We characterize the importance of metallicity on the presence of molecular
hydrogen in damped Lyman-alpha (DLA) systems. We construct a representative
sample of 18 DLA/sub-DLA systems with log N(HI)>19.5 at high redshift
(zabs>1.8) with metallicities relative to solar [X/H]>-1.3(with[X/H]=
logN(X)/N(H)-log(X/H)solar and X either Zn, S or Si). We gather data covering
the expected wavelength range of redshifted H2 absorption lines on all systems
in the sample from either the literature (10 DLAs), the UVES-archive or new
VLT-UVES observations for four of them. The sample is large enough to discuss
for the first time the importance of metallicity as a criterion for the
presence of molecular hydrogen in the neutral phase at high-z. From the new
observations, we report two new detections of molecular hydrogen in the systems
at zabs=2.431 toward Q2343+125 and zabs=2.426 toward Q2348-011. We compare the
H2 detection fraction in the high-metallicity sample with the detection
fraction in the overall sample from Ledoux et al. (2003). We show that the
fraction of DLA systems with logf=log 2N(H2)/(2N(H2)+N(HI))>-4 is as large as
50% for [X/H]>-0.7 when it is only about 5% for [X/H]<-1.3 and about 15% in the
overall sample (with -2.5<[X/H]<-0.3). This demonstrates that the presence of
molecular hydrogen at high redshift is strongly correlated with metallicity.Comment: 4 pages, 3 Postscript figures. Accepted in Astronomy and Astrophysics
  Lette

C. Ledoux

D'Odorico

Dekker

Dessauges-Zavadsky

Foltz

Grevesse

Heinmüller

Hirashita

Kulkarni

Ledoux

Levshakov

Morton

P. Noterdaeme

P. Petitjean

Petitjean

Prochaska

R. Srianand

Sargent

Savage

Srianand

Viegas

English

arXiv

International audienceAims.We characterize the importance of metallicity on the presence of molecular hydrogen in damped Lyman-alpha (DLA) systems. Methods: .We constructed a representative sample of 18 DLA/sub-DLA systems with log N(H i) > 19.5 at high redshift (z_abs > 1.8) and with metallicities relative to solar [X/H] > -1.3 (with [X/H] = log N(X)/N(H)-log (X/H)&sun; and X either Zn, S or Si). We gathered data covering the expected wavelength range of redshifted H2 absorption lines on all systems in the sample from either the literature (10 DLAs), the UVES-archive, or new VLT-UVES observations for four of them. The sample is large enough to allow us to discuss for the first time the importance of metallicity as a criterion for the presence of molecular hydrogen in the neutral phase at high-z. Results: .From the new observations, we report two new detections of molecular hydrogen in the systems at z_abs = 2.431 toward Q 2343+125 and at z_abs= 2.426 toward Q 2348-011. Q 2348-011, From a comparison of the H2 detection fraction in the high-metallicity sample with earlier results, we find that the fraction of DLA systems with log f = log 2N(H2)/(2N(H2) + N(H i)) > -4 is as large as 50% for [X/H] > -0.7, when it is only ~5% for [X/H] < -1.3 and ~15% in the overall sample (with -2.5 < [X/H] < -0.3). This demonstrates that the presence of molecular hydrogen at high redshift is strongly correlated with metallicity.<BR /

Petitjean, Patrick

Ledoux, Cédric

Noterdaeme, Pasquier

Srianand, Raghunathan

HAL: Hyper Article en Ligne

Metallicity as a criterion to select H_2-bearing damped Lyman-alpha systems

Aims.We characterize the importance of metallicity on the presence of molecular hydrogen in 
damped Lyman-α (DLA) systems.
        Methods.We constructed a representative sample of 18 DLA/sub-DLA systems with log N(H 

EDP Sciences OAI-PMH repository (1.2.0)

Metallicity as a criterion to select H

HAL-INSU

Aims: We characterize the importance of metallicity on the presence of molecular hydrogen in damped Lyman- &#945;(DLA) systems. Methods.We constructed a representative sample of 18 DLA/sub-DLA systems with log N(H I) &gt; 19.5 at high redshift ( zabs &gt; 1.8) and with metallicities relative to solar [X/H] &gt; -1.3 (with [X/H] = log N(X)/N(H)-log (X/H) &#x2299; and X either Zn, S or Si). We gathered data covering the expected wavelength range of redshifted H2 absorption lines on all systems in the sample from either the literature (10 DLAs), the UVES-archive, or new VLT-UVES observations for four of them. The sample is large enough to allow us to discuss for the first time the importance of metallicity as a criterion for the presence of molecular hydrogen in the neutral phase at high-z. Results.From the new observations, we report two new detections of molecular hydrogen in the systems at zabs=2.431 toward Q 2343+125 and at zabs= 2.426  toward Q 2348-011. Q 2348-011, From a comparison of the H2 detection fraction in the high-metallicity sample with earlier results, we find that the fraction of DLA systems with log f = log 2N(H2)/(2N(H2) + N(H I)) &gt; -4 is as large as 50% for [X/H] &gt; -0.7, when it is only &#8764;5% for [X/H] &lt; -1.3 and &#8764; 15% in the overall sample (with -2.5 &lt; [X/H] &lt; -0.3). This demonstrates that the presence of molecular hydrogen at high redshift is strongly correlated with metallicity

Petitjean, P.

Ledoux, C.

Noterdaeme, P.

Srianand, R.

