A carbon‐enhanced metal‐poor damped Lyα system: probing gas from Population III nucleosynthesis?★

Abstract

We present high-resolution observations of an extremely metal-poor damped Lyα system (DLA), at zabs= 2.340 0972 in the spectrum of the QSO J0035−0918, exhibiting an abundance pattern consistent with model predictions for the supernova yields of Population III stars. Specifically, this DLA has [Fe/H]≃−3, shows a clear ‘odd–even’ effect, and is C-rich with [C/Fe]=+1.53, a factor of ∼20 greater than reported in any other DLA. In analogy to the carbon-enhanced metal-poor stars in the Galactic halo (with [C/Fe] > +1.0), this is the first known case of a carbon-enhanced DLA. We determine an upper limit to the mass of 12C, M(12C) ≤ 200 M⊙, which depends on the unknown gas density n(H); if n(H) > 1 cm−3 (which is quite likely for this DLA given its low velocity dispersion), then M(12C) ≤ 2 M⊙, consistent with pollution by only a few prior supernovae. We speculate that DLAs such as the one discovered here may represent the ‘missing link’ between the yields of Population III stars and their later incorporation in the class of carbon-enhanced metal-poor stars which show no enhancement of neutron-capture elements (CEMP-no stars).

Full text

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arXiv:1011.0733v2 [astro-ph.CO] 15 Dec 2010
Mon. Not. R. Astron. Soc. 000, 1–13 (2010)Printed 16 December 2010(MN LATEX style file v2.2)
A carbon-enhanced metal-poor damped Lyα system:
Probing gas from Population III nucleosynthesis?⋆
Ryan Cooke1†, Max Pettini1, Charles C. Steidel2, Gwen C. Rudie2,
and Regina A. Jorgenson1
1Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA
2California Institute of Technology, MS 249-17, Pasadena, CA 91125, USA
Accepted . Received ; in original form
ABSTRACT
We present high resolution observations of an extremely metal-poor damped Lyα
system, at zabs = 2.3400972 in the spectrum of the QSO J0035−0918, exhibiting
an abundance pattern consistent with model predictions for the supernova yields of
Population III stars. Specifically, this DLA has [Fe/H]≃ −3, shows a clear ‘odd-even’
effect, and is C-rich with [C/Fe] = +1.53, a factor of ∼ 20 greater than reported in any
other damped Lyα system. In analogy to the carbon-enhanced metal-poor stars in the
Galactic halo (with [C/Fe] > +1.0), this is the first reported case of a carbon-enhanced
damped Lyα system. We determine an upper limit to the mass of12C, M(12C)?
200 M⊙, which depends on the unknown gas density n(H); if n(H)> 1cm−3(which
is quite likely for this DLA given its low velocity dispersion), then M(12C)? 2 M⊙,
consistent with pollution by only a few prior supernovae. We speculate that DLAs
such as the one reported here may represent the ‘missing link’ between the yields of
Pop III stars and their later incorporation in the class of carbon-enhanced metal-poor
stars which show no enhancement of neutron-capture elements (CEMP-no stars).
Key words: galaxies: abundances − galaxies: evolution − quasars: absorption lines
− quasars: individual: J0035−0918 − stars: carbon − stars: Population III
1INTRODUCTION
Damped Lyα systems (DLAs) are the neutral gas reser-
voirs at high redshift that have, by definition, neutral hy-
drogen column densities in excess of 1020.3atoms cm−2(see
Wolfe et al. 2005 for a review). At these high column den-
sities the gas is self-shielded (e.g. Vladilo et al. 2001), re-
sulting in a simple ionization structure which facilitates
the derivation of element abundances. Moreover, the abun-
dances thus derived are independent of the geometrical con-
figuration and thermodynamical state of the gas, and of
most other factors which complicate the analysis of stel-
lar spectra (e.g. Asplund 2005). The largest uncertainties in
DLA abundance studies are due to the effects of line satu-
ration and dust depletion (for some elements), although the
latter of these concerns is found to be minimal when the
⋆Based on data obtained at the W. M. Keck Observatory, which
is operated as a scientific partnership among the California Insti-
tute of Technology, the University of California, and NASA, and
was made possible by the generous financial support of the W. M.
Keck Foundation.
