A&A 385, 802-815 (2002)
DOI: 10.1051/0004-6361:20020198
C. Ledoux1 - J. Bergeron1 - P. Petitjean2,3
1 -
European Southern Observatory, Karl-Schwarzschild Straße 2,
85748 Garching bei München, Germany
2 -
Institut d'Astrophysique de Paris, 98bis Boulevard Arago,
75014 Paris, France
3 -
DAEC, Observatoire de Paris-Meudon,
92195 Meudon Principal Cedex, France
Received 3 August 2001 / Accepted 4 February 2002
Abstract
We analyse a sample of 24 damped Lyman-
(DLA)/moderate DLA systems at
intermediate redshifts,
,
all with measurement of the weak Mn II
absorption lines, to investigate which elemental ratios could possibly be
used as tracers of either dust depletion or nucleosynthesis effects.
We applied a component-by-component analysis to the five systems of the sample
with new observations and, using this procedure, re-analyzed data
gathered from the literature whenever possible. We show that the
standard method which uses column densities integrated over the whole
absorption profiles could substantially underestimate the abundance of
rare elements relative to Fe. We find a correlation between the
observed [Si/Fe] and [Zn/Fe] ratios, present in our sample at the
significance level. This correlation is fully consistent with a dust
depletion sequence only for a Galactic warm disk cloud or halo cloud
depletion pattern. The correlation between [Mn/Fe] and [Zn/Fe], detected at
the
significance level, cannot be accounted for by any dust
depletion sequence: it implies either variations of the intrinsic Mn
abundance relative to Fe from -0.3 to +0.1 dex and/or a relation between
depletion level and metallicity. The correlation between [Mn/Fe] and
metallicity (
significance level) strengthens the assumption
of intrinsic variations of [Mn/Fe] although some marginal correlation
between [Zn or Si/Fe] and [Zn/H] is present as well. Extension of the sample
toward low metallicity is needed to confirm the correlation between
depletion level and metallicity. The variations of [Ti/Fe] vs. [Zn/Fe]
cannot be fitted by a single dust depletion sequence either. We then adopt a
warm disk cloud or halo cloud depletion pattern and compare the resulting
dust-corrected abundance ratios to those observed in Galactic and SMC stars.
At high metallicity, [Fe/H
,
the intrinsic abundance
pattern of Si, Ti, Cr and Mn in DLA absorbers closely follows the trends
observed in Galactic stars and these absorbers should thus have a chemical
evolution similar to that of our Galaxy. At lower metallicity, some absorbers
do follow the trends present in Galactic stars but a substantial fraction
of them have elemental ratios (in particular [Si/Fe
and [Mn/Fe
)
closer to the solar values than Galactic stars.
This could be explained by a larger contribution of type Ia supernovae to the
chemical enrichment of these DLA absorbers than in Galactic stars of similar
metallicity. This metal-poor DLA absorber population could
trace H I-rich dwarf galaxies.
Key words: cosmology: observations - galaxies: halos - galaxies: ISM - quasars: absorption lines
The association of damped Lyman-
(DLA) systems with galaxies, as first
suggested by Wolfe et al. (1986), has been demonstrated at low and
intermediate redshifts by the identification of a small sample of DLA systems
at
(Le Brun et al. 1997). These absorbers have been
extensively used to probe the chemical history and the properties of the
gaseous halos of distant galaxies. Their metallicity was first derived
from high-resolution spectroscopic observations of Zn II
absorption lines as this element is not significantly depleted onto
dust grains in the Galactic interstellar medium (ISM). The derived
N(H I)-weighted mean metallicity of DLA systems at
is about
with a large spread of two orders
of magnitude (Pettini et al. 1997b). The value found at
is only slightly larger,
(Pettini et al. 1999). These low metallicities point toward young
galaxies, although the exact nature of high-redshift DLA systems is still
controversial (see Prochaska & Wolfe 1997a; Ledoux et al. 1998). The wide
range in metal enrichment and lack of significant evolution could be due
to the presence of various populations of galaxies of different
morphological types among the DLA absorbers, thus different
star-formation histories.