Indian Academy of Sciences

A&A 456, L9–L12 (2006)
DOI: 10.1051/0004-6361:20065769
c© ESO 2006
Astronomy
&
Astrophysics
Letter to the Editor
Metallicity as a criterion to select H2-bearing
damped Lyman-α systems
P. Petitjean1,2, C. Ledoux3, P. Noterdaeme3, and R. Srianand4
1 Institut d’Astrophysique de Paris, CNRS – Université Pierre et Marie Curie, 98bis boulevard Arago, 75014 Paris, France
e-mail: petitjean@iap.fr
2 LERMA, Observatoire de Paris, 61 avenue de l’Observatoire, 75014 Paris, France
3 European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Vitacura, Santiago, Chile
4 IUCAA, Post Bag 4, Ganesh Khind, Pune 411 007, India
Received 7 June 2006 / Accepted 16 July 2006
ABSTRACT
Aims. We characterize the importance of metallicity on the presence of molecular hydrogen in damped Lyman-α (DLA) systems.
Methods. We constructed a representative sample of 18 DLA/sub-DLA systems with log N(H i) > 19.5 at high redshift (zabs > 1.8)
and with metallicities relative to solar [X/H] > −1.3 (with [X/H] = log N(X)/N(H)−log (X/H) and X either Zn, S or Si). We gathered
data covering the expected wavelength range of redshifted H2 absorption lines on all systems in the sample from either the literature
(10 DLAs), the UVES-archive, or new VLT-UVES observations for four of them. The sample is large enough to allow us to discuss
for the first time the importance of metallicity as a criterion for the presence of molecular hydrogen in the neutral phase at high-z.
Results. From the new observations, we report two new detections of molecular hydrogen in the systems at zabs = 2.431 toward
Q 2343+125 and at zabs = 2.426 toward Q 2348−011. Q 2348−011, From a comparison of the H2 detection fraction in the high-
metallicity sample with earlier results, we find that the fraction of DLA systems with log f = log 2N(H2)/(2N(H2) + N(H i)) > −4 is
as large as 50% for [X/H] > −0.7, when it is only ∼5% for [X/H] < −1.3 and ∼15% in the overall sample (with −2.5 < [X/H] < −0.3).
This demonstrates that the presence of molecular hydrogen at high redshift is strongly correlated with metallicity.
Key words. galaxies: high-redshift – galaxies quasars: absorption lines – galaxies quasars: individual: Q 2343+125 –
galaxies quasars: individual: Q 2348−011
1. Introduction
Early searches for molecular hydrogen in DLAs, though not
systematic, have led to either low values of or upper limits to
the molecular fraction of the gas. For a long time, only the
DLA at zabs = 2.811 toward Q 0528−250 was known to con-
tain H2 molecules (Levshakov & Varshalovich 1985; Foltz et al.
1988). Ge & Bechtold (1999) searched for H2 in a sample of
eight DLAs using the MMT moderate-resolution spectrograph
(FWHM = 1 Å). Apart from the detection of molecular hy-
drogen at zabs = 1.973 and 2.338 toward Q 0013−004 and
Q 1232+082, respectively, they measured upper limits on f in
the range 10−6−10−4 in the other systems.
A major step forward in understanding the nature of DLAs
through their molecular hydrogen content has recently been
made possible by the unique high-resolution and blue-sensitivity
capabilities of UVES at the VLT. In the course of the first large
and systematic survey for H2 at high redshift, we searched for H2
in DLAs down to a detection limit of typically N(H2) = 2 ×
1014 cm−2 (Petitjean et al. 2000; Ledoux et al. 2003). Out of
the 33 surveyed systems, eight had firm detections of associ-
ated H2 absorption lines. Considering that three detections had
 Based on observations carried out at the European Southern
Observatory (ESO) under prog. ID Nos. 65.O-0158, 65.O-0411,
67.A-0022, 67.A-0146, 69.A-0204, 70.B-0258, 072.A-0346 with
UVES installed at the Very Large Telescope (VLT) on Cerro Paranal,
Chile.
been already known from past searches, H2 was detected in
∼15% of the surveyed systems. The existence of a correlation be-
tween metallicity and depletion factor, measured as [X/Fe] (with
X = Zn, S or Si) was demonstrated (see also Ledoux et al. 2002a)
and the DLA and sub-DLA systems where H2 was detected were
usually among those with the highest metallicities.
However, the high metallicity end of the Ledoux’s sam-
ple was biased by the presence of already known detections.
Therefore, to further investigate this possible dependence with
metallicity and derive the actual molecular content of the high-
redshift gas with highest metallicity, we searched a representa-
tive sample of high metallicity DLAs for H2. The number of
H2 measurements in systems with [X/H]> −1.3 is twice larger in
our sample compared to previous surveys. We describe the sam-
ple and the observations in Sect. 2, present two new detections
of H2 in Sect. 3 and the results of the survey and our conclusions
in Sect. 4.
2. Sample and observations
We have selected from the literature all z > 1.8 DLAs and
sub-DLAs systems (log N(H i) ≥ 19.5) with previously mea-
sured elemental abundances (e.g. Prochaska & Wolfe 2001;
Kulkarni & Fall 2002) larger than [X/H] > −1.3 and accessible
to UVES. The inclusion of sub-DLAs is justified by the fact that
for log N(H i) > 19.5 most of the hydrogen is neutral (Viegas
1995). In addition, Ledoux et al. (2003) have shown that for
Article published by EDP Sciences and available at http://www.edpsciences.org/aa or http://dx.doi.org/10.1051/0004-6361:20065769
L10 P. Petitjean et al.: H2 in DLA systems
Table 1. Metal and molecular content of high-metallicity DLA/sub-DLA systems at zabs > 1.8.
QSO zem zabs log N(H i) 1 [X/H] 1 X log N(H2) 2 log f 3 References
J = 0 J = 1
Q 0013−004 2.09 1.973 20.83 ± 0.05 −0.59 ± 0.05 Zn 17.72–20.00 −1.68+1.07−1.18 a
Q 0112−306 2.99 2.702 20.30 ± 0.10 −0.49 ± 0.11 Si <14.1 <14.0 <−5.55 b
Q 0216+080 2.99 2.293 20.50 ± 0.10 −0.70 ± 0.11 Zn <14.3 <14.5 <−5.39 c
Q 0347−383 3.22 3.025 20.73 ± 0.05 −1.17 ± 0.07 Zn 14.53+0.06−0.06 −5.90+0.11−0.11 b
Q 0405−443 3.02 2.595 21.05 ± 0.10 −1.12 ± 0.10 Zn 18.14+0.07−0.10 −2.61+0.17−0.20 b, d
Q 0458−020 2.29 2.040 21.70 ± 0.10 −1.22 ± 0.10 Zn <14.6 <14.6 <−6.40 e
Q 0528−250 2.77 2.811 21.35 ± 0.07 −0.91 ± 0.07 Zn 18.22+0.11−0.12 −2.83+0.18−0.19 b, d
Q 0551−366 2.32 1.962 20.70 ± 0.08 −0.35 ± 0.08 Zn 17.42+0.45−0.73 −2.98+0.53−0.81 f
Q 1037−270 2.23 2.139 19.70 ± 0.05 −0.31 ± 0.05 Zn <14.0 <14.1 <−5.05 g
Q 1209+093 3.30 2.584 21.40 ± 0.10 −1.01 ± 0.10 Zn <14.9 <15.1 <−5.69 c
Q 1441+276 4.42 4.224 20.95 ± 0.10 −0.63 ± 0.10 S 18.28+0.08−0.07 −2.38+0.18−0.17 h