† email: rcooke@ast.cam.ac.uk
metallicity of the DLA is below ∼ 1/100Z⊙ (Pettini et al.
1997; Prochaska & Wolfe 2002; Akerman et al. 2005).
In recent years, these very metal-poor (VMP) DLAs,
with metallicities [Fe/H]1< −2, have attracted an increas-
ing amount of interest because of their potential for probing
gas which may still bear the chemical imprint of the first
few generations of stars to have formed in the Universe (e.g.
Erni et al. 2006). They are thus an extremely valuable com-
plement, at high redshifts, to local studies of the oldest and
most metal-poor stars in the Galactic halo.
The VMP DLA regime was largely unexplored until
very recently, when it became possible to identify candi-
date metal-poor DLAs in Sloan Digital Sky Survey (SDSS)
quasars, and then measure their chemical composition with
high resolution follow-up spectroscopy. The first high spec-
tral resolution (R>
∼30000, FWHM<
VMP DLAs was compiled by Pettini et al. (2008), whose
main goal was to compare the relative abundances of C, N,
∼10 km s−1) sample of
1We adopt the standard notation: [A/B] ≡ log(NA/NB) −
log(NA/NB)⊙ where NA,Brefers to the number of atoms of ele-
ment A,B.
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2Cooke et al.
and O with the trends observed in VMP halo stars in our
Galaxy. More recently, Penprase et al. (2010) have presented
medium resolution (FWHM ∼ 60km s−1) spectroscopy of
a sample of 27 VMP DLAs to explore the general proper-
ties of DLAs in the VMP regime. However, as Penprase et
al. acknowledge themselves, there are difficulties with ac-
curately measuring column densities from medium (as op-
posed to high) resolution data, given that most DLAs with
metallicities less than 1/100 solar exhibit very low veloc-
ity dispersions, with metal line widths less than 10km s−1
(Ledoux et al. 2006; Murphy et al. 2007; Prochaska et al.
2008). Under these circumstances, line saturation can easily
be overlooked.
Perhaps the most startling result from abundance stud-
ies of VMP DLAs are the near-solar values of [C/O] at low
metallicity (Pettini et al. 2008), in line with the peculiar up-
turn in the [C/O] abundance below [O/H]
tic halo stars discovered by Akerman et al. (2004) and later
confirmed by Fabbian et al. (2009a). Akerman et al. (2004)
attributed this behaviour to an increased C yield from ear-
lier generations of massive stars. Penprase et al. (2010) ex-
tended this work and reported a number of DLAs with su-
persolar [C/O] suggesting that this ratio continues to in-
crease with decreasing [O/H]. A more recent compilation of
high spectral resolution observations of VMP DLAs, how-
ever, suggests that the ratio in fact plateaus at [C/O] ∼ −0.2
(Cooke et al. 2010b).
In contrast to the relatively new interest in VMP DLAs,
studies of metal-poor Galactic halo stars have received on-
going attention for more than two decades, most recently
from the dedicated HK (Beers, Preston, & Shectman 1992)
and HES (Christlieb et al. 2001) surveys. A relevant result
emerging from this work is that nearly one-quarter of all
metal-poor stars with [Fe/H]< −2.0 exhibit a marked car-
bon enhancement, with [C/Fe]> +1.0 (Beers & Christlieb
2005; Lucatello et al. 2006). These are collectively known as
carbon-enhanced metal-poor stars (CEMP stars), and have
been subdivided into four classes based on the abundances
of their neutron-capture elements: (i & ii) The CEMP-s and
CEMP-r classes, with enhancements of elements produced
predominantly by the s-process and r-process respectively;
(iii) the CEMP-rs class, with enhancements in both the s-
and r-process elements; and (iv) the CEMP-no class, which
exhibits no such enhancements.
