A&A 482, L39-L42 (2008)
DOI: 10.1051/0004-6361:200809727
LETTER TO THE EDITOR
R. Srianand1 - P. Noterdaeme2,3 - C. Ledoux2 - P. Petitjean3
1 - IUCAA, Post Bag 4, Ganeshkhind, Pune 411 007, India
2 -
European Southern Observatory, Alonso de Córdova
3107, Casilla 19001, Vitacura, Santiago 19, Chile
3 -
UPMC Paris 06, Institut d'Astrophysique de Paris,
UMR7095 CNRS, 98bis Boulevard Arago, 75014 Paris, France
Received 6 March 2008 / Accepted 20 March 2008
Abstract
We present the first detection of carbon monoxide (CO) in a damped
Lyman-
system (DLA) at
= 2.41837 toward SDSS
J143912.04+111740.5. We also detected H2 and HD
molecules. The measured total column densities (in log units) of
H I, H2, and CO are
,
,
and
,
respectively. The molecular fraction, f = 2N(H2)/(N(H I)+2N(H
2)) = 0.27+0.10-0.08, is the
highest among all known DLAs. The abundances relative to solar of
S, Zn, Si, and Fe are
,
,
,
and
,
respectively,
indicating a high metal enrichment and a depletion pattern onto dust-grains
similar to the cold ISM of our Galaxy. The measured N(CO)/N(H
is much less than the
conventional CO/H2 ratio used to convert the CO emission into
gaseous mass but is consistent with what is measured along
translucent sightlines in the Galaxy. The CO rotational excitation
temperatures are higher than those measured in our Galactic ISM for
similar kinetic temperature and density. Using the C I fine
structure absorption lines, we show that this is a consequence of the
excitation being dominated by radiative pumping by the cosmic
microwave background radiation (CMBR). From the CO excitation
temperatures, we derive
K, while
K is expected from the hot big-bang theory. This is
the most precise high-redshift measurement of
and the
first confirmation of the theory using molecular transitions at high
redshift.
Key words: galaxies: abundances - galaxies: quasars: absorption lines
Damped Lyman-
systems (DLAs) in QSO spectra are characterized by very
high H I column densities, N(H I
cm-2.
The inferred metallicities relative to solar vary between
[Zn/H]= -2.0 and 0 for
(e.g. Pettini et al. 1997; Prochaska & Wolfe 2002).
Therefore DLAs are believed to be located in the close vicinity
of star-forming regions.
The dust content in a typical DLA is less than or
equal to 10% of what is seen in the Galactic
ISM for similar N(H I), however
sufficient for favoring the formation of H2 (Ledoux et al. 2003).
The abundance of H2, the relative populations of the H2 rotational levels, and the fine-structure levels of the C I ground-state are used to derive the physical conditions in the gas, such as temperature, gas pressure, and ambient radiation field (Savage et al. 1977; Black & van Dishoeck 1987; Jenkins & Tripp 2000; Tumlinson et al. 2002). These conditions are believed to be driven by the injection of energy and momentum through various dynamical and radiative processes associated with star formation activity. Thus, detecting H2 in DLAs at high redshifts is an important step forward in understanding the evolution of normal galaxies. Detecting other molecules would pioneer interstellar chemistry studies at high redshift (see e.g. Wiklind & Combes 1995, for the dense ISM component).
In the course of our recently completed Very Large Telescope
survey for H2 in DLAs,
we gathered a sample of 13 H2 absorption systems at
1.8<z<4.3 out of a total of 77 DLAs (Ledoux et al. 2003, 2006;
Petitjean et al. 2006; Noterdaeme et al. 2008). Absorption lines of
HD are detected in one of the DLAs (Varshalovich
et al. 2001) and none show detectable CO absorption.
We noticed a strong preference for H2-bearing DLAs being associated
with C I absorption (Srianand et al. 2005) and having high
metallicities and large
depletion factors (Petitjean et al. 2006; Noterdaeme
et al. 2008).
In the Sloan Digital Sky Survey data base,
we identified a most promising candidate at
towards SDSS J143912.04+111740.5
showing such characteristics.
We were allocated
8 h of director discretionary time
on the Ultraviolet and Visual Echelle Spectrograph (UVES)
at the VLT of the
European Southern Observatory (ESO)
to search for CO in addition to H2.
This observation resulted in the detection of
CO UV absorption lines that have been elusive for more than a quarter
century
(Varshalovich & Levshakov 1981; Srianand & Petitjean 1998;
Cui et al. 2005).
We also detect H2 and HD absorption lines.
In this letter we focus our attention on the
CO excitation. A detailed analysis of the HD absorptions will be
presented elsewhere.