Q 1444+014 2.21 2.087 20.25 ± 0.07 −0.80 ± 0.09 Zn 18.16+0.14−0.12 −1.80+0.21−0.19 b
Q 2116−358 2.34 1.996 20.10 ± 0.07 −0.34 ± 0.11 Zn <14.5 <14.8 <−4.75 c
Q 2206−199 2.56 1.921 20.67 ± 0.05 −0.54 ± 0.05 Zn <14.4 <14.7 <−5.44 c
Q 2230+025 2.15 1.864 20.90 ± 0.10 −0.81 ± 0.10 S <15.4 <15.4 <−4.80 c
Q 2243−605 3.01 2.331 20.65 ± 0.05 −0.85 ± 0.05 Zn <13.8 <13.9 <−6.15 c
Q 2343+125 2.51 2.431 20.40 ± 0.07 −0.89 ± 0.08 Zn 13.69+0.09−0.09 −6.41+0.16−0.16 c
Q 2348−011 3.01 2.426 20.50 ± 0.10 −0.60 ± 0.11 S 18.45+0.27−0.26 −1.76+0.37−0.36 c
1 Neutral-hydrogen column densities and metallicities relative to Solar, [X/H] ≡ log [N(X)/N(H)] − log [N(X)/N(H)] (with X = Zn as the
reference element when Zn ii is detected, or else either S or Si) are from Ledoux et al. (2006a).
2 Molecular-hydrogen column densities are summed up over all J levels in case of detection and upper limits for J = 0 and J = 1 are given in case
of non-detection.
3 Molecular fraction, f = 2N(H2)/(2N(H2) + N(H i)).
a Petitjean et al. (2002), b Ledoux et al. (2003), c this work, d Srianand et al. (2005), e Heinmüller et al. (2006), f Ledoux et al. (2002b), g Srianand
& Petitjean (2001), h Ledoux et al. (2006b).
log N(H i) > 19.5, there is no correlation between the presence
of H2 and the H i column density. That we include ALL known
systems with these criteria guarantees that the sample is repre-
sentative of the population of DLA-subDLA.
We ended up with a sample of 15 DLAs and 3 sub-
DLAs (with log N(H i) = 19.7, 20.10 and 20.25) as pre-
sented in Table 1. The H2 content of ten of these sys-
tems has already been published (Srianand & Petitjean 2001;
Petitjean et al. 2002; Ledoux et al. 2002b, 2003, 2006b;
Srianand et al. 2005 and Heinmüller et al. 2006). Five of
the eight remaining systems have data in the UVES archive
(Q 1209+093: Prog. 67.A-0146 P.I. Vladilo; Q 2116−358:
Prog. 65.O-0158 P.I. Pettini; Q 2230+025: Prog. 70.B-0258, P.I.
Dessauges-Zavadsky; Q 2243−605: Prog. 65.O-0411 P.I. Lopez;
Q 2343+125: Prog. 69.A-0204 and 67.A-0022, P.I. D’Odorico).
New observations of Q 0216+080, Q 2206−199, Q 2343+125,
and Q 2348−011 have been performed with the Ultraviolet
and Visible Echelle Spectrograph (UVES, Dekker et al. 2000)
mounted on the ESO Kueyen VLT-UT2 8.2 m telescope on Cerro
Paranal in Chile. These new observations resulted in two new
detections described in the next section.
The data for each of the eight QSOs were reduced using the
UVES pipeline, which is available as a package (a so-called
context) of the ESO MIDAS data reduction system (see e.g.
Ledoux et al. 2003 for details). Standard Voigt-profile fitting
methods were used to analyze metal lines and molecular lines,
when detected, to determine column densities using the oscilla-
tor strengths compiled in Ledoux et al. (2003) for metal species
and by Morton & Dinerstein (1976) for H2. We adopted the Solar
abundances from Morton (2003) based on meteoritic data from
Grevesse & Sauval (2002).
The characteristics of the sample are summarized in Table 1.
The H i column densities and most of metallicities are from the
compilation by Ledoux et al. (2006a). Slight differences with
previously published values, e.g. Ledoux et al. (2003), are due
to the use of different damping coefficients for H i. The only
known z > 1.8 H2 bearing DLA system out of this sample is
the system at zabs = 2.337 toward Q 1232+082 (Srianand et al.
2000).
3. Two new detections
We report two new detections of H2 in DLAs from our new ob-
servations. Detail analysis and interpretation of the physical con-
ditions in these two DLAs are beyond the scope of the present
work and will be described in a subsequent paper.
3.1. Q 2343+125, zabs = 2 .431
This DLA system was first studied by Sargent et al. (1988).
High resolution data have been described by Lu et al. (1996),
D’Odorico et al. (2002), and Dessauges-Zavadsky et al. (2004).
The profile of the metal lines is spread over more than
250 km s−1 from zabs = 2.4283 to 2.4313, but the strongest com-
ponent is centered at zabs = 2.43127 corresponding to the red
edge of the above redshift range. From Voigt-profile fitting to the
H i Lyman-α, β, and γ lines, we find that the damped Lyman-α
line is centered at zabs = 2.431, and the column density is
log N(H i) = 20.40 ± 0.07, consistent with previous measure-
ment by D’Odorico et al. (2002; log N(H i) = 20.35 ± 0.05).
We use Zn as the reference species for metallicity measure-
ment and find [Zn/H] = −0.89 ± 0.08. This is consistent with
P. Petitjean et al.: H2 in DLA systems L11
Fig. 1. Velocity plots of observed H2 absorption lines from the J = 0 and
J = 1 rotational levels at zabs = 2.43127 toward Q 2343+125. Profiles of
associated Si ii and Fe ii absorptions are shown in the upper panels. The
best-fitting model to the system is overplotted; the location of Voigt-
profile sub-components are shown as vertical dashed lines.
previous findings. Absorption from the J = 1 and probably from
the J = 0 rotational levels of H2 is detected in this system at
zabs = 2.43127 (see Fig. 1). The optically-thin H2 absorption
lines are very weak, i.e. close to, but above, the 3σ detection
limit. A very careful normalization of the spectrum was per-
formed by adjusting the continuum while fitting the lines. The
best-fitting consistent model for H2 is shown in Fig. 1. The to-
tal H2 column density, integrated over the J = 0 and 1 levels,
is estimated to be log N(H2) = 13.69 ± 0.09 (12.97 ± 0.04 and
13.60± 0.10 for J = 0 and 1, respectively). This leads to the low-
est molecular fraction observed up to now, log f = −6.41+0.16−0.16.
We also derive an upper limit on the detection of absorption from
the J = 2 level, log N(H2–J = 2) < 13.1 at the 3σ level.
3.2. Q 2348–011, zabs = 2 .426
There are two DLA systems at zabs = 2.426 and zabs = 2.615 to-
ward Q 2348−011, with total neutral hydrogen column densities
of log N(H i) = 20.50 ± 0.10 and log N(H i) = 21.30 ± 0.08, re-
spectively. Conspicuous H2 absorptions are detected in the zabs 
2.426 DLA system, the only system to be considered here as dis-
playing a high metallicity (see Fig. 2). The molecular lines are
very numerous and strong, but the spectral resolution of our data
is high enough to allow unambiguous detection and accurate de-
termination of the line parameters. Seven H2 components spread
over about 300 km s−1 were used for the H2 fit. It is interest-
ing to note that the strongest metal component (at V = 0 km s−1