For further details of these classes, and the likely ori-
gins of their carbon enhancements, we direct the reader
to Masseron et al. (2010). In short, there is reasonable ev-
idence to suggest that CEMP-s and CEMP-rs stars are
extrinsically polluted by a now extinct asymptotic giant
branch (AGB) companion. The origin of the CEMP-no
class, however, is not yet firmly established. Whilst sev-
eral models invoking mass transfer from an AGB com-
panion could explain the lack of neutron-capture elements
(e.g. Fujimoto, Ikeda, & Iben 2000; Siess, Goriely, & Langer
2004), radial-velocity measurements have not yet confirmed
whether CEMP-no stars are host to binary companions. In-
deed, the apparent difference in the metallicity distributions
between CEMP-no stars and the other CEMP classes, in
the sense that CEMP-no stars are more numerous at lower
metallicity, might suggest that a mechanism other than the
AGB binary transfer scenario produces the C enhancement
in CEMP-no stars (Aoki et al. 2007). In all likelihood, as
<
∼−1.0 in Galac-
pointed out by Masseron et al. (2010), there is a continuous
link that connects some CEMP-no stars with CEMP-s stars,
while the carbon enhancement of other CEMP-no stars may
have a different origin.
Models of core-collapse supernova yields from Popula-
tion III (or near metal-free) stars do entertain high C yields
relative to Fe (e.g. Woosley & Weaver 1995). Population III
enrichment is a particularly intriguing explanation for the
origin of the carbon enhancement in some CEMP stars, since
the three most Fe-poor halo stars all exhibit carbon enhance-
ments relative to iron (Christlieb et al. 2002; Frebel et al.
2005; Norris et al. 2007). Moreover, the fraction of metal-
poor stars that exhibit a carbon-enhancement increases
with decreasing metallicity (Beers & Christlieb 2005). In
this Population III enrichment scenario, the carbon enhance-
ment in the extremely metal-poor regime reflects the initial
composition of the gas from which these stars formed, rather
than resulting from mass transfer from an evolved compan-
ion.
Whilst the physics behind core-collapse supernovae
is poorly constrained, several parameterised models have
been developed to calculate the expected yields from zero-
metallicity progenitors to compare with the observations of
the most Fe-poor CEMP stars, as well as the more ‘nor-
mal’ non-CEMP stars. The most important (and largely
unknown) quantities are the degree of mixing during the
supernova explosion and the amount of fallback onto the
remnant black hole thereafter (Umeda & Nomoto 2003).
Umeda & Nomoto (2003) parameterised both quantaties,
then suitably selected the appropriate values that reproduce
the observed stellar abundance patterns. Heger & Woosley
(2010), on the other hand, parameterise only the mixing pa-
rameter by applying a running boxcar filter over the star
following the explosion. This prescription successfully re-
produces the chemical composition of the extremely metal-
poor (non-CEMP) halo stars from the study by Cayrel et al.
(2004), as well as that of the most Fe-poor CEMP stars
HE0107−5240 (Christlieb et al. 2002) and HE1327−2326
(Frebel et al. 2005). Joggerst, Woosley, & Heger (2009) ex-
tended this work by mapping the one-dimensional mod-
els by Heger & Woosley (2010) onto a two-dimensional
grid to follow Rayleigh-Taylor induced mixing after explo-
sive nuclear burning. Whilst the Joggerst, Woosley, & Heger
(2009) models are physically motivated, they are unable to
reproduce the relatively high levels of nitrogen enrichment
that are observed in the most Fe-poor stars, nor are they
able to produce sufficient Fe-peak elements.
These concerns are alleviated with models that in-
clude rotation(e.g. Meynet, Ekstr¨ om, & Maeder 2006;
Hirschi 2007; Meynet et al. 2010), since rotation induces
additional mixing and mass loss in the pre-supernova
phase of low-metallicity stars. The only simulation avail-
able to date that investigates the effects of both ro-
tational and Rayleigh-Taylor mixing, as well as incor-
porating a realistic prescription of fallback, are those
presented recently by Joggerst et al. (2010a) (see also
Joggerst, Almgren, & Woosley 2010b). By introducing rota-
tion in their zero-metallicity models, Joggerst et al. (2010a)
report an increased nitrogen yield, as well as an increased
Fe-peak element yield, however, these models are not able to
reproduce the high CNO to Fe ratios observed in the most
metal-poor stars. In summary, it is still a matter of some
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A carbon-enhanced metal-poor DLA3
debate whether these extremely metal-poor early Popula-
tion II stars: (i) were borne out of gas which had previously
been carbon-enriched by the first stars; or (ii) received a
CNO top-up from a companion star; and/or (iii) exhibit a
degree of self-pollution from their own nucleosynthesis.
