A&A 402, 487-497 (2003)
DOI: 10.1051/0004-6361:20030294
G. Vladilo - M. Centurión - V. D'Odorico - C. Péroux
Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34131 Trieste, Italy
Received 18 December 2002 / Accepted 27 February 2003
Abstract
We present a study of Ar abundances in 15 damped Ly
systems (DLAs)
in the redshift interval
.
The sample includes 4 UVES/VLT measurements of Ar I column densities presented here for the first time, together with 6 measurements
and 5 upper/lower limits collected from the literature
(UVES/VLT and HIRES/Keck observations).
The majority of DLAs show significant underabundances
of Ar relative to other
-capture elements with common nucleosynthetic origin.
We show that neither dust depletion nor
intervening H II regions inside DLAs
offer a viable justification to these underabundances.
A natural explanation is found in the framework of
photoionization models of H I regions embedded in
an ionizing continuum with varying spectral distribution.
At
the observed Ar deficiencies are large,
[Ar/
dex, suggestive
of a hard, QSO-dominated spectrum.
At
the deficiencies
are instead small, suggestive of a soft, stellar-type spectrum,
though more data
are needed to generalize this high-z result.
Should the change of Ar abundances with redshift
be governed by the
evolution of the UV stellar emission internal to DLAs,
a synchronization of the
star formation in DLAs would be required, with a strong stellar
emission at z > 3, but weak at z < 3.
This variation seems inconsistent with the weak signal of evolution
indicated by abundance
studies of DLAs.
More likely, the change of Ar abundances is induced by the evolution
of the UV metagalactic continuum, in which case
the UV emission internal to DLAs must be small
(i.e. DLAs should have modest star formation rates)
and the external background must become softer at z > 3.
The former requirement is consistent with
the modest evolution of DLAs abundances and
the lack of Ly
and H
emissions associated with DLAs.
The latter requirement is consistent with
the observed evolution of Si IV/C IV ratios in the IGM,
the claims of high escape fraction of UV photons from Ly-break galaxies at
,
and the recent finding that the He II re-ionization seems to occur
between
and
.
Comparison with results from local interstellar studies indicates
that Ar abundances can be used to trace the evolution of the
ionization history of the universe down to z=0, where
[Ar/
dex.
We predict a rise of Ar abundances in the redshift range from
to z=0,
at the epoch at which the metagalactic field of galaxies overcomes
that of quasars.
Key words: intergalactic medium - ultraviolet: galaxies - cosmology: observations - quasars: absorption lines
The abundance of argon in diffuse gas can be measured
from absorption line spectroscopy of
the Ar I 104.8 and 106.6 nm resonance transitions.
Early Copernicus observations indicated that
argon is moderately depleted in the local
interstellar gas (Meyer 1989 and refs. therein), a result confirmed by
recent IMAPS (Sofia & Jenkins 1998; hereafter SJ98)
and FUSE (Jenkins et al. 2000; Lehner et al. 2002)
observations.
Interstellar depletions are generally attributed to the incorporation of atoms into dust grains
and tend to be correlated with the gas column density
(Jenkins 1987; Savage & Sembach 1996).
It is hard to establish whether such a correlation, suggestive of dust, exists or not for Ar
because Ar I lines saturate when
atoms cm-2
at the solar metallicity characteristic of the local interstellar gas.
In fact, SJ98 provided theoretical arguments to show
that Ar depletion is unlikely due to dust since Ar atoms have a low probability of
being incorporated into dust grains.
The Ar deficiency is most likely due to ionization effects since the ratio between
photoionization and recombination rates is typically
one order of magnitude larger for Ar I than for H I
(Fig. 3 in SJ98).
Owing to this property, Lyman continuum photons
with
eV, the Ar I ionization threshold,
will be extremely efficient in ionizing Ar I but not H I,
if they are able to leak through a H I layer.
This characteristic of Ar can be used to probe ionization conditions not only
in local interstellar clouds,
but also in clouds of external galaxies.
In particular, with high resolution spectroscopy of QSO sources
we can study clouds in intervening galaxies
detected as absorption line systems.
Damped Ly
systems (DLAs), are thought to arise in these intervening galaxies
(Wolfe et al. 1995) and offer the
possibility to probe ionization conditions at high redshift
from the study of Ar abundances.
At
the Ar I lines can in fact be detected in the optical range,
where the collecting power of 10-m class telescopes can be exploited
to obtain
high quality QSO spectra.
Thanks to the low metallicity typical of DLAs (Pettini et al. 1999),
the Ar resonance lines can be unsaturated
even at the high column densities,
cm-2,
typical of these systems.
Therefore, DLAs can yield information complementary to that obtained
from studies of local ISM, where the Ar I lines become
readily saturated.