in Fig. 2) has no associated H2 absorption. Strong H2 absorp-
tion is seen at V ∼ −150 and +50 km s−1. All seven molecu-
lar components have associated C i absorption. However, ad-
ditional components are needed to fit the metal absorption
Fig. 2. Absorption profiles at zabs = 2.4263 (taken as zero of the velocity
scale) toward Q 2348−011. The positions of the seven (respectively, 13)
components needed to fit the H2 (respectively, metal line) profiles are
indicated by vertical dashed lines. The resulting best-fitting model to
the system is overplotted.
lines: 9 components for C i and 13 components for the singly
ionized species. H2 absorption from the rotational levels J = 0
to 5 are unambiguously detected. The total H2 column density
integrated over all rotational levels is log N(H2) = 18.45+0.27−0.26,
corresponding to a molecular fraction log f = −1.76+0.37−0.36.
We also derive an upper limit on the column density of
HD molecules, leading to log N(HD)/N(H2) < −3.3.
4. Metallicity as a criterion for the presence
of molecular hydrogen
From Table 1, it can be seen that H2 is detected in nine high-
metallicity systems out of 18. However, for two of the detections,
the corresponding value of log f is lower than most of the up-
per limits derived for other systems. All upper limits are lower
than −4.5 and all detections higher than these upper limits are
higher than −3. We therefore define a system with large (respec-
tively small) H2 content if log f is higher (respectively lower)
than −4.
In Fig. 3 we plot the molecular fraction, log f , versus
metallicity, [X/H], for our representative sample of DLAs with
[X/H] > −1.3 (18 measurements summarized in Table 1) and
measurements by Ledoux et al. (2003) for [X/H] < −1.3
(23 measurements). The log f distribution is bimodal with an ap-
parent gap in the range −5 < log f < −3.5 justifying the above
classification of systems. Note that this jump in log f was al-
ready noticed by Ledoux et al. (2003) and is similar to what is
seen in our Galaxy (Savage et al. 1977; see also Srianand et al.
2005). It is apparent that the fraction of systems with molecular
fraction log f > −4 increases with increasing metallicity. It is
only ∼5% for [X/H] < −1.3 when it is ∼39% for [X/H] > −1.3.
This fraction is even higher, 50%, for [X/H] > −0.7 which is the
median metallicity for systems with [X/H] > −1.3. In addition,
all systems with [X/H] < −1.5 have log f < −4.5.
We conclude that metallicity is an important criterion for
the presence of molecular hydrogen in DLAs. This may not
be surprising as the correlation between metallicity, and the
depletion of metals onto dust grains (Ledoux et al. 2003)
implies that higher metallicity means higher dust content and
therefore higher H2 formation rate. In addition, the presence of
dust implies a greater absorption of UV photons that usually
dissociate the molecule. More generally, Ledoux et al. (2006a)
L12 P. Petitjean et al.: H2 in DLA systems
-2.5 -2.0 -1.5 -1.0 -0.5 0.0
[X/H]
-8
-6
-4
-2
0
lo
g 
f
Fig. 3. Logarithm of the molecular fraction, f = 2N(H2)/(2N(H2) +
N(H i)), versus metallicities, [X/H] = log N(X)/N(H)−log (X/H) with
X either Zn, S or Si, in DLAs from the sample described in this pa-
per ([X/H] > −1.3, see Table 1) and the sample of Ledoux et al. (2003,
[X/H] < −1.3). Filled squares indicate systems in which H2 is detected.
Dashed lines indicate the limits used in the text (log f = −4; [X/H] =
−1.3). The dotted line shows the median of the high-metallicity sample
([X/H] = −0.7).
have shown that a correlation exists between metallicity and
velocity width in DLAs. If the latter kinematic parameter is inter-
preted as reflecting the mass of the dark-matter halo associated
with the absorbing object, then DLAs with higher metallicity
are associated with objects of higher mass where star formation
could be enhanced. All this makes it arguable that, in DLAs,
star-formation activity is probably correlated with the molecular
fraction (Hirashita & Ferrara 2005). It is therefore of prime im-
portance to survey a large number of DLA systems to define their
molecular content better and use this information to derive the
physical properties of the gas and the amount of star-formation
occurring in the associated objects.
Acknowledgements. P.P.J. thanks ESO for an invitation to stay at the ESO head-
quarters in Chile where part of this work was completed. R.S. and P.P.J. grate-
fully acknowledge support from the Indo-French Centre for the Promotion of
Advanced Research (Centre Franco-Indien pour la Promotion de la Recherche
Avancée) under contract 3004-3. P.N. is supported by an ESO student fellowship.
References
Dekker, H., D’Odorico, S., Kaufer, A., Delabre, B., & Kotzlowski, H. 2000, Proc.
SPIE, 4008, 534
Dessauges-Zavadsky, M., Calura, F., Prochaska, J. X., D’Odorico, S., &
Matteucci, F. 2004, A&A, 416, 79
D’Odorico, V., Petitjean, P., & Cristiani, S. 2002, A&A, 390, 13
Foltz, C. B., Chaffee, F. H. Jr., & Black, J. H. 1988, ApJ, 324, 267
Ge, J., & Bechtold, J. 1999, in Highly redshifted Radio Lines, ed. C. L. Carilli,
et al., ASP Conf. Ser., 156, 121
Grevesse, N., & Sauval, A. J. 2002, Adv. Space Res., 30, 3
Heinmüller, J., Petitjean, P., Ledoux, C., Caucci, S., & Srianand, R. 2006, A&A,
449, 33
Hirashita, H., & Ferrara, A. 2005, MNRAS, 356, 1529
Kulkarni, V. P., & Fall, S. M. 2002, ApJ, 580, 732
Ledoux, C., Bergeron, J., & Petitjean, P. 2002a, A&A, 385, 802
Ledoux, C., Srianand, R., & Petitjean, P. 2002b, A&A, 392, 781
Ledoux, C., Petitjean, P., & Srianand, R. 2003, MNRAS, 346, 209
Ledoux, C., et al. 2006a, A&A [arXiv:astro-ph/0606185]
Ledoux, C., Petitjean, P., & Srianand, R. 2006b, ApJ, 640, L25
Levshakov, S. A., & Varshalovich, D. A. 1985, MNRAS, 212, 517
Lu, L., et al. 1996, ApJS, 107, 475
Morton, D. C. 2003, ApJS, 149, 205
Morton, D. C., & Dinerstein, H. L. 1976, ApJ, 204, 1
Petitjean, P., Srianand, R., & Ledoux, C. 2000, A&A, 364, L26
Petitjean, P., Srianand, R., & Ledoux, C. 2002, MNRAS, 332, 383
Prochaska, J. X., & Wolfe, A. M. 2001, ApJS, 137, 21
Sargent, W. L. W., Boksenberg, A., & Steidel, C. C. 1988, ApJS, 68, 539
Savage, B. D., Bohlin, R. C., Drake, J. F., & Budich, W. 1977, ApJ, 216, 291
Srianand, R., & Petitjean, P. 2001, A&A, 373, 816
Srianand, R., Petitjean, P., & Ledoux, C. 2000, Nature, 408, 931
Srianand, R., Petitjean, P., Ledoux, C., Ferland, G., & Shaw, G. 2005, MNRAS,
362, 549
Viegas, S. M. 1995, MNRAS, 276, 268


