A&A 368, L21-L24 (2001)
DOI: 10.1051/0004-6361:20010183
S. D'Odorico 1 - M. Dessauges-Zavadsky1,2
- P. Molaro 3
1 -
European Southern Observatory, Karl-Schwarzschildstr. 2, 85748 Garching bei München, Germany
2 -
Observatoire de Genève, 1290 Sauverny, Switzerland
3 -
Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11, 34131 Trieste, Italy
Received 20 November 2000 / Accepted 1 February 2001
Abstract
We present the first D/H measurement in a damped Ly
(hereafter DLA) system at
towards QSO 0347-3819 obtained from the UVES-VLT spectra. The DLA absorber has a
metallicity of [Zn/H]
and a relatively simple velocity structure, with two
dominating components detected in several metal lines. The hydrogen Lyman series can be followed
down to Ly12 thanks to the high UV-Blue efficiency of UVES. The best fit of the Lyman series lines,
and in particular of Ly
,
Ly8, Ly10 and Ly12, relatively free of local contamination, is
obtained when the D I absorption is included in the two main components. The measured
deuterium column density yields
close to other
low D/H values from Lyman limit systems. The corresponding values for the baryon to photon ratio
and the baryon density derived from D/H are
and
respectively.
Key words: cosmology: observations - quasars: QSO 0347-3819 - nuclear reactions, nucleosynthesis, abundances
Deuterium (D or )
is the only element entirely produced by nuclear reactions in the first
minutes after Big Bang (Wagoner et al. 1967). The D yields are the most sensitive to the nuclear
density at the nucleosynthesis epoch among the primordial light elements 3He, 4He and 7Li,
thus making the D abundance the most sensitive measurement of the baryon density in the universe
(Wagoner 1973; Schramm & Turner 1998).
Deuterium is currently measured in the local interstellar medium (ISM), (D/H)
(Linsky et al. 1993), but since whenever it is cycled through stars it is
completely burned away, extrapolation to the primordial D/H value requires a modeling of the
Galactic chemical evolution. Direct D measurements of primordial material are thus of
high interest. Adams (1976) suggested that almost primordial D could be measured in low metallicity absorption line systems in the spectra of distant quasars (QSOs). This was recently achieved for a few systems, but with conflicting results differing by almost an order of magnitude.
A few systems provide high D/H values (see e.g. Webb et al. 1997), who measure
), while two other systems give a low abundance at
(Burles & Tytler 1998a, 1998b). An even lower D/H estimation was
obtained by Molaro et al. (1999), further discussed by Levshakov et al. (2000). Kirkman et al. (2000) measured an upper limit of
6.76 10-5. The handful of D detections found so far does not allow a firm conclusion. Different arguments favour a low primordial
D/H ratio: the possible H I contamination of the
D I absorption lines and, on the modeling side, the results by
Tosi et al. (1998) which predict for a variety of chemical
evolution scenarios and to be consistent with the Galactic data a
maximum decrease of the primordial D abundance by a factor of 3.
The paucity of suitable absorption systems for accurate D/H measurements is due to the fact that
only absorption line systems with simple velocity structures and with intermediate H I column densities allow the detection of the D I lines. At too low column densities the D I lines are too weak for detection, whereas at high column densities the lines are normally
washed out by the saturation of the H I line. We show here for the first time that in the
latter case the deuterium signature can be successfully detected through the higher members of the
Lyman series, when the target is a damped Ly
system (
(H I)
)
at high
redshift. This approach was first suggested by Khersonsky et al. (1995).
![]() |
Figure 1:
Absorption metal line profiles plotted against velocity for the DLA system at
![]() ![]() ![]() |
Open with DEXTER |
The spectra of QSO 0347-3819 (V=17.3,
)
were obtained during the Commissioning of the Ultraviolet-Visual Echelle Spectrograph (UVES) on the VLT 8.2 m Kueyen telescope at Paranal, in 1999. The instrument is described in D'Odorico et al. (2000). Two exposures of 4500 s each, covering the spectral range from 3650 to 4900 Å and from 6700 to 10000 Å were obtained with a resolution of
6.9 km s-1 and
5.7 kms-1 respectively. The individual spectra were reduced using the UVES data reduction pipeline implemented in the ESO MIDAS package. The final spectrum reaches a S/N varying from 20 to 35.