Most important, if Ar abundances in DLAs are indicative of ionization conditions,
as they are in the local ISM, their study
may yield important constraints on the nature
and relative importance of ionizing sources at high redshift,
a topic of particular interest in cosmology for its effects on the physical state of the
intergalactic medium and for its link to the star formation history
of the universe.
The first detection of Ar I in a DLA system was obtained
for the z=3.39 absorber towards QSO 0000-26
observed with UVES/VLT (Molaro et al. 2001).
The Ar abundance was found
to match that of other -capture elements measured in the same
system, suggesting a low degree of ionization of the gas.
Recently a few other Ar I detections in DLAs have been obtained with UVES/VLT
(Levshakov et al. 2002; López et al. 2002) and HIRES/Keck observations
(Prochaska et al. 2002a,b).
From these works there was no evidence for a general underabundance of Ar
in DLAs, with the notable exception of the z=2.62 system towards QSO 1759+75,
an absorber characterized by complex ionization conditions
(Prochaska et al. 2002a).
From the theoretical point of view, the Ar ionization fractions in DLA systems have been estimated by several authors, assuming different models of radiation field and ionization structure of the absorbers (Izotov et al. 2001; Vladilo et al. 2001; Prochaska et al. 2002a). At variance with other metal species observed in DLAs, the fraction of Ar I is quite sensitive to the adopted model. This makes Ar I an important discriminant of ionization conditions in DLAs. Motivated by this possibility, we have actively searched for new Ar I detections in DLAs. In this paper we present and discuss the results of this search. The new measurements are described in Sect. 2, while in Sect. 3 we review the general properties of Ar abundances in DLAs combining these new data with previous measurements. We show that a significant underabundance of Ar is common among DLAs, contrary to previous belief, and we discuss this result in the framework of photoionization equilibrium models. In Sect. 4 we present evidence for redshift evolution of the Ar abundances in DLAs and we briefly summarize Ar interstellar abundances at z=0. Finally, in Sect. 5 we discuss these findings in the framework of current evolution scenarios for DLAs and for the metagalactic radiation field. The results are summarized in Sect. 6.
Table 1:
Column densitiesa and relative abundances of argon in Damped Ly systems.
The observations were obtained with the Ultraviolet Visual Echelle Spectrograph (UVES)
fed by the Kueyen
telescope
(unit 2 of the ESO VLT). This spectrograph is particularly well suited for observing
the spectral region in the proximity of the ultraviolet atmospheric cutoff (Dekker et al. 2000)
allowing the Ar I lines to be observed at redshifts .
The slit widths were set to 1 arcsec, yielding a mean resolving power
.
The targets, the basic data for the DLAs investigated, and
the new measurements of Ar I column densities, N(Ar I),
are listed in the first 4 rows of Table 1.
A full presentation of the data reduction is given
in a parallel paper focussed
on a study of nitrogen abundances in DLAs (Centurión et al. 2003).
More details on UVES studies of
the systems at z=2.812 in QSO 0528-2505
and z=2.431 in QSO 2343+1232 can be found
in Péroux et al. (2003) and D'Odorico et al. (2002), respectively.
The column densities have been derived with the Voigt-profile fitting routines FITLYMAN
(Fontana & Ballester 1995) implemented in the MIDAS software package.
The oscillator strength for the transition Ar I
104.8 nm,
f=0.2570, was taken
from Federman et al. (1992), the same adopted for the local ISM by SJ98
and in previous work on DLAs by Molaro et al. (2001) and Levshakov et al. (2002).
Oscillator strengths for other
-capture elements
were taken from
Morton (1991), except for: Si II 1304, 1526 (Spitzer & Fitzpatrick
1993) and Si II 1808 (Bergeson & Lawler 1993).
![]() |
Figure 1: Absorption profiles of Ar I and S II lines (see labels) for the DLA system at z=2.8120 (zero velocity) in QSO 0528-2505. |
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Figure 2: Absorption profiles of Ar I and S II lines (see labels) for the DLA system at z=2.3745 (zero velocity) in QSO 0841+129. |
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Figure 3: Absorption profiles of Ar I and S II lines (see labels) for the DLA system at z=2.4762 (zero velocity) in QSO 0841+129. |
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The absorption profiles of Ar I lines are shown in
Figs. 1-4.
Care has been taken in assessing possible contamination
of the Ar I profiles due to Ly interlopers,
which would tend to increase N(Ar I).
Comparison with the radial velocity profiles of low-ionization species falling
inside and outside
the Ly
forest has not revealed evidence of contamination.
In the figures we show examples of the profiles of the S II and/or Si II lines used in the present analysis.
The agreement with the radial velocity profiles of Ar I lines
is generally
excellent.
A slight misalignment between the Ar I and S II lines is only observed in the z=2.3745 system in Q 0841+129.