Metallicity as a criterion to select H<SUB>2</SUB>-bearing damped Lyman-&#945;systems

Crossref

Metallicity as a criterion to select H
                    2

Publications of the IAS Fellows

A&A 456, L9–L12 (2006)
DOI: 10.1051/0004-6361:20065769
c© ESO 2006
Astronomy
&Astrophysics
Letter to the Editor
Metallicity as a criterion to select H2-bearing
damped Lyman-α systems
P. Petitjean1,2, C. Ledoux3, P. Noterdaeme3, and R. Srianand4
1 Institut d’Astrophysique de Paris, CNRS – Université Pierre et Marie Curie, 98bis boulevard Arago, 75014 Paris, France
e-mail: petitjean@iap.fr
2 LERMA, Observatoire de Paris, 61 avenue de l’Observatoire, 75014 Paris, France
3 European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Vitacura, Santiago, Chile
4 IUCAA, Post Bag 4, Ganesh Khind, Pune 411 007, India
Received 7 June 2006 / Accepted 16 July 2006
ABSTRACT
Aims. We characterize the importance of metallicity on the presence of molecular hydrogen in damped Lyman-α (DLA) systems.
Methods. We constructed a representative sample of 18 DLA/sub-DLA systems with log N(H i) > 19.5 at high redshift (zabs > 1.8)
and with metallicities relative to solar [X/H] > −1.3 (with [X/H] = log N(X)/N(H)−log (X/H) and X either Zn, S or Si). We gathered
data covering the expected wavelength range of redshifted H2 absorption lines on all systems in the sample from either the literature
(10 DLAs), the UVES-archive, or new VLT-UVES observations for four of them. The sample is large enough to allow us to discuss
for the first time the importance of metallicity as a criterion for the presence of molecular hydrogen in the neutral phase at high-z.
Results. From the new observations, we report two new detections of molecular hydrogen in the systems at zabs = 2.431 toward
Q 2343+125 and at zabs = 2.426 toward Q 2348−011. Q 2348−011, From a comparison of the H2 detection fraction in the high-
metallicity sample with earlier results, we find that the fraction of DLA systems with log f = log 2N(H2)/(2N(H2) + N(H i)) > −4 is
as large as 50% for [X/H] > −0.7, when it is only ∼5% for [X/H] < −1.3 and ∼15% in the overall sample (with −2.5 < [X/H] < −0.3).
This demonstrates that the presence of molecular hydrogen at high redshift is strongly correlated with metallicity.
Key words. galaxies: high-redshift – galaxies quasars: absorption lines – galaxies quasars: individual: Q 2343+125 –
galaxies quasars: individual: Q 2348−011
1. Introduction
Early searches for molecular hydrogen in DLAs, though not
systematic, have led to either low values of or upper limits to
the molecular fraction of the gas. For a long time, only the
DLA at zabs = 2.811 toward Q 0528−250 was known to con-
tain H2 molecules (Levshakov & Varshalovich 1985; Foltz et al.
1988). Ge & Bechtold (1999) searched for H2 in a sample of
eight DLAs using the MMT moderate-resolution spectrograph
(FWHM = 1 Å). Apart from the detection of molecular hy-
drogen at zabs = 1.973 and 2.338 toward Q 0013−004 and
Q 1232+082, respectively, they measured upper limits on f in
the range 10−6−10−4 in the other systems.
A major step forward in understanding the nature of DLAs
through their molecular hydrogen content has recently been
made possible by the unique high-resolution and blue-sensitivity
capabilities of UVES at the VLT. In the course of the first large
and systematic survey for H2 at high redshift, we searched for H2
in DLAs down to a detection limit of typically N(H2) = 2 ×
1014 cm−2 (Petitjean et al. 2000; Ledoux et al. 2003). Out of
the 33 surveyed systems, eight had firm detections of associ-
ated H2 absorption lines. Considering that three detections had
 Based on observations carried out at the European Southern
Observatory (ESO) under prog. ID Nos. 65.O-0158, 65.O-0411,
67.A-0022, 67.A-0146, 69.A-0204, 70.B-0258, 072.A-0346 with
UVES installed at the Very Large Telescope (VLT) on Cerro Paranal,
Chile.
been already known from past searches, H2 was detected in
∼15% of the surveyed systems. The existence of a correlation be-
tween metallicity and depletion factor, measured as [X/Fe] (with
X = Zn, S or Si) was demonstrated (see also Ledoux et al. 2002a)
and the DLA and sub-DLA systems where H2 was detected were
usually among those with the highest metallicities.
However, the high metallicity end of the Ledoux’s sam-
ple was biased by the presence of already known detections.
Therefore, to further investigate this possible dependence with
metallicity and derive the actual molecular content of the high-
redshift gas with highest metallicity, we searched a representa-
tive sample of high metallicity DLAs for H2. The number of
H2 measurements in systems with [X/H]> −1.3 is twice larger in
our sample compared to previous surveys. We describe the sam-
ple and the observations in Sect. 2, present two new detections
of H2 in Sect. 3 and the results of the survey and our conclusions
in Sect. 4.
2. Sample and observations