The underabundance of Cr relative to Zn has been first attributed to
the selective depletion of Cr onto dust grains
(Pettini et al. 1994)
by analogy to the Milky Way. Indeed, the high elemental abundance ratios
[Zn/Cr] (
Zn)/N(H
Cr)/N(H)]) and/or [S/Cr]
in DLA systems with detected H2 molecules demonstrates the presence of
dust in these DLA
absorbers (Ledoux et al. 2002; Petitjean et al. 2002). However,
this interpretation has been challenged by Lu et al. (1996)
and Prochaska & Wolfe (1997b). These authors studied the abundances of a
large number of elements and suggested that the observed relative
abundances may reflect the nucleosynthesis pattern of type II supernovae
enrichment. It seems that several groups now tend to agree that both
effects are present and that the [Cr/Zn] ratio is primarily an indicator of
dust depletion
(Pettini et al. 1997a; Kulkarni et al. 1997; Vladilo 1998; Prochaska & Wolfe 1999).
The main goal of this paper is to analyze a large sample of DLA systems at
intermediate redshift to investigate these issues further. We search for
a clear depletion pattern, in particular for elements less depleted than Fe
in the Galactic ISM other than Zn, as Si. We then concentrate our analysis
on Mn and Ti, elements which have not been extensively studied so far. Our
sample of 24 DLA/moderate-DLA absorbers combines new high spectral resolution
data on five systems at
with previously published data
on DLA/moderate-DLA systems at
.
In all cases, the
wavelength range of the expected, associated Mn II transition lines
has been observed.
We describe our observations in Sect. 2 and present our analysis of individual absorbers and our measurements in Sects. 3 and 4, emphasizing the importance of component-by-component analysis. In Sect. 5, we discuss the abundance pattern observed in DLA systems. As explained in Sect. 6, we then correct the observed abundance ratios for dust depletion effects to finally draw our conclusions in Sect. 7.
UV spectra were retrieved from the HST Faint Object Spectrograph (FOS) archive
to determine the H I column density from the Lyman-
line of the
metal-rich absorbers toward Q0058+019 (PHL938), Q0453-423
and Q(PKS)2128-123. The observation log is summarized in
Table 1.
Quasar | mag![]() |
![]() |
Telescope | Instrument | Wavelength | Resolution | Exposure | Comment |
range (Å) | FWHM (Å) | time (s) | ||||||
0058+019 | 17.16(V) | 1.959 | ESO 3.6 m | CASPEC | 3510-4797 | 0.16 | 21600 | |
HST | FOS | 1572-2311 | 0.84 | 1590 | G190H | |||
0453-423 | 17.06(V) | 2.661 | ESO 3.6 m | CASPEC | 3680-5096 | 0.16 | 16200 | |
HST | FOS | 1572-2311 | 0.84 | 1700 | G190H | |||
HST | FOS | 2223-3277 | 1.19 | 2440 | G270H | |||
1122-168 | 16.20(R) | 2.400 | VLT-UT2 | UVES | 3350-3875 | 0.10 | 26400 | Dic#1 |
VLT-UT2 | UVES | 4780-6815![]() |
0.10 | 26400 | Dic#1 | |||
VLT-UT2 | UVES | 3875-4780 | 0.10 | 27000 | ||||
2128-123 | 16.11(V) | 0.501 | ESO 3.6 m | CASPEC | 3510-4797 | 0.16 | 21600 | |
HST | FOS | 1572-2311 | 0.84 | 5221 | G190H |
In the cases of Q0058+019 and Q2128-123, it was necessary to correct
for scattered light by bringing the cores of the Lyman-
lines to the
zero flux level (correction <10%).
The signal-to-noise ratio in the adjacent continuum is of the order of 5 and
10 for Q0058+019 and Q2128-123 respectively. For Q0453-423, no
flux is actually observed in the G190H spectrum as is also the case shortward
of about 3020 Å in the G270H spectrum, thereby revealing the existence
of an optically thick Lyman-limit system at
along
the line of sight.
High-resolution, high signal-to-noise ratio spectra were obtained with the
UV and Visible Echelle Spectrograph (UVES) at the Kueyen VLT-UT2 8.2
m telescope on Cerro Paranal Observatory during UVES Science Verification
(SV; February 6-17, 2000). Combining two standard dichroic settings: Dic#1
(grisms B346 nm, R580 nm) and Dic#2 (grisms B437 nm, R860 nm), to observe with
both spectrograph arms, the full optical range was covered from 3050 to
10000 Å with only a small gap between the two red CCDs
(see Table 1). A slit width of
(resp.
)
in the Blue (resp. Red) was used in good seeing conditions and of
when the seeing was poor, yielding a spectral resolution
.
