A&A 480, 349-357 (2008)
DOI: 10.1051/0004-6361:20078607
P. Petitjean1 - C. Ledoux2 - R. Srianand3
1 - Institut d'Astrophysique de Paris, CNRS and UPMC Paris 6, UMR7095,
98bis boulevard Arago, 75014 Paris, France
2 -
European Southern Observatory, Alonso de Córdova
3107, Casilla 19001, Vitacura, Santiago, Chile
3 -
IUCAA, Post Bag 4, Ganesh Khind, Pune 411 007, India
Received 4 September 2007 / Accepted 18 December 2007
Abstract
Aims. We study the oxygen and nitrogen abundances in the interstellar medium of high-redshift galaxies.
Methods. We use high resolution and high signal-to-noise ratio spectra of damped Lyman-
(DLA) systems detected along the line-of-sight to quasars to derive robust abundance measurements from unsaturated metal absorption lines.
Results. We present results for a sample of 16 high-redshift DLAs and strong sub-DLAs (log N(H I) > 19.5, 2.4 <
<3.6) including 13 new measurements. We find that the oxygen to iron abundance ratio is pretty much constant with [O/Fe]
+0.32
0.10 for -2.5 < [O/H] < -1.0 with a small scatter around this value. The oxygen abundance follows quite well the silicon abundance within
0.2 dex, although the silicon abundance could be slightly smaller for [O/H] < -2. The distribution of the [N/O] abundance ratio, measured from components that are detected in both species, is somehow double peaked: five systems have [N/O] > -1 and nine systems have [N/O] < -1.15. In the diagram [N/O] versus [O/H], a loose plateau is possibly present at [N/O]
-0.9, which is below the so-called primary plateau as seen in local metal-poor dwarf galaxies ([N/O] in the range -0.57 to -0.74). No system is seen above this primary plateau whereas the majority of the systems lie well below with a large scatter. All this suggests a picture in which DLAs undergo successive star-bursts. During such an episode, the [N/O] ratio decreases sharply because of the rapid release of oxygen by massive stars, whereas inbetween two bursts, nitrogen is released by low and intermediate-mass stars with a delay and the [N/O] ratio increases.
Key words: ISM: abundances - Galaxy: abundances - galaxies: quasars: absorption lines
The production of nitrogen in stars is the subject of strong interest as it is difficult to explain consistently the nitrogen abundances measured in different astrophysical environments. New abundance measurements in the interstellar medium (ISM) of our Galaxy, as well as in external galaxies, are of prime importance in these discussions. This could help decide what the sites of the nitrogen production are. It is generally believed that the main contributors are the long-lived intermediate mass stars that are progenitors of asymptotic giant branch (AGB) stars. The contribution from massive stars is still uncertain.
The main nucleosynthetic pathway for the production of nitrogen
is the CNO cycle, which takes place in the stellar H-burning layers.
Nitrogen is thought to have both primary and secondary origins depending
on whether the seed carbon and oxygen nuclei are produced by the star itself
(primary) or are already present in the ISM from which the star
forms, in which case, carbon and oxygen seeds are left-overs from previous generations of stars
(secondary). Secondary production can happen in the H-burning layers of all stars
as the carbon seed is present, by definition, in these layers.
Primary nitrogen is produced when carbon, synthesized in the helium-burning
shell of the star, penetrates into the hydrogen-burning upper shell, where
it is transformed into nitrogen by the CNO cycle. This happens
in intermediate mass stars (4
7) during
the AGB phase (Henry et al. 2000).
Large uncertainties affect the theoretical predictions of both the primary and
secondary nucleosynthesis of nitrogen in low and intermediate-mass stars and the possible
contribution of primary nitrogen from massive stars (Woosley & Weaver 1995; Marigo 2001;
Maeder & Meynet 2002).
It is believed that the determination of the nitrogen abundance in objects with
metallicities spread over a large range may help to answer this question.
Therefore, it is usual to derive metallicities in nearby
low-metallicity emission line galaxies (e.g. Nava et al. 2006;
Izotov et al. 2006) or low-metallicity stars in our Galaxy (Spite et al. 2005).