We have selected from the literature all z > 1.8 DLAs and
sub-DLAs systems (log N(H i) ≥ 19.5) with previously mea-
sured elemental abundances (e.g. Prochaska & Wolfe 2001;
Kulkarni & Fall 2002) larger than [X/H] > −1.3 and accessible
to UVES. The inclusion of sub-DLAs is justified by the fact that
for log N(H i) > 19.5 most of the hydrogen is neutral (Viegas
1995). In addition, Ledoux et al. (2003) have shown that for
Article published by EDP Sciences and available at http://www.edpsciences.org/aa or http://dx.doi.org/10.1051/0004-6361:20065769
L10 P. Petitjean et al.: H2 in DLA systems
Table 1. Metal and molecular content of high-metallicity DLA/sub-DLA systems at zabs > 1.8.
QSO zem zabs log N(H i) 1 [X/H] 1 X log N(H2) 2 log f 3 References
J = 0 J = 1
Q 0013−004 2.09 1.973 20.83 ± 0.05 −0.59 ± 0.05 Zn 17.72–20.00 −1.68+1.07−1.18 a
Q 0112−306 2.99 2.702 20.30 ± 0.10 −0.49 ± 0.11 Si <14.1 <14.0 <−5.55 b
Q 0216+080 2.99 2.293 20.50 ± 0.10 −0.70 ± 0.11 Zn <14.3 <14.5 <−5.39 c
Q 0347−383 3.22 3.025 20.73 ± 0.05 −1.17 ± 0.07 Zn 14.53+0.06−0.06 −5.90+0.11−0.11 b
Q 0405−443 3.02 2.595 21.05 ± 0.10 −1.12 ± 0.10 Zn 18.14+0.07−0.10 −2.61+0.17−0.20 b, d
Q 0458−020 2.29 2.040 21.70 ± 0.10 −1.22 ± 0.10 Zn <14.6 <14.6 <−6.40 e
Q 0528−250 2.77 2.811 21.35 ± 0.07 −0.91 ± 0.07 Zn 18.22+0.11−0.12 −2.83+0.18−0.19 b, d
Q 0551−366 2.32 1.962 20.70 ± 0.08 −0.35 ± 0.08 Zn 17.42+0.45−0.73 −2.98+0.53−0.81 f
Q 1037−270 2.23 2.139 19.70 ± 0.05 −0.31 ± 0.05 Zn <14.0 <14.1 <−5.05 g
Q 1209+093 3.30 2.584 21.40 ± 0.10 −1.01 ± 0.10 Zn <14.9 <15.1 <−5.69 c
Q 1441+276 4.42 4.224 20.95 ± 0.10 −0.63 ± 0.10 S 18.28+0.08−0.07 −2.38+0.18−0.17 h
Q 1444+014 2.21 2.087 20.25 ± 0.07 −0.80 ± 0.09 Zn 18.16+0.14−0.12 −1.80+0.21−0.19 b
Q 2116−358 2.34 1.996 20.10 ± 0.07 −0.34 ± 0.11 Zn <14.5 <14.8 <−4.75 c
Q 2206−199 2.56 1.921 20.67 ± 0.05 −0.54 ± 0.05 Zn <14.4 <14.7 <−5.44 c
Q 2230+025 2.15 1.864 20.90 ± 0.10 −0.81 ± 0.10 S <15.4 <15.4 <−4.80 c
Q 2243−605 3.01 2.331 20.65 ± 0.05 −0.85 ± 0.05 Zn <13.8 <13.9 <−6.15 c
Q 2343+125 2.51 2.431 20.40 ± 0.07 −0.89 ± 0.08 Zn 13.69+0.09−0.09 −6.41+0.16−0.16 c
Q 2348−011 3.01 2.426 20.50 ± 0.10 −0.60 ± 0.11 S 18.45+0.27−0.26 −1.76+0.37−0.36 c
1 Neutral-hydrogen column densities and metallicities relative to Solar, [X/H] ≡ log [N(X)/N(H)] − log [N(X)/N(H)] (with X = Zn as the
reference element when Zn ii is detected, or else either S or Si) are from Ledoux et al. (2006a).
2 Molecular-hydrogen column densities are summed up over all J levels in case of detection and upper limits for J = 0 and J = 1 are given in case
of non-detection.
3 Molecular fraction, f = 2N(H2)/(2N(H2) + N(H i)).
a Petitjean et al. (2002), b Ledoux et al. (2003), c this work, d Srianand et al. (2005), e Heinmüller et al. (2006), f Ledoux et al. (2002b), g Srianand
& Petitjean (2001), h Ledoux et al. (2006b).
log N(H i) > 19.5, there is no correlation between the presence
of H2 and the H i column density. That we include ALL known
systems with these criteria guarantees that the sample is repre-
sentative of the population of DLA-subDLA.
We ended up with a sample of 15 DLAs and 3 sub-
DLAs (with log N(H i) = 19.7, 20.10 and 20.25) as pre-
sented in Table 1. The H2 content of ten of these sys-
tems has already been published (Srianand & Petitjean 2001;
Petitjean et al. 2002; Ledoux et al. 2002b, 2003, 2006b;
Srianand et al. 2005 and Heinmüller et al. 2006). Five of
the eight remaining systems have data in the UVES archive
(Q 1209+093: Prog. 67.A-0146 P.I. Vladilo; Q 2116−358:
Prog. 65.O-0158 P.I. Pettini; Q 2230+025: Prog. 70.B-0258, P.I.
Dessauges-Zavadsky; Q 2243−605: Prog. 65.O-0411 P.I. Lopez;
Q 2343+125: Prog. 69.A-0204 and 67.A-0022, P.I. D’Odorico).
New observations of Q 0216+080, Q 2206−199, Q 2343+125,
and Q 2348−011 have been performed with the Ultraviolet
and Visible Echelle Spectrograph (UVES, Dekker et al. 2000)
mounted on the ESO Kueyen VLT-UT2 8.2 m telescope on Cerro
Paranal in Chile. These new observations resulted in two new
detections described in the next section.
The data for each of the eight QSOs were reduced using the
UVES pipeline, which is available as a package (a so-called
context) of the ESO MIDAS data reduction system (see e.g.
Ledoux et al. 2003 for details). Standard Voigt-profile fitting
methods were used to analyze metal lines and molecular lines,
when detected, to determine column densities using the oscilla-
tor strengths compiled in Ledoux et al. (2003) for metal species
and by Morton & Dinerstein (1976) for H2. We adopted the Solar
abundances from Morton (2003) based on meteoritic data from
Grevesse & Sauval (2002).
The characteristics of the sample are summarized in Table 1.
The H i column densities and most of metallicities are from the
compilation by Ledoux et al. (2006a). Slight differences with
previously published values, e.g. Ledoux et al. (2003), are due
to the use of different damping coefficients for H i. The only