The calibrated data for Q1122-168 were made available to the ESO community in May 2000. The data reduction was performed by the SV team using the UVES pipeline (Ballester et al. 2000), available as a context of the MIDAS software. The main characteristics of the pipeline is to perform a precise inter-order background subtraction, especially for master flat-fields, and to allow for an optimal extraction of the object signal rejecting cosmic rays and performing sky-subtraction at the same time. The pipeline results were checked step by step. We then converted the wavelengths of the reduced spectra to vacuum-heliocentric values and scaled, weighted and combined individual 1D exposures using the NOAO onedspec package of the IRAF software. The resulting unsmoothed spectra were re-binned to the same wavelength step (0.0415 Å pix-1), yielding a signal-to-noise ratio per resolution element of 100 (resp. 37) at 5500 Å (resp. 4000 Å).
High-resolution spectroscopy data of three quasars, Q0058+019, Q0453-423 and Q2128-123, were obtained with the CASPEC echelle spectrograph at the ESO 3.6 m telescope at La Silla Observatory on November 18-21, 1995 and September 23-26, 1997. The observational details are summarized in Table 1. A spectral range of 1200 Å was covered per setting. The instrumental resolution FWHM is 0.16 Å (or 11 km s-1) and the accuracy in the wavelength calibration is of the order of one tenth of the resolution.
The data were reduced and analysed using the echelle package of MIDAS. The object-flux weighted median of individual frames and cosmic-ray removal were performed simultaneously. The sky spectrum was difficult to extract in the blue due to the small spacing between orders. We thus carefully fitted the zero level to the bottom of the saturated lines in the extracted 1D spectra. The uncertainty in the flux zero level is about 5%. The mean signal-to-noise ratio of the final spectra is typically 10, 17 and 28 down to about 4000 Å, for Q0058+019, Q0453-423 and Q2128-123 respectively.
Ionic column densities were derived from a least-squares technique using the
fitlyman program (Fontana & Ballester 1995) in which Voigt profiles
are convolved with the instrumental point spread function.
The metal line profiles
were fitted consistently assuming the same velocity for all the ions of
a given component and a pure turbulent broadening. Whenever available, we used
updated oscillator strength values (see Savage & Sembach 1996), or
else the compilation by Morton (1991), except for
the Fe II
2344, 2374 lines for which we used
the laboratory measurements of Bergeson et al. (1996). The column
densities of Mg I, Mg II, Si II, Ca II,
Ti II, Cr II, Mn II, Fe II and Zn II were
derived simultaneously and the results are shown in Table 2.
We now comment on the five individual systems of the sample with new observations.
The total neutral hydrogen column density of this absorber was derived from
the available HST FOS spectrum.
We get H I
.
We adopted the column density estimated by
Pettini et al. (2000)
for Fe II
as it is based on the high signal-to-noise ratio detection of the weak,
unsaturated Fe II
2249, 2260 lines in a Keck HIRES
spectrum. Our Mn II measurement yields [Mn/Fe
,
which
is slightly sub-solar. A
detection of Ti II
3384
was reported by Churchill et al. (2000a) using new Keck HIRES data.
The measured column density,
Ti II
(Churchill, private communication),
leads to an elemental ratio [Ti/Fe
.
As mentioned in Sect. 2.1, the existence of an optically thick Lyman-limit system at
prevents a direct determination
of the total H I column density of the
and 1.1496
strong metal-line systems. However, in both systems,
Fe II
2600
Mg II
2796) is of order unity which, together with the strength of
the Mg II lines,
Mg II
2796)>1 Å,
implies that N(H I) is well in excess of a few 1019 cm-2.
Both absorbers should thus be moderate-DLA if not DLA
systems (see Bergeron & Stasinska 1986; Rao & Turnshek 2000).
For the system at
,
the Fe II line profile
is complex spanning about 550 km s-1. A simultaneous least-square fit of
the Fe II
2249, 2260 and Fe II
2344
profiles (the latter constraining the line parameters of the components 5 to 8; see Table 2) yields a total Fe II column
density,
Fe II
,
thus probably a high
metallicity ([Fe/H]>0.5).
For the system at
,
we
measured
Fe II
.
The Mn abundances relative
to Fe derived using the method outlined in Sect. 4
are over-solar, [Mn/Fe
and
at
and 1.150 respectively (see Table 3).