A complementary approach is to derive metallicities directly at high redshift in
damped Lyman-
(DLA) systems (Pettini et al. 2002; Prochaska et al. 2002;
Centurión et al. 2003) that have metallicities typically in the range
-2.5
-1. Due to the large H I column densities
(log N(H I)
1020 cm-2) and the conspicuous presence of metals, DLAs
are believed to arise in high-redshift galaxies or at least to be located close
to regions where star-formation occurs.
It is usual to discuss these issues using the diagram giving the nitrogen to oxygen
abundance ratio versus the oxygen abundance.
In the case of secondary production, the ratio of
nitrogen to oxygen abundances increases with increasing oxygen metallicity,
whereas for primary production, the ratio remains constant as nitrogen tracks
oxygen. In H II regions of nearby galaxies, a trend in the [N/O] abundance ratio is
seen with a primary plateau at low oxygen abundances and a secondary behavior for
abundances [O/H] > -1 (van Zee et al. 1998; Izotov & Thuan 1999).
DLA systems have a very different behavior in this diagram with
[N/]
(see below) measurements well below the primary plateau (Pettini et al. 2002;
Centurión et al. 2003). This is probably related to the star-formation history
of these objects.
Oxygen and nitrogen abundances can be derived directly from the
N(O I)/N(H I) and N(N I)/N(H I) column density ratios.
Because of efficient charge exchange reactions that link the neutral species together,
the ionization corrections are negligible for log N(H I) > 19.5 (Viegas 1995).
Accurate O I and N I column densities are, however, difficult to derive.
The main reason is that the absorption lines are located in the
Lyman-
forest and are often blended. In addition, and this is especially
true for O I, absorption lines are easily saturated. For N I the situation
is less dramatic because the nitrogen abundance is smaller and N I has
two triplets around
1134 and
1200 Å. On the contrary, the main O I
absorption feature at
1302 is a single line and is often saturated.
Other weaker absorption lines are found much further in the blue, therefore deep in the
Lyman-
forest and in a wavelength range where good a signal-to-noise ratio (SNR) is difficult
to obtain. This is why, in previous studies, the oxygen abundance has often been
replaced by that of another
-element, such as sulfur or silicon (Pettini et al. 2002;
Centurión et al. 2003; Henry & Prochaska 2007).
Using silicon instead of oxygen, Centurión et al. (2003)
claimed that 75% of DLA systems show a mean value [N/Si] = -0.87 with
0.17 dex dispersion, corresponding approximately to the primary plateau observed
locally, and 25% are clustered around [N/Si] = -1.5 with even less
dispersion (0.05 dex), the transition between low and high [N/Si] values happening
at [N/H] = -2.8 (note that the latter transition may occur at [N/H] = -3, see
Molaro et al. 2003).
Here we present the (N/O) vs. (O/H) diagram using robust determinations of oxygen and nitrogen metallicities derived from unsaturated absorption lines selected from the largest UVES sample of DLA systems available up to now (Ledoux et al. 2003, 2006a). In Sect. 2 we present our sample. We describe the O and N abundance measurements in Sect. 3 and discuss the results and implications in Sects. 4 and 5.
Table 1: Results of Voigt-profile fitting.
Most of the systems in our sample were selected from the follow-up of the Large
Bright QSO Survey (Wolfe et al. 1995) and observed at the VLT
with UVES between 2000 and 2004 in the course of a systematic search
for molecular hydrogen at
(Petitjean et al. 2000; Ledoux et al. 2003).
Our sample comprises
61 bona-fide DLA systems (
H I
)
and 13
strong sub-DLA systems with total neutral hydrogen column densities
in the range
H I)<20.3. Characteristics of the systems
(metallicities, N(H I) column densities, kinematics), except
for four sub-DLAs with log N(H I) = 19.7-19.8), are given in
Ledoux et al. (2006a). The absorption line analysis was performed in a homogeneous manner
using standard Voigt-profile fitting techniques adopting the
oscillator strengths compiled by Morton (2003). Total column
densities were derived as the sum of the column densities measured in
individual components of the line profiles. Average gas phase metallicities
relative to solar,
[X/H
X)/N(H
X)/N(H
,
were calculated using solar abundances from Lodders (2003).
As we are interested in measuring the [O/H] and [N/H] abundances from the
N(O I), N(N I) and N(H I) column densities,
we have to select systems where we can derive accurate column densities, e.g.,
where at least one transition from both N I and O I is not strongly saturated.
Note that the transitions from these elements are mostly redshifted in the
Lyman-
forest and we require in addition
that at least one unsaturated transition be free of blending.