known z > 1.8 H2 bearing DLA system out of this sample is
the system at zabs = 2.337 toward Q 1232+082 (Srianand et al.
2000).
3. Two new detections
We report two new detections of H2 in DLAs from our new ob-
servations. Detail analysis and interpretation of the physical con-
ditions in these two DLAs are beyond the scope of the present
work and will be described in a subsequent paper.
3.1. Q 2343+125, zabs = 2 .431
This DLA system was first studied by Sargent et al. (1988).
High resolution data have been described by Lu et al. (1996),
D’Odorico et al. (2002), and Dessauges-Zavadsky et al. (2004).
The profile of the metal lines is spread over more than
250 km s−1 from zabs = 2.4283 to 2.4313, but the strongest com-
ponent is centered at zabs = 2.43127 corresponding to the red
edge of the above redshift range. From Voigt-profile fitting to the
H i Lyman-α, β, and γ lines, we find that the damped Lyman-α
line is centered at zabs = 2.431, and the column density is
log N(H i) = 20.40 ± 0.07, consistent with previous measure-
ment by D’Odorico et al. (2002; log N(H i) = 20.35 ± 0.05).
We use Zn as the reference species for metallicity measure-
ment and find [Zn/H] = −0.89 ± 0.08. This is consistent with
P. Petitjean et al.: H2 in DLA systems L11
Fig. 1. Velocity plots of observed H2 absorption lines from the J = 0 and
J = 1 rotational levels at zabs = 2.43127 toward Q 2343+125. Profiles of
associated Si ii and Fe ii absorptions are shown in the upper panels. The
best-fitting model to the system is overplotted; the location of Voigt-
profile sub-components are shown as vertical dashed lines.
previous findings. Absorption from the J = 1 and probably from
the J = 0 rotational levels of H2 is detected in this system at
zabs = 2.43127 (see Fig. 1). The optically-thin H2 absorption
lines are very weak, i.e. close to, but above, the 3σ detection
limit. A very careful normalization of the spectrum was per-
formed by adjusting the continuum while fitting the lines. The
best-fitting consistent model for H2 is shown in Fig. 1. The to-
tal H2 column density, integrated over the J = 0 and 1 levels,
is estimated to be log N(H2) = 13.69 ± 0.09 (12.97 ± 0.04 and
13.60± 0.10 for J = 0 and 1, respectively). This leads to the low-
est molecular fraction observed up to now, log f = −6.41+0.16−0.16.
We also derive an upper limit on the detection of absorption from
the J = 2 level, log N(H2–J = 2) < 13.1 at the 3σ level.
3.2. Q 2348–011, zabs = 2 .426
There are two DLA systems at zabs = 2.426 and zabs = 2.615 to-
ward Q 2348−011, with total neutral hydrogen column densities
of log N(H i) = 20.50 ± 0.10 and log N(H i) = 21.30 ± 0.08, re-
spectively. Conspicuous H2 absorptions are detected in the zabs 
2.426 DLA system, the only system to be considered here as dis-
playing a high metallicity (see Fig. 2). The molecular lines are
very numerous and strong, but the spectral resolution of our data
is high enough to allow unambiguous detection and accurate de-
termination of the line parameters. Seven H2 components spread
over about 300 km s−1 were used for the H2 fit. It is interest-
ing to note that the strongest metal component (at V = 0 km s−1
in Fig. 2) has no associated H2 absorption. Strong H2 absorp-
tion is seen at V ∼ −150 and +50 km s−1. All seven molecu-
lar components have associated C i absorption. However, ad-
ditional components are needed to fit the metal absorption
Fig. 2. Absorption profiles at zabs = 2.4263 (taken as zero of the velocity
scale) toward Q 2348−011. The positions of the seven (respectively, 13)
components needed to fit the H2 (respectively, metal line) profiles are
indicated by vertical dashed lines. The resulting best-fitting model to
the system is overplotted.
lines: 9 components for C i and 13 components for the singly
ionized species. H2 absorption from the rotational levels J = 0
to 5 are unambiguously detected. The total H2 column density
integrated over all rotational levels is log N(H2) = 18.45+0.27−0.26,
corresponding to a molecular fraction log f = −1.76+0.37−0.36.
We also derive an upper limit on the column density of
HD molecules, leading to log N(HD)/N(H2) < −3.3.
4. Metallicity as a criterion for the presence
of molecular hydrogen
From Table 1, it can be seen that H2 is detected in nine high-
metallicity systems out of 18. However, for two of the detections,
the corresponding value of log f is lower than most of the up-
per limits derived for other systems. All upper limits are lower
than −4.5 and all detections higher than these upper limits are
higher than −3. We therefore define a system with large (respec-
tively small) H2 content if log f is higher (respectively lower)
than −4.
In Fig. 3 we plot the molecular fraction, log f , versus
metallicity, [X/H], for our representative sample of DLAs with
[X/H] > −1.3 (18 measurements summarized in Table 1) and
measurements by Ledoux et al. (2003) for [X/H] < −1.3