We have carefully checked that this is not due to an error in the
wavelength calibration. Some weak contamination could be responsible
for this shift, in which case the derived Ar column density could be
slightly overestimated, probably within the large error
bar of this measurement. This would not affect the conclusion
that Ar is significantly underabundant in this system.
Care has been taken in assessing systematic errors due to the uncertainty
of the continuum, which are largest for the two DLAs in Q 0841+129
(Figs. 2 and 3).
Even in these two cases the N(Ar I) error bars
are sufficiently large to encompass the continuum errors.
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Figure 4: Absorption profiles of Ar I and S II lines (see labels) for the DLA system at z=2.4312 (zero velocity) in QSO 2343+1232. |
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The 4 new measurements presented here represent a significant increase over the 6 N(Ar I) measurements in DLAs previously obtained with UVES/VLT or HIRES/Keck observations, also shown in Table 1. When also upper and lower limits of N(Ar I) are considered (last 5 rows of Table 1), the combined sample sums up to a total of 15 DLAs. Care has been taken in comparing the abundances derived by different authors. The f value of the Ar I 104.8 nm line adopted by us is 2% smaller than the updated Morton (1991) value adopted by Jenkins et al. (2000) and Lehner et al. (2002) for ISM studies and by Prochaska et al. (2002a,b) for DLAs studies. The value is 5% larger than the old Morton (1991) value adopted by López et al. (2002). We did not attempt to correct for these differences, given the presence of some saturation in the lines. The differences are in all cases well within the measurement errors quoted by the authors.
For the sake of comparison with previous work,
in Fig. 5 we plot the [Ar/Fe] versus
[Fe/H] abundance ratios for the combined sample of DLA systems.
The Ar/Fe ratio is likely to be affected by nucleosynthesis evolution,
dust depletion and ionization effects. Disentangling these effects is extremely
difficult, making the
interpretation of the data shown in Fig. 5 quite uncertain.
As we explain below, the ratio of Ar relative to
-capture elements
is best suited for casting light on ionization effects alone.
In Cols. 5-7 of Table 1 we give the Ar abundances
normalized to those of O, S, and Si,
representative of -capture elements
measured in DLAs.
Most of the systems have [Ar/O, Si, S] ratios below the solar value,
with underabundances of about -0.7 dex
in several cases.
Only the z=3.39 DLA towards Q0000-26 shows solar ratios.
The [Ar/O, Si, S] ratios show therefore a significant scatter, with differences as high
as 0.8 dex.
All upper and lower limits are consistent with this range of values.
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Figure 5:
[Ar/Fe] versus [Fe/H] abundance ratios in Damped Ly ![]() |
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Because O, Si, S, and Ar are all -capture elements,
the [Ar/O], [Ar/Si], and [Ar/S] ratios
are expected to be only weakly dependent on the detailed
chemical history undergone by each system.
In fact, when at least two measurements of [Ar/O, Si, S] are available
for a given absorber,
one can see in Table 1 that the ratios are in good agreement between them.
The only exception is the z=3.025 system in Q0347-383,
for which O and Si abundances show a difference of 0.2 dex. However,
we do not believe that nucleosynthesis is a likely source for explaining
the significant underabundances found in the
[Ar/O, Si, S] ratios.
We now consider both dust and ionization effects as possible
sources of these underabundances.
The evidence that Ar is unlikely to be incorporated in dust
is mostly based on theoretical arguments
(SJ98). In fact, local interstellar
measurements of N(Ar I) are feasible only
in low-column density lines of sight
(
), which are not
expected to have a significant dust content.
The Ar measurements in DLAs provide the possibility to test
the existence of Ar dust depletion in high redshift galaxies,
in a regime of higher H I column densities.
We can estimate the dust content of individual DLAs making use of
the [Zn/Fe] ratio, given the large differential depletion between Zn and Fe,
well known from local ISM studies (Savage & Sembach 1996) and also supported from studies of DLAs
(Pettini et al. 1994; Vladilo 1998; Prochaska & Wolfe 2002)
.
In Fig. 6 we plot [Ar/S] versus [Zn/Fe]
for the subsample of Table 1 with
available Fe and Zn data.
Should Ar be incorporated into dust, we would expect
the [Ar/S] ratio to become more and more underabundant with increasing [Zn/Fe], because S is known to be undepleted in the interstellar gas (Savage & Sembach 1996).
However, this type of trend is not seen in Fig. 6. On the
contrary, severe underabundances of Ar are present even when [Zn/Fe] is relatively
small, i.e. when the amount of dust is negligible.
This empirical test indicates that a
mechanism different from dust depletion is responsible for the Ar underabundances in DLAs, lending support to the
theoretical arguments given by SJ98.
Among the abundances reported in Table 1 only
the [Ar/Si] ratio is possibly affected by dust depletion, given the fact that O and S
are known to be undepleted from local ISM studies.