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() REFERENCES: ( ![]() |
This DLA system was recently studied by de la Varga et al. (2000) using
ESO 3.6 m CASPEC and Keck HIRES data. From an HST FOS spectrum, these
authors derived a total H I column density
H I
from a fit of the Lyman-
and Lyman-
lines. In our very high signal-to-noise ratio UVES spectra,
we measured accurate column densities for Fe II, Mn II
and Ti II, and placed a stringent upper limit on the column density of
Zn II (see Table 2).
The Ca II lines are optically thin and their profile closely
follows that of the Mg I
2852 line from -100 to +200 km s-1 (see Fig. 1).
![]() |
Figure 1:
Low-ionization line profiles from the
![]() |
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The Fe abundance derived from the weaker transitions of Fe II is
[Fe/H
.
The non-detection of Zn II
yields [Zn/Fe]<+0.29 and <+0.39 for components 3+4 and 9
respectively (see Tables 2 and 3). This DLA system thus
appears to be essentially dust-free (see also de la Varga et al. 2000).
The Mn II column densities we derived for the components 3+4 and 9
lead to sub-solar elemental ratios, [Mn/Fe
and
[Mn/Fe
respectively.
Note that these values estimated for
individual components are 0.3 to 0.4 dex higher than those integrated over
the whole profile as given by de la Varga et al. (2000). This is due to
a non-negligible contribution to the total Fe II column density
of components detected in Fe II but not in Mn II. This
clearly demonstrates that systematic of this kind can dominate the
abundance ratio variations in DLA systems. The Ti abundance is derived from
a simultaneous fit of
the Ti II
3073, 3242, 3384 lines. We get
[Ti/Fe
and [Ti/Fe
for
components 3+4 and 9 respectively.
Our determination of the total H I column density from the HST
FOS spectrum is H I
.
Although the
Lyman-
line has damped wings, this value is lower than the
threshold usually adopted for DLA systems (
20.3). Since
H I
corresponds to an optical depth at the
Lyman-limit
,
ionization corrections should be small
(e.g. Viegas 1995). Furthermore, we concentrate
our discussion below and the relative abundances of Fe, Mn and Ti and these elements have
negligible differential ionization corrections
(see Vladilo et al. 2001).
In our ESO 3.6 m CASPEC spectrum, the line profiles
from low-ionization species show two separated components
spanning
km s-1, the strongest component
being located on the profile red side.
A two velocity-component model was used to fit the Fe II and
Mg II doublets and the Mg I line detected in the observed
wavelength range, 3600-4100 Å (see Table 2). The velocity
broadening of the main component is constrained by
the Fe II
2586 profile, whereas that of the
satellite component is determined from the strong Mg II doublet.
Since the latter Fe II line may be saturated, we give only a lower
limit on the column density:
Fe II)>14.08 in the
main component. From the non-detection of the Mn II lines, we
get [Mn/Fe]<-0.03 (see Table 3). From
Ti II)<11.21 (Churchill, private communication), we derive
[Ti/Fe]<-0.29.
In order to investigate the abundance pattern of DLA systems and study
dust depletion effects, we have built a sample of 24 DLA/moderate DLA (i.e.
H I)>20.3/
H I)<20.3) systems
at 0.3 <
,
all with measurements of the Mn II
absorption lines. The sample, as shown in Table 3, includes
seven DLA candidates selected on the basis of their large
Fe II
2600
Mg II
2796
equivalent width ratio. We give the measured abundances of Si, Ti, Cr, Mn and
Zn relative to Fe. The previously analyzed Mn samples are those of
Lu et al. (1996), which includes seven Mn II systems
at
of which three are associated with DLA
candidates, and Pettini et al. (2000), which includes
six Mn II systems at
of which one is in
common with those of Lu et al. In our sample, there are nine Ti II
detections and three upper limits for systems at
.
The Ti abundance measurements in the
DLA systems toward Q0058+019, Q0454+039, Q1622+238 and
Q2128-123 were kindly made available to us by C. W. Churchill. They were
derived from Keck HIRES spectra obtained for another
project (Churchill et al. 2000a,b). The sample of
six Ti II absorbers detected by Prochaska et al. (2001) covers a
somewhat higher redshift range,
.