(23 measurements). The log f distribution is bimodal with an ap-
parent gap in the range −5 < log f < −3.5 justifying the above
classification of systems. Note that this jump in log f was al-
ready noticed by Ledoux et al. (2003) and is similar to what is
seen in our Galaxy (Savage et al. 1977; see also Srianand et al.
2005). It is apparent that the fraction of systems with molecular
fraction log f > −4 increases with increasing metallicity. It is
only ∼5% for [X/H] < −1.3 when it is ∼39% for [X/H] > −1.3.
This fraction is even higher, 50%, for [X/H] > −0.7 which is the
median metallicity for systems with [X/H] > −1.3. In addition,
all systems with [X/H] < −1.5 have log f < −4.5.
We conclude that metallicity is an important criterion for
the presence of molecular hydrogen in DLAs. This may not
be surprising as the correlation between metallicity, and the
depletion of metals onto dust grains (Ledoux et al. 2003)
implies that higher metallicity means higher dust content and
therefore higher H2 formation rate. In addition, the presence of
dust implies a greater absorption of UV photons that usually
dissociate the molecule. More generally, Ledoux et al. (2006a)
L12 P. Petitjean et al.: H2 in DLA systems
-2.5 -2.0 -1.5 -1.0 -0.5 0.0
[X/H]
-8
-6
-4
-2
0
lo
g 
f
Fig. 3. Logarithm of the molecular fraction, f = 2N(H2)/(2N(H2) +
N(H i)), versus metallicities, [X/H] = log N(X)/N(H)−log (X/H) with
X either Zn, S or Si, in DLAs from the sample described in this pa-
per ([X/H] > −1.3, see Table 1) and the sample of Ledoux et al. (2003,
[X/H] < −1.3). Filled squares indicate systems in which H2 is detected.
Dashed lines indicate the limits used in the text (log f = −4; [X/H] =
−1.3). The dotted line shows the median of the high-metallicity sample
([X/H] = −0.7).
have shown that a correlation exists between metallicity and
velocity width in DLAs. If the latter kinematic parameter is inter-
preted as reflecting the mass of the dark-matter halo associated
with the absorbing object, then DLAs with higher metallicity
are associated with objects of higher mass where star formation
could be enhanced. All this makes it arguable that, in DLAs,
star-formation activity is probably correlated with the molecular
fraction (Hirashita & Ferrara 2005). It is therefore of prime im-
portance to survey a large number of DLA systems to define their
molecular content better and use this information to derive the
physical properties of the gas and the amount of star-formation
occurring in the associated objects.
Acknowledgements. P.P.J. thanks ESO for an invitation to stay at the ESO head-
quarters in Chile where part of this work was completed. R.S. and P.P.J. grate-
fully acknowledge support from the Indo-French Centre for the Promotion of
Advanced Research (Centre Franco-Indien pour la Promotion de la Recherche
Avancée) under contract 3004-3. P.N. is supported by an ESO student fellowship.
References
Dekker, H., D’Odorico, S., Kaufer, A., Delabre, B., & Kotzlowski, H. 2000, Proc.
SPIE, 4008, 534
Dessauges-Zavadsky, M., Calura, F., Prochaska, J. X., D’Odorico, S., &
Matteucci, F. 2004, A&A, 416, 79
D’Odorico, V., Petitjean, P., & Cristiani, S. 2002, A&A, 390, 13
Foltz, C. B., Chaffee, F. H. Jr., & Black, J. H. 1988, ApJ, 324, 267
Ge, J., & Bechtold, J. 1999, in Highly redshifted Radio Lines, ed. C. L. Carilli,
et al., ASP Conf. Ser., 156, 121
Grevesse, N., & Sauval, A. J. 2002, Adv. Space Res., 30, 3
Heinmüller, J., Petitjean, P., Ledoux, C., Caucci, S., & Srianand, R. 2006, A&A,
449, 33
Hirashita, H., & Ferrara, A. 2005, MNRAS, 356, 1529
Kulkarni, V. P., & Fall, S. M. 2002, ApJ, 580, 732
Ledoux, C., Bergeron, J., & Petitjean, P. 2002a, A&A, 385, 802
Ledoux, C., Srianand, R., & Petitjean, P. 2002b, A&A, 392, 781
Ledoux, C., Petitjean, P., & Srianand, R. 2003, MNRAS, 346, 209
Ledoux, C., et al. 2006a, A&A [arXiv:astro-ph/0606185]
Ledoux, C., Petitjean, P., & Srianand, R. 2006b, ApJ, 640, L25
Levshakov, S. A., & Varshalovich, D. A. 1985, MNRAS, 212, 517
Lu, L., et al. 1996, ApJS, 107, 475
Morton, D. C. 2003, ApJS, 149, 205
Morton, D. C., & Dinerstein, H. L. 1976, ApJ, 204, 1
Petitjean, P., Srianand, R., & Ledoux, C. 2000, A&A, 364, L26
Petitjean, P., Srianand, R., & Ledoux, C. 2002, MNRAS, 332, 383
Prochaska, J. X., & Wolfe, A. M. 2001, ApJS, 137, 21
Sargent, W. L. W., Boksenberg, A., & Steidel, C. C. 1988, ApJS, 68, 539
Savage, B. D., Bohlin, R. C., Drake, J. F., & Budich, W. 1977, ApJ, 216, 291
Srianand, R., & Petitjean, P. 2001, A&A, 373, 816
Srianand, R., Petitjean, P., & Ledoux, C. 2000, Nature, 408, 931
Srianand, R., Petitjean, P., Ledoux, C., Ferland, G., & Shaw, G. 2005, MNRAS,
362, 549
Viegas, S. M. 1995, MNRAS, 276, 268


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Metallicity as a criterion to select H2 bearing Damped Lyman-alpha
  systems

Metallicity as a criterion to select H2 bearing Damped Lyman-alpha systems

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