In the penultimate column of Table 1 we give the amount
of Si/Ar differential depletion expected for DLAs with
available Zn data, estimated following the procedure
of Vladilo (2002).
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Figure 6:
[Ar/S] versus [Zn/Fe] abundance ratios in Damped Ly ![]() |
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Abundance measurements in DLAs are performed using dominant ionization states of metals in H I regions, which can be neutral (e.g. N I, O I, and Ar I) or singly ionized (e.g. Si II, S II, Fe II, and Zn II), depending on the ionization potential of each element. The predominance of such species in H I regions is well known from local ISM studies and can be easily explained in terms of photoionization equilibrium models of DLAs (Howk & Sembach 1999; Vladilo et al. 2001, hereafter V&01; Prochaska et al. 2002b). Ionization corrections for the abundances can be estimated, in the framework of these models, as a function of the intensity and spectrum of the adopted ionizing continuum. Unfortunately, singly ionized species can also arise in intervening H II regions, if they happen to lie inside the Damped systems. We discuss models with and without an intervening H II region.
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Figure 7:
The O I/Si II ratio in Damped Ly ![]() |
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The presence of intervening H II gas in DLAs would
yield an excess of ionized species, but not of neutrals.
As a consequence, they would create
an (apparent) underabundance of the Ar/Si and Ar/S ratios,
derived by comparing Ar I with
Si II or S II. On the other hand,
the Ar/O ratios, derived from neutral species, are not expected to be altered by this
effect. Therefore, if intervening H II gas exists, we should find
.
The only two [Ar/O] ratios in our sample do not support this possibility
(see Table 1). Since
this sample is too small - and does not include cases with significant
Ar underabundances -
we have also investigated the behaviour of the O I/Si II ratio,
for which 9 DLAs measurements exist,
2 of them shown in Table 1 and the others found in the literature
(Prochaska & Wolfe 1999; Dessauges-Zavadsky et al. 2001;
Pettini et al. 2002; Prochaska et al. 2002a).
The resulting [O/Si] ratios are shown in Fig. 7.
The mean value of this sample is
<
dex.
A few O I measurements are derived from partially saturated 130.2 nm lines,
and in these cases the real [O/Si] ratio could be even higher.
A little excess of [O/Si] may be expected due to differential
dust depletion between the two elements (Savage & Sembach 1996).
Only one system shows a modest underabundance which can
be explained in the framework of the ionization balance of H I regions
discussed below. The general lack of O/Si underabundance
indicates that intervening H II gas is uncommon in DLAs.
On the basis of this result, DLA ionization models which consider
intervening H II regions
as inherent to DLAs ionization structure, such as
the models discussed by Izotov et al. (2001), are not considered hereafter.
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Figure 8:
The [Ar/S] ratio in Damped Ly ![]() |
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In the first case (H1 model) the Al III is produced
by the hard photons, which are able to penetrate
H I layer in depth.
In the second case (S2 model) the Al III originates in
a partially ionized interface created by the soft
radiation field, which is not able to penetrate the neutral gas in depth.
Both types of models are able to reproduce the
observed trend Al III/Al II versus N(H I)
with a proper choice of the ionization parameter, U.
For model H1, the requirement to fit the observed
Al III/Al II versus N(H I) trend implies that
the ionization parameter must vary as
.
However, the model H1 cannot be computed at arbitrarily high values
of N(H I), owing to uncertainty in the thermal
solution of the photoionization computations.
For model S2 the trend Al III/Al II versus N(H I)
can be reproduced with a constant ionization parameter.
We refer to V&01 for more details.
After constraining the ionization parameter in such a way to match the Al III/Al II trend,
the H1 and S2 models can be used to predict
the behaviour of Ar/
ionic ratios as a function of N(H I).
In Figs. 8 and 9 we show the
predictions for the Ar I/S II and Ar I/Si II ratios, respectively,
derived with model H1 (dashed line) and S2 (dotted line),
for a gas with solar abundance ratios.
Both models predict that
deviations from the solar value induced by ionization become
less important with increasing N(H I).
The comparison of observations with model predictions
in Figs. 8 and 9 yields the following results.
1. The [Ar/S] and [Ar/Si] ratios, when available for a given DLA,
give consistent indications
on the ionization conditions.
For instance, both ratios
of the z=3.390 system in QSO 0000-26
are in agreement with model S2.
For the z=2.462 system in QSO 0201+365
the ratios yield consistent results,
slightly above the H1 line,
when the Ar/Si dust correction term is taken into account
(Table 1; dust corrections are negligible in the other cases).
The concordant indications obtained from different ratios, each one measured
and modelled independently, indicates that the results are,
at least, self consistent.
2. Only a few [Ar/S, Si] ratios lie in the proximity of the S2 curve,
suggestive of a soft ionizing continuum.
Most of the data lie instead below the S2 model, with several cases
close to the H1 curve, suggestive of a hard continuum.