The relative abundances in the gaseous phase of DLA systems are usually determined from column densities integrated over the entire absorption profiles. This procedure leads to unbiased results only when all the components observed in the profiles of strong absorption lines are also detected in weaker transition lines. However, the optical depth of most of the Fe II lines are larger than those of the Mn II, Ti II, Cr II and Zn II transitions. As the overall profiles of the weaker transitions are usually less extended than those of the Fe II lines, the above procedure tends to overestimate the column density of Fe II as compared to other species. For this reason, we have used a different approach. After performing the Voigt profile fits of all of the absorption profiles, we summed up only the column densities of the components detected in at least one of the Mn II, Ti II, Cr II or Zn II ions (note that these ions, when observed, are usually detected at the same time). These components correspond most often to the strongest part of the profiles.
Our procedure tends to minimize the impact of limitations due to finite signal-to-noise ratios whereas the standard procedure can lead to underestimate the abundances of rare elements relative to Fe by -0.1up to -0.3 dex (as is the case for the DLA system toward Q1122-168). For DLA systems at intermediate redshifts, we also applied this procedure to all the data taken from the literature when a detailed modeling was available (see Table 3). However, for the estimate of the absolute [Fe/H] abundances, we include all the components of Fe II for comparison with previous studies.
We present in Table 3 the abundance of Fe and elemental abundance
ratios relative to Fe for our sample. From the analysis of a DLA sample
at intermediate redshift, Pettini et al. (1999) showed that there is no
significant cosmic evolution of DLA metallicity ([Zn/H]) in our
redshift range,
.
The elemental abundance ratios
presented in Fig. 2 and Table 4 do not exhibit
any significant evolution with redshift either.
![]() |
Figure 2:
Observed abundance ratios of Si, Ti, Cr, Mn and Zn relative to
Fe versus redshift for DLA systems at
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Ratio | DLA systems
![]() |
SMC
![]() |
Galactic stars![]() |
|||
![]() |
![]() |
ISM | stars | [Fe/H]>-0.5 | [Fe/H![]() |
|
[Mn/Fe] | -0.05 ![]() |
-0.18 ![]() |
-0.07 ![]() |
+0.04 ![]() |
-0.06 ![]() |
-0.26 ![]() |
[Si/Fe] | +0.39 ![]() |
+0.36 ![]() |
+0.82 ![]() |
+0.14 ![]() |
+0.08 ![]() |
+0.28 ![]() |
[Ti/Fe] | +0.12 ![]() |
+0.31 ![]() |
+0.28 ![]() |
+0.16 ![]() |
+0.07 ![]() |
+0.23 ![]() |
[Cr/Fe] | +0.14 ![]() |
+0.15 ![]() |
+0.12 ![]() |
+0.10 ![]() |
+0.00 ![]() |
-0.02 ![]() |
[Zn/Fe] | +0.48 ![]() |
+0.45 ![]() |
+0.73 ![]() |
+0.08 | +0.05 ![]() |
+0.12 ![]() |
![]() ![]() ![]() et al. 1997, and refs. therein). Differences in the adopted Solar system abundances are accounted for. ![]() ![]() |
We examine below the observed abundance patterns to identify the elements which would clearly help separating the effects of star-formation history and depletion of refractory elements onto dust grains.
In DLA systems at
,
Zn and Cr are both observed to be
overabundant relative to Fe (see Fig. 2). The spread in [Zn/Fe]
ranges from a solar ratio up to overabundances of about +0.8, with a mean
of about +0.5 (see Table 4). It is unclear whether
an overabundance of Zn relative to Fe can be explained by stellar
nucleosynthesis models. The channels of Zn production in massive stars are not
yet fully understood and the predictions of standard nucleosynthesis models
lie at least a factor of 2 below the observed stellar abundances of Zn
(Timmes et al. 1995; Thielemann et al. 1996; however see
Umeda & Nomoto 2002). Additional processes might produce
important amounts of Zn, such as axi-symmetrically deformed explosions in type
II supernovae (Nagataki 1999) and p-processing in
the neutrino-driven wind following the supernovae explosions
(Hoffman et al. 1996), but quantitative results depend on uncertain
parameters. Moreover, in Galactic thin disk stars the observed abundance of Zn
follows that of Fe with [Zn/Fe
and in Galactic thick disk and
halo stars the observed [Zn/Fe] ratio is in the
range (0, +0.2) (see Prochaska et al. 2000, and references therein). Finally, in the Galactic ISM, Zn is
mostly undepleted even in dust-rich regions.