QSO 0347-3819 shows a damped Ly
system at
which has been studied in
detail by Centurion et al. (1998), Ledoux et al. (1998) and from HIRES-Keck observations by
Prochaska & Wolfe (1999). The UVES observations however provide the first high-quality data in
the UV (
Å) and in the near-IR (
Å). They allow the abundance measurements
of new features such as O, P, Ar and Zn, in addition to the N, S, Si and Fe abundances measured in the
previous studies. Full abundance analysis of the DLA system at
will be
presented in a future paper. Here we focus on the deuterium detection and the D/H ratio measurement.
Comp |
![]() |
Ident | ![]() |
b |
[cm-2] | [km s-1] | |||
2... | 3.024637 | H I | 20.13 ![]() |
21.5 ![]() |
D I | 15.48 ![]() |
14.1 ![]() |
||
3... | 3.024856 | H I | 20.43 ![]() |
23.0 ![]() |
D I | 15.78 ![]() |
16.2 ![]() |
||
7... | 3.025659 | H I | 19.35 ![]() |
14.7 ![]() |
The absorption profiles of the DLA system at
are characterized by two
dominating components (2 and 3) separated by about 20 km s-1 with the red one slightly
stronger than the blue as it can be seen from the non-saturated metal lines in Fig. 1.
The strong and saturated metal lines reveal that additional material in smaller amount is present
redwards the two main features (components 4, 5, 6 and 7). The region on the blue side of the main
absorption components is sharp and relatively free from material with only one weak component
(component 1) at about -30 kms-1. In total seven components are needed to fit the metal lines
absorption profiles, with two main components containing about 80% of the total column density per
transition.
![]() |
Figure 2:
Damped Ly![]() |
Open with DEXTER |
![]() |
Figure 3:
Ly![]() |
Open with DEXTER |
The DLA system at
is a very good candidate for the deuterium analysis since it shows a relatively simple velocity structure dominated by two strong components, a low metallicity of [Zn/H]
,
indicating that the measured D/H will be representative of primordial D/H and the hydrogen Lyman series lines can be followed down to Ly12.
We used a
minimization routine fitlyman (Fontana & Ballester 1995) in MIDAS
to fit Voigt profiles to the observed absorption profiles, and obtain for each fitted absorption
component the wavelength, the column density N, the Doppler parameter b and the corresponding
errors.
In the case of high hydrogen column densities like in DLA systems, we expect the neutral and low ionization metal lines to trace the H, therefore we model the Lyman series (Ly
to Ly12)
absorption profiles with three features corresponding to the two main components (2 and 3) and the
reddest component (7) as determined from the metal lines (Fig. 1). The contribution of
the weaker component 7 is required only to better constrain the fit on the red edge of the Lyman
lines. The relative intensities of these three major hydrogen components were scaled with the metal
lines assuming they have approximatively the same abundance ratios from component to component.
![]() |
Figure 4: Same as Fig. 3. The fit in this case is performed by considering the two main components for D and the three components for H (see text for details) |
Open with DEXTER |
Starting from this basis we obtained the final H I column densities and b-values (free
parameters) by fitting simultaneously the lines of the Lyman series and by assuming the
same redshift for the three H components as for the metal lines. The Ly
absorption profile
provides a very good constraint to the total H I column density and the Ly8, Ly9, Ly10 and
Ly12 profiles (which are the ones free from strong contamination) to the b-values. From the best
fit (Figs. 2 and 3, Table 1) we derived a total H I column density
of
(H I)
in close agreement with the Pettini et al. (1994) value of
.