From the above sample and under these conditions,
we selected 13 sytems where we could measure N(O I) and
N(N I) column densities accurately.
From the literature, we added three measurements at
= 3.390, 2.844 and
3 toward, respectively, Q 0000-263 (Molaro et al. 2001), Q 1946+769 and QXO 0001
(Prochaska et al. 2002).
As a consequence of our selection criteria, we did not include the system at
= 4.466 toward Q 0307-4945 (Dessauges-Zavadsky
et al. 2001) as the complex O I and N I
velocity profiles, spread over more than 200 km s-1,
are highly saturated and/or blended.
Neither did we include the systems at
= 2.076 and 2.456 toward, respectively,
Q 2206-199 and Q 1409+095 (Pettini et al. 2002) because the oxygen abundance can only
be ill-defined from strongly saturated O I
1302 absorption lines.
In the case of Q 2206-199, the strongest N I feature is also blended
(Molaro et al. 2003).
Ionization may be a concern when deriving the (N/O) abundance ratio from
the N(N I)/N(O I) column density ratio (Viegas 1995; Prochaska et al. 2002).
If the gas is neutral then charge exchange reactions are fast enough so that
O I and N I are both tied to H I and there is no need for correction for
log N(H I) > 19.5 (Viegas 1995).
In case of the presence of enough hard photons (or cosmic rays) the ionization balance
could be displaced towards higher ionization species and both O I and N I
could be over-ionized compared to H I.
However, for most DLA systems, hard photons come predominantly from the background ionization
field. The ionization parameter for this field at z = 3 is U<10-4
for typical densities expected for DLAs (n > 1 cm-3; see e.g. Petitjean et al. 1992),
and ionization corrections are again small.
Indeed, Prochaska et al. (2002)
inspected about twenty DLAs and concluded that the ionization correction for [N/]
is at most of the order of +0.1 dex. This amount should probably be added to the measurement
errors. In the following we did not apply any correction as we show that
there is no correlation between [N/O] and log N(H I).
We performed in a usual manner Voigt profile fitting of absorption lines associated
to the 13 systems we selected from our DLA sample (see e.g. Ledoux et al. 2006a;
Erni et al. 2006).
The overall decomposition in subcomponents is derived from the simultaneous fit
of numerous absorption lines from different species
(most importantly Zn II, Fe II and Si II) redshifted
outside the Lyman-
forest and free from blending.
We used the resulting component structure to detect N I and O I
features and confirm that they are not blended.
Column densities obtained for each of the components are given in Table 1.
Fits are shown in Figs. 8-10. The fits to the absorption lines in the
= 3.025
system towards Q 0347-383 can be seen in Ledoux et al. (2003).
Errors for individual components are the 1
errors from Voigt
profile fitting, as given by FitLyman.
Following the usual procedure, [N/H] and [O/H] abundances
are obtained by adding the column densities in all
detected individual components.
For O I, all the systems are dominated by one strong
component
in which we do not expect much ionization correction
(see the next section for details on each of the systems).
It is possible that in some of the weak components the gas
is partially ionized and that some ionization correction should be applied
to these components. However, this ionization correction is probably smaller
than 0.2 dex in the weakest components and these components
do not account, in total, for more than 20% of the total column density.
We therefore believe that the error on [O/H] due to the procedure is less than
0.2 dex for the whole system (see also the discussion in Prochaska et al. 2002).
For N I, the above error is less as the transitions are weaker and the
species are always detected in less components.
For the [N/O] ratio, we use the column densities in the components where both species are detected. Note that when N I is detected in only one component, this is always the strongest O I component. It is interesting to note that, where N I and O I are detected in several components, as toward Q 2332-094, the [N/O] ratios are similar in all components.
We adopt the oxygen and nitrogen solar abundances from Lodders (2003):
12 + (O/H) = 8.69 and 12 + (N/H)
= 7.83.
Note that the main discussion on the production of these elements is
little dependent on the exact adopted values.
Depletion onto dust-grains is known to be much smaller in DLA systems
compared to the ISM of our Galaxy, mostly because of smaller
(by a factor of at least ten) metallicities. Depletions should
therefore be much smaller than in the diffuse galactic ISM clouds
where it has been shown that, apart from local effects, the [N/O] ratio
is similar to the solar ratio (Knauth et al. 2006).