The first possibility, that at least some CEMP stars
were borne out of carbon-enhanced gas, is given additional
support from the discovery reported here of an extremely
metal-poor DLA at zabs= 2.3400972 exhibiting a carbon en-
hancement akin to that measured in Galactic CEMP stars.
No other case was known until now, given (i) the rarity of
DLAs with [Fe/H]< −2, which lie in the tail of the metallic-
ity distribution of DLAs (e.g. Pontzen et al. 2008), and (ii)
the difficulty in measuring [C/H] in DLAs, where the rele-
vant absorption lines are often saturated even in the VMP
regime. We speculate that this DLA may be the ‘missing
link’ between the first few generations of stars and some of
the CEMP stars in the Galactic halo.
The paper is organized as follows. Section 2 summa-
rizes the observations and data reduction. We analyse the
absorption lines in the damped Lyα system and deduce cor-
responding element abundances in Section 3. In Section 4
we scrutinize potential issues that could masquerade as a
carbon enhancement. After ruling out these possibilities, we
discuss in Section 5 possible origins of this enhancement,
and consider the implications of our finding for models of
CEMP stars. Finally, we summarize our results and draw
our conclusions in Section 6.
2 OBSERVATIONS AND DATA REDUCTION
The mr = 18.89, zem = 2.42 QSO J0035−0918 was selected
for our on-going survey for VMP DLAs (Cooke et al. 2010b)
on the basis of its SDSS spectrum which shows a DLA at
zabs ≃ 2.340 with no apparent associated metal lines. Such
cases are highly suggestive of narrow and weak metal ab-
sorption lines which are undetectable at the coarse resolu-
tion and signal-to-noise ratio (S/N) of the discovery SDSS
spectra.
Follow-up observations of J0035−0918 were made
withthe Magellan Echellette
(Marshall et al. 2008) on the Magellan ii Clay telescope on
the nights of 2008 December 30 and 31 in good conditions
with 1arcsec seeing. We took 2 × 2400s exposures using
the standard setup with 1×1 binning and a 1.0arcsec slit
giving a resolving power R ≡ λ/∆λ ≃ 4100. Standard
calibrations were taken following the recommendations by
Simcoe, Hennawi & Williams2. The data were reduced
using a custom set of IDL routines written by G. D. Becker
and described in Becker et al. (2006). The MagE spectrum
confirmed the VMP DLA nature of the zabs = 2.3400972
absorption system, by showing clear damping wings to
the Lyα absorption line (see top panel of Figure 1) and
unusually weak associated metal lines.
Encouraged by these initial indications, we subse-
quently observed J0035−0918 on the night of 2009 De-
cember 9 with the High Resolution Echelle Spectrograph
(MagE) spectrograph
2see http://web.mit.edu/rsimcoe/www/MagE/mage.html
(Vogt et al. 1994) on the Keck i telescope under good con-
ditions with sub-arcsecond seeing, for a total integration
time of 16200s, divided into 6 exposures of 2700s, result-
ing in a signal-to-noise ratio near 4500˚ A of S/N ≃ 18.
We used the 1.148arcsec wide slit which, when uniformly
illuminated, provides a nominal spectral resolution R ≡
λ/∆λ = 37000. From our spectra, we measure R ≃ 41000
which corresponds to a velocity full-width at half maximum
FWHM = 7.3 km s−1, sampled by ∼ 3 pixels. We employed
the UV cross-disperser which covers the wavelength range
3100–6000˚ A with ∼ 70˚ A-wide gaps near 4000˚ A and 5000˚ A.
The HIRES spectra were reduced with the makee data
reduction pipeline developed by Tom Barlow, which per-
forms the usual steps of flat-fielding, order tracing, back-
ground subtraction, and extraction of the final 1-D spec-
trum. The data were wavelength calibrated by reference to
the spectrum of a ThAr lamp, and mapped onto a vacuum
heliocentric wavelength scale. The extracted spectra were
merged and then normalized by dividing out the QSO con-
tinuum and emission lines, using the software package uves
popler, maintained by Michael Murphy3. Following this
step, all available metal absorption lines associated with the
DLA were extracted in ±150km s−1windows around the
pixel with highest optical depth. A further fine correction to
the continuum was then applied if necessary.
