reliable
abundance measures for other chemical elements because
all the observed lines are heavily saturated.
In the component at lower redshift (
,
km s-1 in Fig. 4),
the ratio [Si III/Si II]
implies that ionisation corrections are small at this
velocity. We observe transitions due to Si II and Fe II
that are not saturated, from which
the relative abundance [Si/Fe]
is
obtained. On the other hand, C II and O I are
slightly saturated but not going to zero, this results in
the upper limits:
and
.
Reliable measures of O and C abundances are quite
rare. We discuss the implications of our result in the
following section.
-capture elements are mainly produced by type II
SNe which should dominate in the early stages of the
chemical evolution of galaxies, while type I SNe
contribute iron peak elements later on. Therefore, the
[
/Fe] abundance ratio can be used to trace the
chemical evolution history and, to a certain extent, the
nature of galaxies.
Oxygen and sulphur are more reliable estimators of
-element abundances than silicon which is subject
to dust depletion.
Abundance studies of carbon (e.g. Tomkin et al. 1995)
indicate that in the disk of our Galaxy [C/Fe] and
[
/Fe] show similar trends with [Fe/H].
Measures of C and O in DLASs are generally complicated by
the fact that often the only available lines are
C II
and O I
which
most of the times are heavily saturated.
In the DLAS described in Sect. 3.3, we constrain
the values of the O/Fe and C/Fe ratios, considering
a single component which is only mildly saturated and not
going to zero.
We derive that the C/Fe ratio is consistent with
solar while the O/Fe and Si/Fe ratios are consistent
among them and show a very small enhancement.
The average iron abundance of the system is about 1/100
solar.
Our result, together with the recent measures by
Molaro et al. (2000) for the DLAS in the spectrum of Q0000-26,
indicates that there is no evidence for the [O/Fe] ratio
to be over-solar in DLAS. This is at a variance with
what is observed in the atmosphere of Galactic stars at
the same metallicity (but see also Dessauges-Zavadsky et al. 2001).
The abundance pattern which is closest to the above data is that of an old starburst, as is observed at the boundaries of our galactic disk, although, in general, for larger iron abundances (Chiappini et al. 1999).
Copyright ESO 2002