Adding other components to the hydrogen fit with low column densities as the ones observed in the
stronger metal lines, component 1 at -30 km s-1 and components 4, 5 and 6 at slightly higher
redshifts than the two main components, does not change significantly neither the total H I
column density of the two main components nor the fit on the blue and red wings of the Lyman
lines. Figure 3 however clearly shows that the fit with three hydrogen components
systematically fails to reproduce the edge of the blue wing in the higher members of the hydrogen
Lyman series, Ly,
Ly8, Ly10 and Ly12, at about -82 kms-1 from the two H main
components (2 and 3) which is the expected displacement of the corresponding D lines.
We then add to the model the deuterium by assuming its contribution only to the two main components (2 and 3). In the fitting procedure we assumed the same redshift for the two fitted components as for H and we left the column densities and the broadening parameters b as free parameters.
The best fit performed over D I Ly8 and Ly10 improves the
of the hydrogen fit by a
factor of 3. It reproduces nicely the blue wing of the Ly8, Ly10 and Ly12 absorption profiles
and is consistent with Ly
(see Fig. 4). It gives a deuterium abundance of
To check the stability of our result when the assumption that H and D mimic the metal structure is relaxed, we made a fit of the Ly,
Ly
,
Ly8, Ly10 and Ly12 lines with three H and D components by keeping both their redshifts and column densities as free parameters. We obtained a satisfactory fit with shifts of the components with respect to the values given in Table 1 of less than 4 kms-1. The derived D/H ratio was comprised within our D/H error range. A fit with only one strong H component and the corresponding D component gave also a D/H ratio within our D/H error range.
Three arguments support the interpretation of the absorption feature detected on the blue wing of
some of the higher members of the H I Lyman series, Ly,
Ly8, Ly10 and Ly12, as
the D I absorption with the same two main components as detected in the metal lines of the
DLA system.
First, the absorption in the blue wing of the Lyman series lines could also be explained by a
hydrogen interloper associated with the damped Ly
system with a column density between
1015.7 and 1016.2, a b-value between 15 and 25 km s-1 and placed between
-80 and -100 km s-1. However, from the density distribution of the Ly
clouds in the
forest at
(see e.g. Kim et al. 2000), the probability to have such a cloud at that
position is smaller than 1/1000. The lack of any metal component at this velocity in the strong and
saturated lines of the DLA system (see Fig. 1) provides an additional evidence for
discarding this possibility.
Secondly, the contamination on the blue wing of the Lyman series lines by different H I
interlopers at different redshifts which would mimic the same deuterium abundance as derived from
the D I Ly,
Ly8, Ly10 and Ly12 lines is even more unlikely.
Finally, the D and H column densities and broadening parameters b resulting from the fits are consistent one relatively to other in the two fitted components (2 and 3): the derived D/H ratios are the same and the b(D I)/b(H I) ratios are close to what is expected in the thermally dominated case.
This measurement of D is the first made in a DLA system. It shows that the DLA systems with their low metallicity ISM are a very promising class of absorbers for measurements of the D/H ratios at high redshifts, when it is possible to measure the higher members of the Lyman series. A systematic program of measurements using UVES data is under way.
The derived D/H ratio of
is close to the low values obtained by
Burles & Tytler in Lyman limit systems (1998a, 1998b) and it makes the claim of the primordial low
D/H ratio more robust. Taken at face this ratio gives a baryon to photon ratio,
,
of
6.3 10-10 (Burles et al. 2000). This
implies a helium abundance (in mass) of
and a lithium abundance of
which are both larger than presently allowed by observations of He in
extragalactic H II regions and of Li in halo stars (Izotov & Thuan 1998; Bonifacio &
Molaro 1997). On the other hand an
of
6.3 10-10 corresponds to a
present-day baryon density of
which remains significantly lower than the
derived from CMB
anisotropy (Jaffe et al. 2000).
Acknowledgements
We are indebted to the UVES project team for the high quality of the spectra obtained early in the operation of the instrument. We like to thank J. X. Prochaska for making available the profiles of the stronger metal lines and for comments on an earlier version of the manuscript.