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Figure 9:
The [Ar/Si] ratio in Damped Ly ![]() |
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3. Even if the DLA with highest N(H I) in our sample (the z=3.390 system in QSO 0000-26) does show negligible ionization effects, there is no evidence for the systematic decrease of ionization effects with increasing N(H I) predicted by the models. The trend could be smeared out because different DLAs are embedded in different types of ionizing continuum, with one possible case of extremely intense continuum, as we mention in the next point.
4. The z=2.812 system in QSO 0528-25 is peculiar
since its redshift is larger than the emission redshift
of the QSO,
.
Most likely the system has a large, positive radial velocity
and is very close to the QSO. In this case,
the large [Ar/
] underabundance at high N(H I) observed in this system
(open symbol in Figs. 8 and 9)
could be due to the effect of a strong, QSO-type
radiation field enhanced relative to the
diffuse background. A detailed analysis of this DLA will be presented in a
separate paper (Péroux et al. 2003).
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Figure 10:
The [Ar/![]() ![]() ![]() |
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We now present evidence for redshift evolution
of Ar abundances, based on measurements in DLAs
in the range
and on local ISM measurements, representative of the conditions at z=0.
In Fig. 10 we show
all [Ar/]
measurements
and limits given in Table 1 plotted versus z.
For the limits, we adopt the most stringent limit on [Ar/
]
available.
For the measurements,
we define [Ar/
]
as the mean value of available [Ar/O], [Ar/S], and [Ar/Si] data, with error bars
obtained from error propagation of individual data.
Only for the z=3.025 absorber in QSO 0347-38 the individual error bars
are smaller than the difference between the [Ar/O] and [Ar/Si] measurements.
In this case
we adopt an error bar sufficiently large to encompass both
measurements.
The [Ar/]
ratios in Fig. 10
increase with redshift, from
-0.7 dex at
to
0 dex at
.
The trend is supported by all the 10 measurements and the 5 limits,
four of which happen to be stringent.
A linear regression of [Ar/
]
versus z for the 10 measurements
yields a slope
at 99.92% confidence level (cl).
If the z=2.812 system in Q 0528-25 is excluded,
the linear regression yields
at 99.94% cl.
The trend versus z is particularly remarkable given the lack of
any trend versus N(H I) as discussed above.
This result strongly suggests that Ar abundances are subject to evolution,
even if the sample is still small at
to allow firm conclusions to be drawn. In favour of the
existence of the trend, we note that
the linear regression analysis still yields consistent results, i.e.
at 98.91% cl,
after excluding the data point at z=3.4 (8 measurements;
z=2.812 system also excluded).
To our knowledge, the Ar/
ratio shows the strongest redshift variation
of relative abundances found so far in DLAs.
Ionic ratios predicted to be
less affected by the ionization field do not show such variations.
For instance, the Si/Fe ratio shows a nearly
constant value, although with significant scatter,
over the large redshift interval
(Fig. 26 in Prochaska & Wolfe 2002). Once corrected for
dust effects, the Si/Fe ratio does show some increase
with redshift (Fig. 3 in Vladilo 2002) which, however, can be explained
in term of chemical evolution
(Fig. 5 in Calura et al. 2003).
As mentioned before, we do not expect
significant chemical evolution for the Ar/
ratio.
The interpretation of the observed evolution
of Ar/
versus redshift is discussed in Sect. 5.
Interstellar lines of sight with
through the Milky Way or nearby galaxies
are representative of the behaviour of DLAs at z=0.
Most interstellar Ar abundances have been obtained
for the Milky Way.
Unfortunately, at the solar metallicity typical of the Milky Way,
Ar I interstellar absorptions are saturated
when
and
reliable measurements can be obtained
only in lines of sight
with column densities somewhat lower than in DLAs.
This difference is taken into account in the discussion below (Sect. 5).
Milky-Way measurements of [Ar/]
are available
in 4 lines of sight,
yielding
dex (Jenkins et al. 2000; Lehner et al. 2000).
Since the local ISM has solar metallicity, the Ar underabundance
can also be estimated from the [Ar/H] ratios, which are available for 3 other lines
of sight, yielding
,
-0.4, and -0.2 dex (SJ98).
Limits obtained from saturated Ar I lines are consistent with
these values.
The median value of all available measurements is -0.2 dex,
significantly larger than the typical [Ar/
] ratio
in DLAs at
.
The largest Ar deficiencies are found in the Galactic plane. Only one,
observed towards Cen (
dex),
is comparable to those measured in DLAs at
.
According to SJ98, this large deficiency
requires the cloud to be located close to an ionizing star
or to be embedded in soft X-ray radiation produced by local hot gas.