We thus infer that, except maybe for elemental
ratios [Zn/Fe
(see also Sect. 6), dust
depletion should be the main effect governing the observed [Zn/Fe] abundance
pattern of DLA systems.
In DLA absorbers, the observed [Cr/Fe] ratio shows very little scatter
whatever the metallicity (see Table 3),
(see Fig. 2) or [Zn/Fe] (see Fig. 3).
![]() |
Figure 3: Observed abundance ratios of Si, Ti, Cr and Mn relative to Fe versus observed [Zn/Fe]. For completeness, absorbers with Ti and/or Mn but no Zn measurements are shown assuming that [Zn/Fe]<0.95. Meaningful double limits for [Ti/Fe] and [Mn/Fe] ratios are also displayed. Examples of depletion sequence are superimposed to the data for various dust compositions: the ISM depletion patterns of Galactic cold disk clouds (solid lines) and Galactic warm disk (long dash) and warm halo clouds (short dash) from Welty et al. (1999). Each of these sequences is given for intrinsically solar, over-solar and/or sub-solar ratios. An intrinsic [Zn/Fe]=+0.1 ratio is typical of those observed in Galactic halo and SMC stars (see Table 4). |
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In DLA absorbers, the abundance of Mn relative to Fe spans a larger range
than previously found from smaller samples. For a substantial fraction of our
sample, we get
Mn/Fe
,
but there are also five systems at
with [Mn/Fe]>0 (see Fig. 2). Of the latter, only
one is a confirmed DLA absorber with
H I)>20.3. As can
be seen in Table 4, the mean abundance of Mn,
an odd-Z Fe-peak element, in DLA systems is close to that observed
in Galactic metal-poor halo stars
(McWilliam et al. 1995; Ryan et al. 1996) and the cases with
[Mn/Fe
are similar to those of SMC stars. The dust depletion level
of Mn is about the same as that of Fe in Galactic warm halo clouds and
slightly lower in Galactic warm disk clouds (Savage & Sembach 1996). From
the spread of values of [Mn/Fe] in DLA absorbers together with the small
relative ionization correction factors for these elements (as well as
for Ti: see e.g. Vladilo et al. 2001), we thus infer that
both nucleosynthesis and dust depletion effects are needed to account for this
elemental ratio.
The abundances of the two -elements Si and Ti are larger than solar
in DLA absorbers, with a few cases slightly sub-solar for Ti
(see Fig. 2). These elements are also overabundant in Galactic
halo and SMC stars (see Table 4). The overabundance of Si can be a
consequence of nucleosynthesis effects
(Lu et al. 1996; Pettini et al. 2000) since in Galactic chemical
evolution models the abundance of
-elements is enhanced as compared to
solar for metallicities
Fe/H
(e.g. Timmes et al. 1995; McWilliam 1997). However, it is also
consistent with a dust depletion pattern similar to that observed in Galactic
warm disk and halo clouds (see Sect. 6). As Ti is observed to be
more depleted onto dust grains than Fe in Galactic warm halo clouds,
nucleosynthesis effects are most probably required to account for the
observed over-solar [Ti/Fe] ratios of DLA systems, and this, whatever the dust
content. However, dust depletion could be the dominant factor for the few
cases with observed, sub-solar [Ti/Fe] ratio.
When a uniform depletion fraction is assumed, the intrinsic abundances
are simply given by:
A simple method which takes into account the dust composition was developed
by Vladilo (1998). Various depletion patterns can be used with
different fractions, ,
of an element X locked onto dust grains.
From Vladilo's Eqs. (17) and (18), introducing explicitly in Eq. (18) the
intrinsic [Zn/Fe] elemental ratio to allow for non-solar values, we derive the
following general relation:
To compute a "depletion sequence'', [X/Fe
versus [Zn/Fe
,
we need to assume a value for [Zn/Fe
.
Since in Galactic stars of low metallicity and in SMC stars this ratio is
observed to be slightly over-solar,
Zn/Fe
(see Table 4), we discuss in Sect. 7 the effect of a
slight overabundance of Zn relative to Fe on the dust depletion correction.
We present in Fig. 3 the observed abundance ratios of Si, Ti, Cr and
Mn relative to Fe versus [Zn/Fe
together with depletion sequences
for various depletion patterns, namely those of Galactic cold disk
clouds, Galactic warm disk and Galactic warm halo clouds as provided by
Welty et al. (1999). The observations show a correlation between
[Si/Fe
and [Zn/Fe
,
detected at the
significance level using a Kendall rank correlation test. A similar trend is
present in the higher redshift sample analyzed by Prochaska & Wolfe (1999).