Table 2:
Metal abundances in Damped Lyman-
systems.
Q 0000-263: We adopt the column densities from Molaro et al. (2001). The oxygen abundance
is derived from unsaturated O I925,950 absorption lines.
The nitrogen abundance is derived from the unsaturated
953 line and the
1134 triplet. The system is modeled with only one component.
Q 0102-190 (Fig. 8, top panel): The structure of the system is derived from the fit of Fe II
and Si II lines. N(N I) is derived from a 1199 optically thin feature
corresponding in redshift with the strongest O I component.
We add all O I column densities (obtained from optically thin
976,1039 features) to calculate the O abundance. Note, however,
that the red satellites do not contribute much. The [N/O] ratio is taken as the
column density ratio in the main central component.
Q 0112-306 (Fig. 8): The two-component structure of the system is very well defined
from Si II and Fe II absorption lines. This is why we are confident
that the log N(O I) column density derived from the moderately saturated
1302 absorption is
robust. N I is detected through an optically thin
1199 feature.
Q 0347-383: This system has been studied by Levshakov et al. (2002) and
Prochaska et al. (2002). The UVES data have also been reanalyzed by Ledoux
et al. (2003). N(N I) is constrained by optically thin 1134
features and N(O I) by
950 and
974. Note that
Levshakov et al. (2002) derived log N(N I) = 14.89, 0.25 dex larger than
our result. We believe this is because the latter authors do not restrict their
fit to optically thin lines.
Q 0841+129 (Fig. 8): This is a beautiful one-component system with several
optically thin N I components (see also Centurión et al. 2002).
N(O I) is derived from
the consistent fit of optically thick 1302 and 1039 features
and the moderately saturated
950 absorption.
Centurión et al. (2003) found a N I column density 0.16 dex
larger. This is likely due to the difference in data quality.
Q 0913+072 (Fig. 8, bottom panel): The absorption profile is made of two
closely blended components of similar strength.
An upper limit on log N(N I) is obtained from 1199
(a feature is present but below the 3
detection limit, see also
Erni et al. 2006). Upper limits are derived from the noise
in the adjacent continuum; when two components are present we conservatively
consider each component independently.
N(O I) is derived from optically thin
1039.
Q 1108-077 (Fig. 9, top panel): This is a one-component system.
An upper limit on N(N I) is obtained from 1134.
N(O I) is derived from an optically thin
1039 feature.
Q 1337+113,
= 2.508 (Fig. 9): This is again a single component
system with well constrained N I and O I
column densities from optically thin, respectively,
1200 and
1039
absorption lines.
Q 1337+113,
= 2.796 (Fig. 9): There are two components
in this system. N(N I) and N(O I) are both well defined
from optically thin, respectively,
1134,1200 and
976 features.
The component at
= 2.79581 is the strongest and is the
only component detected in N I. It is possible that N I is
somewhat ionized in the component at
= 2.79557.
As done for all systems, we add the O I
column densities to calculate the oxygen abundance but take the N/O ratio from
the component at
= 2.79581.
Q 1340-136 (Fig. 9, bottom panel): This is a beautiful one-component system with optically thin transitions.
Q 1409+095,
= 2.456 (Fig. 10, top panel): Although there may be
some absorption at the wavelengths corresponding to the
N I transitions, we derive only an upper limit on N(N I)
in this two-component system (see also Pettini et al. 2002).
The O I column density is well defined by optically thin
1039 absorption.
Q 1409+095,
= 2.668 (Fig. 10): The consistency of
the
1199,1200 and
1134 features makes us
believe that N I is present in this one-component system.
The O I column density is well defined by optically thin
1039 and
950 absorptions.
Q 1946+769: We use the measurements by Prochaska et al. (2002).
Q 2059-360 (Fig. 10): N I and O I are detected in
the three components of this system in, respectively, optically thin
1134,1200 and
950,976 transitions. It is important
to note that the [N/O] ratio is similar for the three components.
This supports the assumption made in this paper that ionization
corrections are negligible.
Q 2332-094 (Fig. 10, bottom panel): The system is complex but dominated by two main
components. We add all O I column densities but the contribution of the
three satellite components is negligible. N I is detected in a single component
corresponding to the strongest O I component at
= 3.05723
and we use the [N/O] ratio in this component.