The existence of soft X-ray emitting gas in hot
cavities of the local H I distribution is well known
and the line of sight to
Cen
intersects the Local Bubble (Kuntz & Snowden 2000; Sfeir et al. 1999)
and, probably, a further hot cavity associated with Loop I (Iwan 1980; Centurión & Vladilo 1991).
Therefore, the severe Ar deficiency measured towards
Cen may
well represent a particular, rather than general, interstellar condition.
Among the local ISM measurements, those taken at high Galactic latitude
have better chance to reflect the influence of radiation fields external to the Milky Way.
Only three lines of sight have been investigated at
,
yielding
,
-0.18, and -0.27 dex (Jenkins et al. 2000; Lehner et al. 2000),
consistent with the median value of the full sample of Milky-Way
measurements, -0.2 dex.
To our knowledge, the only Ar I interstellar absorption detected in a nearby
galaxy is the one obtained by Vidal-Madjar et al. (2000) from a FUSE spectrum of IZw18 taken with a large entrance aperture covering the whole galaxy.
Analysis of the absorption spectrum yields
dex (Izotov et al. 2001),
even though a value somewhat closer to solar can be derived by taking into account
the effects of large-scale velocity fields integrated in the field of view (Levshakov et al. 2001).
In summary, with the available data set
it seems reasonable to take [Ar/
dex
as a representative interstellar value of the local universe.
The comparison with the low values observed
in DLAs at
indicates that the Ar abundance must significantly increase from
to z=0.
The present study indicates that the full range of
[Ar/] values observed in DLAs
can be reproduced in the framework of photoionization models
of H I regions by properly tuning the spectrum of the ionizing continuum.
Even if the models considered are quite idealized, it is clear that the
magnitude of the underabundance is related to the shape of
the ionizing spectrum, [Ar/
dex being suggestive
of a hard, QSO-type continuum,
while [Ar/
] closer to solar value being suggestive of a soft,
stellar-type continuum
.
The unique capability of Ar I
to discriminate between different sources of ionization
at work in DLAs offers the opportunity
to cast light on the origin of the ionizing continuum in the associated galaxies,
which in principle will include both internal contributions,
due to stellar emission,
and external ones, due to integrated metagalactic radiation.
The origin of the integrated UV background has
been the object of intensive investigation, with
QSOs and star-forming galaxies being
considered as important contributors to the UV background
at various redshifts
(Haardt & Madau 1996;
Songaila 1998; Steidel et al. 2001; Bianchi et al. 2001; Haehnelt et al. 2001).
From these studies a picture is emerging
in which the intensity and spectral shape of
the UV background evolves with redshift.
These studies are necessarily linked to those of the global
star formation rate in the universe,
which also shows evidence for evolution
(Madau et al. 1998; Lanzetta et al. 2002).
Both types of change (UV background and star formation rate)
are expected to affect the ionization state
and the Ar abundances in DLAs.
The trend of Fig. 10
suggests that the ionizing continuum in DLAs becomes gradually harder
from
up to
.
The interstellar Ar abundances
in the local universe suggest that the continuum should become softer
from
to z=0.
We discuss the possible implications of such evolutionary trends.
The interpretation of the
modest or negligible underabundances seen at ,
suggestive of a softer ionizing spectrum,
is less straightforward.
The decline of space density of quasars for z>3 (Shaver et al. 1996)
may lead to a softening of the UV continuum
if quasars are gradually replaced by stars as dominant sources of ionization.
However,
we do not know whether the bulk of the soft photons required
is internal (e.g. stars inside the DLAs)
or external
(e.g. Lyman-break galaxies).
We briefly discuss these two possible scenarios, taking also into account
the possibility that the softening of the ionizing continuum at z>3 may
reflect an evolution of the optical depth of the IGM.
UV continuum at z > 3 dominated by internal stars.
An internal origin of the flux
requires a strong synchronization of stellar emissivity
among DLAs, in such a way that internal stellar photons
dominate at z>3 and become negligible at z<3 in most cases.
This requirement
is rather stringent since it would imply a strong decrease of the star formation rate
in DLAs at ,
not easy to understand given
the weak evidence for redshift evolution of metallicities
(Pettini et al. 1999; Prochaska & Wolfe 2000; Vladilo et al. 2000)
and relative abundances (Vladilo 2002) in DLAs.
In addition, high redshift DLAs are likely to have low dust content and this
makes more critical the requirement that
the UV continuum is dominated by internal starlight. In fact,
we know from Milky Way studies that the main contribution to
the diffuse interstellar radiation is given by photons
scattered by dust (see Bowyer 2001).
If the dust content is low, the UV photons would tend
to escape from the system rather than ionize the H I region(s) inside the system.
UV continuum at z > 3 dominated by external galaxies.
In this scenario
the external continuum dominates the internal one
in the whole redshift interval explored,
with a spectral distribution gradually changing from hard, QSO-type at
z < 3 to soft, stellar-type at z > 3.