Such a correlation is fully consistent with a dust depletion sequence.
The best agreement between model and observations is obtained for the
Galactic warm disk and warm halo cloud patterns. These patterns are similar
to those observed in the ISM of the SMC (Welty et al. 2001). The
Galactic warm halo cloud pattern is also close to that observed in
the Magellanic Bridge (Lehner et al. 2001) where the overall
metallicity may be even lower than in the SMC. In addition, the data allow
for intrinsic elemental ratios
slightly over-solar, 0<[Si/Fe
.
A good depletion sequence
fit is then also obtained for
[Si/Fe
Zn/Fe
.
We thus conclude
that, although some nucleosynthesis effects could be present
(see Sect. 7), the variations of the observed [Si/Fe]
elemental ratio should mainly be due to dust depletion.
Being fairly constant down to low depletion levels, the [Cr/Fe
ratio is not well fitted by a depletion sequence, although a large fraction of
the data points are roughly consistent with dust depletion if an intrinsic
overabundance of Cr relative to Fe of +0.1 dex is considered (see
Fig. 3). This small overabundance is not observed in Galactic
metal-poor stars (McWilliam et al. 1995), whereas it is present in
both the ISM and the stars of the SMC. Although current chemical
evolution models reproduce satisfactorily a constant
[Cr/Fe
(see e.g. Timmes et al. 1995; Goswami & Prantzos 2000), they do not account for
a slight overabundance of Cr relative to Fe.
The [Mn/Fe
ratios cannot be fitted by any dust depletion
sequence (see Fig. 3), even if non-solar values are considered for
the intrinsic [Mn/Fe
ratio. The observed values can only be
recovered if one assumes possible variations of this intrinsic elemental
ratio in the range -0.3 to +0.1 dex.
For the 17 DLA systems with both Mn and Zn measurements, there is a trend for
an increase of [Mn/Fe
with increasing
[Zn/Fe
,
detected at the
significance level using a
Kendall rank correlation test (limits are taken into account as
true detections). This trend may be the consequence of a relation between
depletion level and metallicity. To test this suggestion, we investigate
the variations of [Zn/Fe
and [Si/Fe
versus [Zn/H
,
Si being much less depleted than Mn, Cr and Fe
in the Galactic ISM. They are presented in the upper panels of
Fig. 4.
![]() |
Figure 4:
Observed abundances of Zn and Si (upper panels),
Cr and Mn (lower panels) relative to Fe versus the observed abundances of Zn,
i.e. metallicities. Trends of increasing depletion level, traced by the
observed [Zn/Fe] and [Si/Fe] ratios, with metallicity are present at
the ![]() ![]() |
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Although the number of Ti measurements is small, the variation
of [Ti/Fe
with [Zn/Fe
cannot be fitted either by a
single dust depletion sequence. As for Mn, this suggests a variation of
the intrinsic [Ti/Fe
ratio. Since Ti is more depleted onto
dust grains than Fe, the positive values of [Ti/Fe
observed
in most cases would imply, as for the other
-element in our study,
Si, an over-solar, intrinsic [Ti/Fe
elemental ratio (see below).
We now discuss the intrinsic elemental ratios derived
from Eq. (2) for the 24 DLA absorbers of our sample at
.
Since the depletion pattern of Galactic cold disk
clouds does not fit the correlation between [Si/Fe
and [Zn/Fe
for an intrinsic elemental
ratio [Si/Fe
,
we use the dust depletion patterns of
either Galactic warm disk or warm halo clouds. We present
in Fig. 5 the dust-corrected abundance ratios versus metallicity,
as given by [Fe/H
.
![]() |
Figure 5:
Dust-corrected, i.e. intrinsic, abundance ratios of Si, Ti, Cr
and Mn relative to Fe versus dust-corrected Fe abundances, i.e. metallicities.