3PROFILE FITTING AND ABUNDANCE
ANALYSIS
Table 1 lists all the metal absorption lines in the zabs =
2.3400972 DLA detected in our HIRES spectrum of
J0035−0918, together with a few interesting transitions
which are below the detection limit of the data. For each line,
we give the rest-frame wavelength and oscillator strength
that we adopted for this work, as well as the measured equiv-
alent width and associated random and systematic errors.
The latter were determined by shifting the continuum by
±σ/√n (where σ is the 1σ error spectrum, and n is the
number of independent resolution elements over which the
equivalent width integration was carried out), and recalcu-
lating the equivalent width. For the undetected transitions,
we quote the 3σ limiting rest-frame equivalent width, us-
ing as a guide the profile of the weakest absorption line,
Feii λ1608, which we detect at the 5σ confidence limit (see
Table 1). Examples of metal absorption lines are reproduced
in Fig. 1.
As can be appreciated from inspection of Table 1 and
Fig. 1, the metal lines in this DLA are very weak and narrow,
with equivalent widths W0 < 55m˚ A (the strongest line being
Ciiλ1334), and with the absorption taking place in a single
velocity component with FWHM< 10km s−1. The weakness
of the absorption limits our detection to the intrinsically
most abundant elements of the periodic table, C, N, O, Al,
Si, and Fe; on the other hand, the wide wavelength coverage
of the echelle spectra, which reach well into the far-UV, gives
access to several transitions of differing f-values for most of
these elements.
3uves popler is available from
http://astronomy.swin.edu.au/∼mmurphy/UVES popler
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4Cooke et al.
Figure 1. Selected absorption lines in the zabs= 2.3400972 DLA towards J0035−0918. The data are shown with black histograms,
while the red continuous lines are model fits to the line profiles. Top panel: Portion of the MagE spectrum of J0035−0918 encompassing
the damped Lyα line, together with the theoretical Voigt profile (red line) for a neutral hydrogen column density log[N(Hi)/cm−2] =
20.55 ± 0.1. The remaining panels display portions of the HIRES spectrum near metal lines of interest, together with model profiles
generated with vpfit as described in Section 3.1. This DLA consists of a single absorption component at zabs= 2.3400972 with a small
velocity dispersion, b =√2σ = 2.4km s−1. The weak absorption feature centred at +26km s−1in the Siiiλ1260.4221 panel is probably
Feiiλ1260.533 absorption in the DLA, although its strength is below our 3σ detection limit. The red wings of both Ciiλ1036 and Oiλ988
are blended with a weak Lyα forest line indicated by a continuous blue line. In all plots the y-axis scale is residual intensity.
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A carbon-enhanced metal-poor DLA5
Table 1. Metal lines in the zabs= 2.3400972 DLA towards
J0035−0918
IonWavelengtha
(˚ A)
fa
Wb
(m˚ A)
0
δWc
(m˚ A)
0
δWd
(m˚ A)
0,cont
Cii
Cii
Ni
Ni
Ni
Nii
Oi
Oi
Oi
Oi
Oi
Oi
Alii
Siii
Siii
Siii
Siii
Siii
Sii
Sii
Feii
Feii
1036.3367
1334.5323
1134.1653
1134.4149
1134.9803
1083.9937
971.7382
988.5778
988.6549
988.7734
1039.2304
1302.1685
1670.7886
989.8731
1193.2897
1260.4221
1304.3702
1526.7070
1253.805
1259.5180
1260.533
1608.4509
0.118
0.1278
0.0146
0.0278
0.0416
0.111
0.0116
0.000553
0.0083
0.0465
0.00907
0.048
1.740
0.171
0.582
1.18
0.0863
0.133
0.0109
0.0166
0.0240
0.0577
39
54
2
2
1
1
< 5.3e
6.2
12.5
< 4.6e
24
< 6.0e
23
38
23
42
15
15
28
39
21
34
< 2.0e
3.7
< 2.5e
10
...
1.7
1.6
...
3
...
2
3
2
2
2
2
3
2
2
2
...
0.8
...
2
...
0.5
0.5
...
1
...
1
1
1
1
0.5
1
2
0.5
1
1
...
0.3
...
0.5
aLaboratory wavelengths and f-values from Morton (2003).
bRest-frame equivalent width.
cRandom error on the equivalent width W0.
dSystematic error on W0 from 1σ uncertainty in the continuum
placement.
e3σ upper limit on the rest-frame equivalent width.