This requires
(i) the internal emissivity from stars to be generally modest,
(ii) the UV background must become softer at z>3.
These requirements
seem easy to accommodate within our current views of DLAs,
LBGs and studies of metagalactic UV radiation.
In fact, the modest stellar emissivity of DLAs
is consistent with an origin in galaxies characterized by modest star formation rates,
as suggested by studies of chemical evolution
of DLAs (Calura et al. 2003).
A low stellar emissivity is also suggested by searches for
Ly emissions
(see Fynbo et al. 1999; Fynbo et al. 2000)
and H
emissions from DLAs
(Teplitz et al. 1998; Bunker et al. 1999; Bouché et al. 2001)
which have led, in general, to negative or modest
detections.
A softening of the spectrum of the metagalactic
UV continuum above
is supported by studies of Si IV/C IV ratios in metal absorption systems (Savaglio et al. 1997; Songaila 1998; see
however Kim et al. 2002; Levshakov et al. 2002b).
If the escape fraction of UV photons from Lyman-break galaxies (LBGs)
at z>3 is high, as suggested by Steidel et al. (2001), then the change
of the spectrum could be due to the rise of the contribution from LBGs
parallel to the decline of the space density of quasars.
Perhaps more interesting is the possibility that the softening of the
continuum is related to a change of the optical depth of the IGM,
such as the one expected during the He II reionization epoch.
After this reionization, photons with
eV are free to propagate
through the IGM and can
easily affect the relative abundance of Ar I. In fact,
the ratio of the ionization over recombination rates
is one order of magnitude higher for Ar I than for H I at
eV
(Fig. 3 in SJ98).
Evidence for the detection of the He II reionization has been recently reported by
Bernardi et al. (2002) and Theuns et al. (2002).
The coincidence between the redshift interval in which the Ar abundances change
(Fig. 10) and the interval indicated by Theuns et al. (2002)
for the He II reionization to occur (
)
is indeed quite remarkable.
In any case it is clear that, from a purely observational point of view,
the cumulative evidence for a change of the physical conditions of the IGM
around
starts to be compelling.
The hypothesis that Ar abundances are governed
by radiation fields external to DLAs implies that
the H I regions must be directly exposed to such radiation.
This bears implication on the optical depth of the
C IV/Si IV layers in DLAs, which are
kinematically disconnected from the layers of low ionization
(Wolfe & Prochaska 2000) and probably envelope the low ionization gas,
in a way similar to what observed in the Milky Way, where
C IV and Si IV layers are stratified at large distances from
the Galactic disk. In fact,
in order to prevent absorption from external radiation,
the high-ionization layers in DLAs should have a negligible
Lyman discontinuity. This in turn implies that the neutral
hydrogen column density associated with the highly
ionized gas should be
atoms cm-2.
The local ISM studies suggest that
strong Ar deficiencies might be expected in DLAs
if regions of hot, soft X-ray emitting gas exist in
proximity of the H I region sampled by the line of sight.
In principle, this possibility could be used to
interpret the trend seen in Fig. 10 as entirely due to
internal sources dominating at all redshifts (stars at z>3 and
soft-X bubbles at ). However, in addition to the
drawbacks of the "internal scenario" mentioned in Sect. 5.1,
this would also require a very large filling factor of hot gas in DLAs at
2.4 < z < 3.
Since in the Milky Way we have detected only
one strong Ar deficiency of internal origin out of seven measurements,
such cases do not need to be frequent in DLAs.
The local ISM ionization, representative of the
conditions at z=0, is governed
by the total contribution of ionizing sources internal and external
to the Milky Way.
The origin of the external background in the far UV band
is not well understood, even though it is clear that
(i) some external contribution does exist at high Galactic latitude,
(ii) sources with soft continuum,
such as Galactic starlight scattered by dust and integrated light from galaxies,
do give some contribution;
(iii) sources with hard continuum, such as
integrated QSOs emission and He II Ly emission from the IGM,
are not dominant
(Paresce & Jakobsen 1980; Bowyer 2001; Henry 2002).
At low Galactic latitude most of the internal background
is due to starlight scattered by dust.
Therefore the ionizing continuum in the ISM
at either low and high latitudes
is expected to be soft and, as a consequence, the local Ar deficiencies to be small.
The modest underabundances of Ar found in the local ISM,
with median value -0.2 dex, are
consistent with this expectation.
The fact that local Ar abundances are measured in lines of sight
with H I column density lower than in DLAs
does not affect this result.
In fact, these lines of sight are less self-shielded
and should be more affected by ionization.
A hard continuum would create in any case
severe Ar underabundances, which are instead uncommon.
If we take the Ar measurements in the
local ISM as representative of the universe at z=0,
the present results indicate that the Ar abundances in DLAs
must increase from -0.7 dex at
to
-0.2 dex
at z=0.