Both UVES spectrographic arms were used with standard dichroic settings and
central wavelengths of 390 nm and 580 nm (or 610 nm) for the blue and red
arms, respectively. The resulting wavelength coverage was 330-710 nm with a
small gap between 452 and 478 nm. The CCD pixels were binned
and the
slit width adjusted to
,
yielding a resolving power
of R=45 000 under seeing conditions of
.
The total exposure
time on source exceeded 8 h.
The data were reduced using the UVES pipeline version 3.3.1 based on the
ESO common pipeline library system (Møller Larsen et al. 2007). Wavelengths
were rebinned to the heliocentric rest frame and individual scientific
exposures co-added. In the following, we adopt the solar reference
abundances from Morton (2003).
![]() |
Figure 1:
A sample of molecular and heavy element absorption lines associated
with the damped Lyman-![]() ![]() ![]() ![]() |
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An asymmetric Lyman-
absorption line, together with
the corresponding Lyman-
and
absorption lines,
indicates the presence of multiple components.
The simultaneous fit to the three H I absorption features gives log N(H I)
(cm-2) = 17.8, 19.25, 19.20,
19.40, and
at the velocities of v =
-892, -594, -382, -117, and 0 km s-1 relative to
.
The component with maximum H I column density
(at v = 0 km s-1) also exhibits H2, HD, and CO absorption lines
spread over
50 km s-1 (see Fig. 1).
Transitions from the J=3 H2 rotational level are detected
in six distinct components. Both HD and CO are detected in
three and two of these components, respectively. Transitions
from J=0 and 1 H2 rotational levels are highly saturated,
but accurate integrated column densities can be derived from
damped wings.
Absorption lines from N I, O I, C I, C I*,
C I**, Mg I, Ar I, S I, S II, Si II,
Fe II, Zn II, Al II, and Ni II are seen spread over
up to 950 km s-1. The total gaseous abundances
relative to solar are
,
,
,
and
for S, Zn, Si, and Fe, respectively.
The abundances of S and Zn are consistent
with the gas abundance being close to solar, while the relative
depletions of Si
and Fe are similar to those in cold gas in the diffuse
Galactic ISM.
Absorption lines from the three C I ground-state fine-structure levels
are detected in numerous transitions in the main H I component.
In Fig. 1 we show a few transitions from H2
(J=0, 1 and 3), S I,
C I, HD and CO. By fitting the damping wings of J=0 and 1 transitions, we measured log N(H2,
and
log N(H2,
.
We derived an excitation temperature of
T01 = 105+42-32 K,
which is usually a good indicator of the average kinetic temperature of the gas.
The mean molecular fraction of the gas is
f = 2N(H2)/[2N(H2)+N(H I)
] = 0.27+0.10-0.08.
This is the highest
value measured to date in a high-z DLA.
![]() |
Figure 2:
Voigt profile fits to 12[]CO A-X bands detected at
![]() ![]() |
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Carbon monoxide absorption
was detected in several bands (see Fig. 2).
We used the four bands that are redshifted outside the
Lyman-
forest to derive CO column densities.
Absorptions from different CO rotational levels are located
close to each other, but the resolution of the data is high
enough to deblend them.
A single-component Voigt profile fit gives
for the main CO component. This is coincident within error (<1 km s-1)
with the strongest S I component (see Fig. 2).
A weaker S I satellite component is present at
.
We therefore added a second CO component with a redshift
fixed to the value of the second S I component.
Doppler parameters were left free to vary and the best fit was obtained for b = 1.5 km s-1, but
the column density value in the main component is not very sensitive to the exact value for b>0.5 km s-1.
Results are shown in Fig. 2. Column densities are
log N(CO
,
13.
,
and
,
respectively, for J=0, 1 and 2
in the main component. Column densities for J=0 and 1 are
and
respectively for the second component when b = 1.5 km s-1.
We measured N(CO)/N(H
.
This is similar to or slightly higher than what is
measured along the
Galactic sightlines with similar molecular fraction and
along Galactic sightlines with similar N(H2)
(see Figs. 4 and 5 of Burgh et al. 2007).
This is much less than the CO/H2 ratio of about 10-4
derived for dense molecular clouds (e.g. Lacy et al. 1994).
This strongly suggests that the physical conditions in the gas
are similar to those in the diffuse Galactic ISM.
![]() |
Figure 3:
The CO excitation diagram.
A straight line with slope 1/(
![]() ![]() ![]() ![]() ![]() ![]() |
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The excitation temperatures derived from the population ratios of the different
rotational levels are
K,
K, and
K for the
main component where the errors come from the fitting uncertainties.
Additional rms deviations come from uncertainties in the continuum
placement and from the allowed range for the Doppler parameter of the
second component. We estimate these to be
0.21,
0.37, and
0.18 K around the three excitation temperatures.
The populations of the three rotational levels are thus consistent with
a single excitation temperature,
K (see Fig. 3),
suggesting that a single mechanism controls the level populations.