The observed [Zn/Fe] ratio was used as a depletion indicator with the
dust
depletion patterns of Galactic warm disk (asterisks) and warm halo
clouds (squares). Errors are typically ![]() |
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As discussed in Sect. 6.2, the importance of
nucleosynthesis effects is already revealed by the different behaviours of
the observed abundances of Mn and Cr relative to Fe. The fractions of these
two elements locked onto dust grains are similar (and also close to that for
Fe) and they have comparable solar abundances relative to Fe. However, whereas
the [Cr/Fe
ratio is fairly constant whatever the metallicity, the
correlation observed between [Mn/Fe
and metallicity
(Fig. 4) implies an increase of the intrinsic abundance of
Mn relative to Fe with metallicity.
To compare the intrinsic abundance patterns observed in DLA systems with those
of Galactic and SMC stars, we also give in Fig. 5 the
relation between the abundance ratios and metallicity observed in Galactic
thin disk and halo stars (from the compilation of values by Prochaska et al. 2000) as well as the values for SMC stars (from
the appendix of Welty et al. 1997). For the trend shown for Mn
in Galactic stars, we took into account the hyper-fine structure splitting
effects of the Mn I lines (Prochaska & McWilliam 2000) which leads to a
slightly higher [Mn/Fe] mean ratio, now equal to -0.05 at [Fe/H
.
We note that the intrinsic abundances of the two -elements, Si and Ti,
relative to Fe show a similar trend versus metallicity, although the dust
depletion pattern used to determine these intrinsic elemental ratios was
derived from the correlation present between [Si/Fe
and
[Zn/Fe
only. This strengthens our choice of warm dust depletion
patterns and dust depletion correction method.
For the few DLA absorbers of fairly high
metallicity, [Fe/H
,
all four elemental
ratios: [Si/Fe
,
[Ti/Fe
,
[Cr/Fe
and
[Mn/Fe
follow the trends present in Galactic stars and, at
[Fe/H
,
their abundances relative to Fe are about equal
to the solar values. The chemical evolution of these DLA systems appears
to be similar to that of our Galaxy.
At the metallicity of the SMC, [Fe/H
,
the
DLA dust-corrected abundances of Si, Ti and Cr relative to Fe, and in a lesser
extent that of Mn, are close to those of SMC stars (see Fig. 5
and Table 4) with
[Si/Fe
Ti/Fe
Cr/Fe
and
[Mn/Fe
.
However, at lower metallicity, [Fe/H
,
for a
substantial fraction of DLA absorbers the intrinsic abundances of Si, Ti and
Mn relative to Fe are closer to the solar values than for Galactic stars
and show little variation with decreasing metallicity. The evolution of
[Mn/Fe
with metallicity is well accounted for by
the odd-even effects for the Fe-peak elements using metallicity dependent
yields (for Galactic models see e.g. Timmes et al. 1995; Goswami & Prantzos 2000). Furthermore, the
systems with [Si/Fe
Ti/Fe
(resp.
>+0.10) are indeed also those with [Mn/Fe
(resp. <-0.15), i.e. there is an anti-correlation between [Mn/Fe
and [Si/Fe
or [Ti/Fe
.
This strongly suggests that
the contribution of type Ia supernovae to the chemical enrichment of the
metal-poor DLA systems with [Si/Fe
Ti/Fe
and [Mn/Fe
is larger than for Galactic stars of
comparable metallicity. This conclusion is consistent with the
interpretations of Matteucci et al. (1997)
and Legrand et al. (2000) based on other abundance ratios, which favor
at least for some DLA absorbers an episodic or slow star-formation scenario.
If they have experienced instantaneous starbursts, these DLA absorbers should
thus be older than 50 Myr (see Matteucci & Recchi 2001). Since there is
a wide spread of galactic morphological types among the identified
DLA absorbers (Le Brun et al. 1997), the metal-poor DLA absorbers with
abundance ratios fairly close to the solar values could indeed trace the
H I-rich dwarf galaxy population.
Among the goals for future work, we identify the extension of DLA samples at low [Zn/H] to test the possible correlation between depletion level and metallicity, and observations of S and Zn in the same DLA systems to confirm the lack of depletion for Zn and distinguish further between different star-formation histories from a comparison of [S/Fe or Zn] in DLA absorbers and in Galactic/SMC stars.
Acknowledgements
We are grateful to C. W. Churchill for making available to us unpublished data on the Ti abundances of four DLA systems and N. Prantzos for fruitful discussion. We thank R. Srianand for the reduction of the echelle spectrum of Q0453-423. CL acknowledges support from an ESO postdoctoral fellowship. This work was supported by the European Community Research and Training Network: "The Physics of the Intergalactic Medium''.