This predicted increase is consistent
with the evolution of the relative contribution of QSOs
and galaxies to the UV background in the same redshift interval, derived from
number density studies of the Lyman
forest.
According to these studies
the contribution of galaxies overcomes that of QSOs after
(Bianchi et al. 2001).
If this is true, we predict to detect a
rise of Ar abundances at similar, intermediate redshifts.
From the analysis of the combined sample of Ar abundance, obtained from Ar I absorptions in DLAs (10 measurements and 5 limits), we have derived the following results.
Most DLAs show significant Ar underabundances
relative to other elements (O, Si, and S), with
deficiencies as low as
-0.8 dex in some cases.
Departures from cosmic abundances of this magnitude are not expected for elements that share a common nucleosynthetic history. We have considered dust and ionization effects as possible explanations of the observed underabundances.
From a study of [Ar/]
versus [Zn/Fe] ratios in DLAs
we have shown that dust depletion does not give a viable explanation.
This result is consistent with the lack of
dust depletion expected for Ar
from theoretical studies of the local interstellar medium.
As far as ionization is concerned, we have first considered the possible effects of intervening H II regions inside the DLAs and, on the basis of a study of O I/Si II ratios, we have concluded that such H II regions must be uncommon and cannot explain the observed underabundances.
The full range of the observed Ar abundances can instead be reproduced by photoionization models of H I regions embedded in an ionizing continuum with variable spectral distribution. By using models tuned to match the Al III/Al II ratios measured in DLAs we find that the largest deficiencies of Ar can be explained by a hard, QSO-dominated continuum, while the modest deficiencies by a soft, stellar-type continuum.
We have found evidence for
a redshift dependence of the Ar abundances in DLAs.
At
,
where most of the
measurements are concentrated, the deficiencies are strong
(-0.8/ -0.6 dex).
At
the deficiencies are smaller (>-0.5 dex), with the Ar abundance
becoming solar at
.
More measurements at
are required to understand
how general this latter result is.
The strong Ar deficiencies at
indicate that the ionization is dominated
by a QSO-type spectrum, which we associate with a
QSO-dominated metagalactic background.
The modest deficiencies measured at higher
redshifts suggest a predominance
of a soft, stellar-type spectrum at z>3.
We have considered an origin of this soft spectrum
in both starlight internal to DLAs and
galactic emission external to DLAs.
In both cases, the gradual change of the ionizing continuum from
to
poses strong requirements on the nature of DLAs and/or the
origin of the metagalactic background.
If the redshift variation of Ar abundances is due to the evolution of internal, stellar emission, it requires a synchronization of evolution of DLAs, with most systems having strong star formation rates at z > 3 and weak at z < 3. This seems to be at odd with the lack of clear signal of evolution of DLAs abundances. Also the need for dust at very high redshift, required to maintain an internal, diffuse source of UV radiation, seems to be difficult to reconcile with the low dust content expected in the early evolutionary stages of DLAs.
If the trend is induced by the evolution of the metagalactic ionizing
background, it requires (i) the H I regions in DLAs
to be directly exposed to the external background,
(ii) the contribution from
internal emission to be modest, and
(iii) the external background to become softer at z>3.
The first requirement would imply that the Si IV/C IV layers
associated to DLAs should be optically thin to ionizing radiation.
The second requirement is
consistent with previous claims of low star-formation rates
based on abundance studies and
searches for Ly
and H
emission in DLAs.
The third requirement is consistent
with the evolution of the metagalactic spectrum inferred from
Si IV/C IV measurements in the IGM,
with claims that LBGs at z>3 may have
a high escape fraction of ionizing
photons, and with the recent finding that
the He II reionization does occur between
and
.
We have summarized the results of studies of
Ar abundances in the local ISM taken as representative of a DLA system at z=0.
The typical underabundance of Ar is -0.2 dex, consistent with the expectation that the ionization
in the local universe is dominated by sources
with soft continuum. Only in one case a relatively
strong underabundance of Ar is found, which may originate
in gas embedded in hot, soft-X ray emitting gas known to
exist in the local ISM.
Taken together, the measurements of Ar abundances in DLAs
and in the local universe appear
to offer a new tool for probing the redshift evolution of the
ionization conditions in the universe.
Present observational limitations prevent us to use
this tool to probe intermediate redshifts for
determining the epoch at which QSOs stop being dominant
contributors to the metagalactic UV background. This epoch is
estimated to be at
from recent studies of the
Ly
forest. An independent assessment
based on Ar abundances will be possible from
observations with UV spectrographs fed by
space-born telescopes with large collecting areas
filling the gap of Ar I measurements between z=0 and
.
Acknowledgements
We have benefitted from useful discussions with P. Molaro. We thank the referee for suggestions that have improved the presentation of this work. CP is supported by a Marie Curie fellowship.