3.1Column Densities
We begin our abundance analysis by measuring the column
density of neutral hydrogen. To this end, we used the MagE
spectrum of the QSO, the relevant portion of which is repro-
duced in the top panel of Fig. 1, because at zabs= 2.3400972
the damped Lyα line falls on a gap between two of the CCDs
in the HIRES detector mosaic. Even at the coarser resolu-
tion of MagE (compared to HIRES), the broad damped Lyα
line is fully resolved and no loss of accuracy results in the
derivation of N(Hi). Fitting a Voigt profile to the line, we
deduced log[N(Hi)/cm−2] = 20.55±0.10; the corresponding
theoretical Voigt profile is overlaid on the MagE spectrum
in the top panel of Fig. 1.
In the next step, we determined the Doppler parameter
of the absorbing gas, b (km s−1), and the column density of
the metal ions, N(X) (cm−2), by fitting the corresponding
line profiles with vpfit4, which simultaneously fits multiple
Voigt profiles to several atomic transitions, returning the
values of N(X) and b, together with the associated errors,
that minimize the χ2between the data and the model. We
tied the redshift and the Doppler parameter to be the same
for all of the absorption lines listed in Table 1, which is
justified if the neutrals and first ions are kinematically asso-
ciated with the same gas (we relax this assumption later).
With these constraints, vpfit converged to a best-fitting
4vpfit is available from http://www.ast.cam.ac.uk/∼rfc/vpfit.html
Table 2. Ion column densities in the z = 2.3400972 DLA
towards J0035−0918
IonlogN(X)/cm−2
Hi
Cii
Ni
Oi
Alii
Siii
Sii
Feii
20.55 ± 0.10
15.47 ± 0.15
13.51 ± 0.06
14.96 ± 0.08
11.73 ± 0.05
13.41 ± 0.04
? 13.08
12.98 ± 0.07
model consisting of a single absorption component with red-
shift z = 2.3400972 ± 0.0000008 and Doppler parameter
b = 2.36 ± 0.08 km s−1. The corresponding column den-
sities are listed in Table 2. Examples of the theoretical line
profiles generated by vpfit are shown superimposed on the
data in Fig. 1.
The weakest feature in our data is Siiλ1259. When
this line is included in the vpfit fitting procedure (see
bottom right panel of Fig. 1), we derive a column density
logN(Sii)/cm−2= 13.08 ± 0.10. However, since this ab-
sorption line is only significant at the ∼ 4.5σ level, we con-
servatively consider the above value to be an upper limit to
the column density of Sii.
With the usual assumption that the ions observed are
the dominant stage of the corresponding elements in Hi gas,
so that corrections for unseen ion stages and/or the presence
of ionized gas are negligible (we review this assumption in
Section 3.2 below), it is straightforward to deduce the abun-
dances of the elements concerned by simply dividing the
column densities in Table 2 by N(Hi). Comparison with the
solar abundance scale of Asplund et al. (2009) then leads
to the abundance pattern listed in Table 3 and illustrated
graphically in Fig. 2.
3.2 Ionization Corrections
In DLAs, it is usually assumed that the metals within the ab-
sorbing Hi gas reside in a single dominant ionization stage,
Xn, so that the total abundance of an element is given by
[X/H] = [Xn/Hi] + IC(X) (1)
where the ionization correction, IC(X), is typically negligi-
ble. In general, it is safe to assume IC(X)≃ 0.0 for gas with
high N(Hi), since the gas is self-shielded from ionizing radi-
ation (e.g. Vladilo et al. 2001). Of course, if the gas does not
reside in a single dominant ionization stage, or some amount
of Xn is associated with Hii gas, we may respectively under-
or over-estimate the abundance of a given element [positive
or negative IC(X)].
To gauge the extent of such corrections, we used the
cloudy photoionization software (Ferland et al. 1998) to
model the DLA as a slab of gas with uniform density
in the range −3 < log[n(H)/cm−3] < 3, exposed to the
Haardt & Madau (2001) metagalactic ionizing background
and the cosmic microwave background, both at the red-
shift of the DLA. Adopting the solar abundance scale of
Asplund et al. (2009), we globally scaled the metals to log
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