QXO 0001: We use the measurements by Prochaska et al. (2002). Note that only one digit is given by these authors for the absorption redshift.
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Figure 1: [O/Fe] abundance ratio versus oxygen abundance [O/H]. The dashed and dotted lines are drawn for convenience. |
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Figure 1 gives the [O/Fe] abundance ratio versus the oxygen abundance. In principle, this ratio should be corrected for depletion of metals onto dust-grains. However, this correction is known to be negligible for [X/H] < -1.2 (Prochaska & Wolfe 2003; Ledoux et al. 2003; Wolfe et al. 2005). To support this idea, we plot in Fig. 2 the abundance ratio [S/Fe] versus the sulfur abundance [S/H] for all systems in our DLA sample (Ledoux et al. 2006a). Sulfur and iron are known to be, respectively, little and strongly depleted onto dust-grains. It is apparent that the scatter of the [S/Fe] values is much less below [S/H] = -1.2 than above. This behavior is a consequence of the largest depletion of iron onto dust-grains for [S/H] > -1.2.
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Figure 2: Sulfur to iron abundance ratio, [S/Fe], versus sulfur abundance relative to solar, [S/H]. The lines are drawn for convenience. |
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As the oxygen abundance is difficult to measure, the Si abundance is often used instead. Prochaska & Wolfe (2002) and Dessauges-Zavadsky et al. (2007) have found that the [Si/Fe] ratio is pretty constant amongst DLA systems around a value of +0.43 with a relatively small dispersion. We plot in Fig. 3 the [O/H] abundance versus the [Si/H] abundance. It is apparent that the two abundances correlate well. There may be a slight tendency for oxygen to be more abundant than Silicon for [O/H] < -2 but there are only four measurements there.
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Figure 3: Oxygen abundance relative to solar, [O/H], versus silicon abundance [Si/H]. Dotted line is for [O/H] = [Si/H]. |
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Since we will use two different notations, we would first like to clarify them.
The (N/O) abundance ratio is simply the ratio of the nitrogen to oxygen
abundances, (O/H) = log N(O I)-log N(H I) and the same for
(N/H). This ratio is often plotted versus the oxygen abundance written as
(O/H) + 12. It is, however, very convenient, and usual, to refer to solar abundances,
[O/H] = (O/H)-(O/H).
In that case, solar abundances should be specified. We use:
12 + (O/H)
= 8.69 and 12 + (N/H)
= 7.83.
Therefore, [N/O] = (N/O) + 0.86.
What is most important in the following is the value assigned
to the so-called ``primary plateau''. Nava et al. (2006) observationally
derive a primary plateau at (N/O)
= -1.43 with objects distributed within
a range of -1.54 to -1.27. In that case, [N/O]
= -0.57.
From measurements in blue compact dwarf (BCD) galaxies, Izotov & Thuan (2004),
however, find slightly smaller values with a plateau
at about (N/O)
= -1.6 or [N/O]
= -0.74.
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Figure 4: [N/O] abundance ratio versus neutral hydrogen column density log N(H I). The vertical dotted line indicates the standard definition of DLA systems (log N(H I) > 20.3). |
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Figure 5: Histogram of [N/O] abundance ratios for the 13 systems with definite measurements in our sample. |
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Figure 6:
Nitrogen to oxygen abundance ratio, [N/O],
versus nitrogen abundance [N/H]. [X/H] = (X/H) - (X/H)![]() |
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In Fig. 4 the [N/O] abundance ratio is plotted versus the neutral hydrogen column density. It can be seen that there is no apparent correlation between the two parameters. The smallest [N/O] value is found for a system with log N(H I) > 20.3 and one of the largest values is found for the system with the lowest log N(H I). This gives additional confidence in the assumption that the ionization correction is negligible when deriving the abundances from N(O I), N(N I) and N(H I) (Viegas 1995).
There is a possible dichotomy between systems with [N/O] -0.9 and -1.4.
It can be seen on Fig. 5 that the distribution of the [N/O] abundance ratio
has two peaks, one around [N/O]
-1.35 and one around [N/O]
-0.9.
This could be, however, a consequence of small number statistics.
Out of 13 systems with definite measurements,
7 have [N/O] < -1.15 and 5 have [N/O] > -1.0. If we add
the two upper limits with [N/O] < -1.2, we find that the number of systems
with [N/O] < -1.15 is about two times larger than those with [N/O] > -1.0.