This excitation temperature is more than twice higher than the excitation
temperature measured in the diffuse Galactic ISM (see Fig. 4),
at similar
H2 T01 (Burgh et al. 2007). Since we have seen that the physical
conditions in the DLA are similar than in the diffuse Galactic ISM,
we can suspect
that the process responsible for this high excitation temperature
is specific to the high redshift of the system.
Interestingly, the population ratios in the second component are also consistent
with this value of
,
albeit with large errors.
![]() |
Figure 4: Comparison of rotational excitation temperatures of CO and H2. Filled circles are for the measurements from Galactic diffuse ISM (Burgh et al. 2007). The filled triangle is for our measurement at z = 2.4183towards SDSS J143912.04+111740.5. |
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The presence of absorptions from the three fine-structure levels of the
C I ground state allows us to probe the physical state
of the gas further (Srianand et al. 2000).
By simultaneous Voigt profile fitting of the absorption lines, we derived
log N(C I
,
log N(C I
,
and log N(C I
for the
velocity component that corresponds to the strongest CO component.
Assuming no contribution from the CMBR, we obtain a conservative
hydrogen density range
of 87-135 cm-3 and 52-84 cm-3from the population ratios N(C I*)/N(C I)
and N(C I**)/N(C I),
respectively.
A more realistic range, 45-62 cm-3, is obtained if we use appropriate
excitation by the CMBR with a temperature expected from the hot big-bang
theory.
From the observed f value, we derived a
conservative (<25 cm-3) and a realistic
(<12 cm-3) value for the H2 density
(
.
The above values are strict upper limits, because we have ignored the
contribution of UV photons from stars in this galaxy
to the C I excitation.
We ran the statistical equilibrium radiative transfer code RADEX,
available on line (van der Tak et al. 2007), and found that
for the kinetic temperature, T = 105 K, derived from H2,
the collisional contributions to T01 and T12 are 5%
and
for
cm-3.
The corresponding values are
3% and
1%
for
cm-3. Thus the collisional excitation
of CO by H2 is negligible. Collisions with H contribute little
to the CO excitation compared to collisions with H2 in astrophysical conditions
(Green & Thaddeus 1976; Shepler et al. 2007).
This means that the CO excitation is dominated by
CMBR, so we conclude that
K.
![]() |
Figure 5:
Measurements of T(CMBR) at various z.
The star with errorbars is for the measurement based on
CO presented here. Our earlier measurement using
fine-structure lines of neutral carbon,
![]() ![]() ![]() |
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The CMBR is an important source of excitation for those species
with transitions in the sub-millimeter range. This is the
case for atomic species whose ground state splits into
several fine-structure levels and of molecules that can
be excited in their rotational levels. If the relative
level populations are thermalized by the CMBR, then the
excitation temperature gives the temperature of the black-body
radiation. It has long been proposed to measure the relative
populations of such atomic levels in quasar absorption lines
to derive
at high redshift (Bahcall & Wolf 1968).
In Fig. 5 we combine our precise measurement of
,
the 51 new upper limits obtained using C I
and C I* absorption lines detected towards QSOs in
our UVES sample (Srianand et al. 2005; Noterdaeme et al.
2007a,b; Ledoux et al. 2006), and measurements
reported from the literature
(Meyer et al. 1986; Songaila et al.
1994; Lu et al. 1996; Ge et al. 1997; Roth & Bauer 1999; Molaro
et al. 2002; Cui et al. 2005).
Upper limits are obtained assuming CMBR as the only source of excitation.
Our precise measurements using CO and the new upper limits using C I
are consistent with the adiabatic evolution of
expected
in the standard big-bang model (Fig. 5).
The CN molecule has proven to be a remarkable thermometer of the CMBR in
our Galaxy.
It has been used for precise measurement of
in different
directions (Meyer et al. 1985; Kaiser et al. 1990).
Wiklind & Combes (1996) obtained
K at z=0.885using the absorption lines of CS, H13CO+, and N2H+.
Carbon monoxide in diffuse gas
provides an interesting possibility for measurements at high redshift,
as the rotational energies between different rotational
levels are close to
at
.
Following careful selection of the target, based on intensive observations at ESO-VLT, we have made the first detection of carbon monoxyde molecules in the diffuse ISM at high redshift. The analysis presented here pioneers interstellar chemistry studies at high redshift and demonstrates that, together with the detection of other molecules such as HD or CH, it will be possible to tackle important cosmological issues.
Acknowledgements
We warmly thank the Director Discretionary Time allocation committee and the ESO Director General, Catherine Cesarsky, for allowing us to carry out these observations. R.S. and P.P.J. gratefully acknowledge support from the Indo-French Center for the Promotion of Advanced Research (Centre Franco-Indien pour la Promotion de la Recherche Avancée) under contract No. 3004-3. P.N. is supported by an ESO PhD studentship.