A similar dichotomy has already been noted in the [N/
]
ratio
by Centurión et al. (2003). They find, however, that 75% of systems
cluster around [N/
]
-0.87 and the remaining 25% cluster around
[N/
]
-1.45.
Also, note that the dichotomy present in our data is less pronounced than what is
claimed by Centurión et al. (2003) because of a large dispersion of the
values below [N/O]
-1.2 (see also Henry & Prochaska 2007).
One could wonder if these differences could be due to our sample
being biased against high oxygen metallicity systems compared to the Centurión
et al. sample. We do not think this is the case because there is no obvious
correlation between [O/H] and [N/O].
In addition, Centurión et al. (2003) claimed that there is a transition between the two
regimes at [N/H] -2.8 with most, if not all, of the [N/
] < -1.5
systems having [N/H] < -2.8. Molaro et al. (2003) place this transition
at [N/H]
-3.0 and argue that this could be understood as the point at
which nitrogen production from AGB stars begins to dominate that of massive stars.
Although all the points with [N/O]
-1.2 have
[N/H] < -2.4, we find two systems with [N/O] > -1.2 and
[N/H]
-3.0. Again, below [N/O] < -1, the scatter in the measurements
is larger than above this limit.
Note, however, that the four points with [N/H] < -3.2 have [N/O] < -1.4
so that the figure is not inconsistent with some kind of break below [N/H] = -3.2.
More data are needed below this limit before any firm conclusion can be drawn.
The classical plot giving [N/O] versus [O/H] is shown in Fig. 7.
Several features have to be noted here. First, all the DLA measurements
are located in the region delineated by the usual primary and secondary
lines. Recall that, by definition, if nitrogen is a primary element, it should be
produced at the same time as oxygen and the [N/O] ratio should be a constant
whatever the oxygen metallicity is. The primary plateau observed locally
is derived from measurements of abundances in nearby low-metallicity
galaxies: [N/O]
in the range -0.57 to -0.74.
If nitrogen is a secondary element, its
production is favored at large oxygen abundance and the [N/O] ratio
should increase with increasing oxygen abundance.
The fact that all the DLA systems are found in this region may indicate
that at least part of the systems are in the transition zone as a consequence
of delays in the release of heavy elements from intermediate mass stars.
This is what would be expected if during a starburst,
at the beginning, high mass stars eject material with low [N/O] ratio
and, after some time, intermediate mass stars eject material with higher
ratio. The pathway of one particular system in the diagram during his lifetime
could be a line starting from the left-bottom corner of the figure towards the
up-right direction.
Secondly, DLA measurements
are all below the local primary plateau ([N/O]
-0.57 to
-0.74). If we believe that the five top-most [N/O] DLA measurements define
what can be considered as a plateau, this plateau is at [N/O]
-0.9,
therefore at least -0.15 dex below the local primary plateau.
Note that this has not been recognized by studies based on [N/Si] or
[N/S] ratios (Centurión et al. 2002; Pettini et al. 2007).
We understand this conclusion is based on small number statistics and should
be confirmed with more data. One possibility to explain this would be
that DLAs reach the primary plateau only for [O/H] > -1.
In the lower panel of Fig. 7, we plot
(N/O) vs. (O/H) + 12 for DLAs (squares) and local measurements
(dots and crosses). The local measurements are from dwarf-irregular galaxy HII regions (van Zee
& Haynes 2006), HII regions in spiral galaxies (van Zee et al. 1998), and
metal-poor emission line galaxies (Nava et al. 2006; Izotov et al. 2006).
The same remark as above is, of course, to be made here.
It is apparent that DLA measurements are all below the local
measurements. If a plateau is to be seen in DLA measurements, this is
at (N/O) -1.75.
Other points are pretty well scattered in the diagram and we find it difficult to confirm the
presence of a second plateau in Fig. 7, as claimed by
Centurión et al. (2003).
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Figure 7:
Upper panel: nitrogen to oxygen abundance ratio, [N/O],
versus oxygen abundance [O/H]. [X/H] = (X/H)-(X/H)![]() |
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Figure 8: N I and O I absorption lines in the DLA systems: Q 0102-190, z = 2.926; Q 0112-306, z = 2.418; Q 0841+129, z = 2.476 and Q 0913+072, z = 2.618 from top to bottom, respectively. Model fits are overplotted. |
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Figure 9: N I and O I absorption lines in the DLA systems: Q 1108-077, z = 3.608; Q 1337+113, z = 2.508; Q 1337+113, z = 2.796 and Q 1340-136, z = 3.118 from top to bottom, respectively. Model fits are overplotted. |
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Figure 10: N I and O I absorption lines in the DLA systems: Q 1409+095, z = 2.456; Q 1409+095, z = 2.668; Q 2059-360, z = 3.083 and Q 2332-094, z = 3.057 from top to bottom, respectively. Model fits are overplotted. |
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We show that the scatter in the [O/Fe] measurements is small, around
a mean value of [O/Fe] 0.32
0.10. This small scatter is probably
evidence for similar nucleosynthetic histories and efficient mixing of the gas.
In the same range of metallicity,
[O/Fe] in metal-poor stars is found to be about 0.7 dex if the measurements
are not corrected for 3D effects and 0.4 dex if corrected (Cayrel et al. 2004).
In the [N/O] versus [O/H] diagram, we find that about a third of the measurements
are located close to but below the so-called local primary plateau.
If a plateau is to be defined for DLAs, its position is at
[N/O] -0.9, which is lower than the plateau measured locally
in metal-poor dwarf galaxies, [N/O]
-0.57 to -0.74.
All our measurements are located below the local plateau, which is not the case for measurements in
metal-poor halo stars (Spite et al. 2005):
about half of the stars have [N/O] > -0.9. However, this may be due to small
number statistics as a quarter of the DLA systems compiled by Henry & Prochaska (2007) have
a [N/Si] ratio larger than the value corresponding to the primary plateau
for these elements.
We find it difficult to confirm the presence of a second plateau at a lower value, as claimed by
Centurión et al. (2003), although the [N/O] distribution is probably
double peaked around [N/O]
-0.9 and [N/O]
-1.35.
This bimodality in the [N/O] distribution could also be due to small number statistics
although a possible bimodality in the [N/
]
distribution has already been noticed
by Prochaska et al. (2002)
and Centurión et al. (2003); see also Henry & Prochaska (2007).
Oxygen is mainly produced in short-lived massive stars and released into
the ISM by type II supernovae explosions. Nitrogen is mainly produced
in long-lived intermediate mass stars and ejected into the ISM by stellar winds.
This implies delays in the ejection of nitrogen into the ISM even if it is
primary. This is probably why most of the DLA measurements are below the local
primary plateau. We note that the approximately constant observed [O/Fe] ratio does not
support the suggestion by Pettini et al. (2002) that iron should
have the same evolutionary time-scale as nitrogen, thus
DLAs deficient in nitrogen should also be deficient in iron.
However, their argument holds only for part of the iron production,
the part produced by type Ia supernovae that have the same time-scale as
intermediate-mass stars. Indeed, models with constant star-formation over a large range of
duration produce a [Si/Fe] 0.3 ratio about constant over a large
metallicity range (Henry & Prochaska 2007). These models cannot reproduce the scatter
in the measurements but one could easily claim that constant star-formation rate is
a simplistic assumption and that scatter can arise from different
star-formation histories (see also Mollá et al. #Moll&).
As a consequence, considering an isolated galaxy, the [N/O] ratio could decrease sharply during a starburst when oxygen is released by SNe (e.g. Contini et al. 2002). The scatter in the [N/O] ratio measurements could then be due to the intensity of the different bursts of star formation, explaining the different [N/O] values. After some delay, corresponding to the lifetime of intermediate mass stars, the [N/O] ratio could increase because of the release of most of the nitrogen. The bimodality of the [N/O] distribution and the larger number of systems with a low [N/O] ratio are a consequence of the delay between the releases of oxygen and nitrogen, because the duration of the release is small compared to the life-time of the stars (see also Ledoux et al. 2006b). It is clear that increasing the sample size would be important to improve our knowledge on these issues.
Acknowledgements
We thank the referee, Paolo Molaro, for a thorough reading of the manuscript and useful comments. R.S. and P.P.J. gratefully acknowledge the Indo-French Centre for the Promotion of Advanced Research (Centre Franco-Indien pour la Promotion de la Recherche Avancée) under contract No. 3004-3. We thank Elisabeth Flam for useful discussions.