1. Detection of Pristine Gas Two Billion Years After the Big Bang
Michele Fumagalli, et al.
Science 334, 1245 (2011);
DOI: 10.1126/science.1213581
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2. RESEARCH ARTICLES
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ing clathrin-mediated endocytosis. In analyzing 22. M. Lindhagen-Persson, K. Brännström, M. Vestling, Service Award fellowship F32 NS067782-02; the Cure
human GWAS data we also uncovered sugges- M. Steinitz, A. Olofsson, PLoS ONE 5, e13928 Alzheimer’s Fund; NIH grants K08AG034290, P30AG10161,
tive associations between AD and three other (2010). R01AG15819, and R01AG17917; the Kempe foundation;
and Alzheimerfonden. This paper is dedicated to Nell Matheney,
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Zhao Xindi, Rochelle Glatt, Matyas Topolszki, their
24. S. D. Stamenova, R. Dunn, A. S. Adler, L. Hicke,
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Our yeast model provides a tool for identifying 29. G. Gaidos, S. Soni, D. J. Oswald, P. A. Toselli, K. H. Kirsch,
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Detection of Pristine Gas Two Billion Wb,0). BBN theory also predicts that there was
negligible production of the heavy elements
with abundance ratios X/H < 10−10, because
Years After the Big Bang the physical conditions now typical of stellar
interiors did not yet exist (1).
The analysis of gas observed in absorption
Michele Fumagalli,1* John M. O'Meara,2 J. Xavier Prochaska3
along the lines of sight to high-redshift quasars,
distant galaxies that host supermassive black
In the current cosmological model, only the three lightest elements were created in the first few holes, is a powerful probe of the BBN yields.
minutes after the Big Bang; all other elements were produced later in stars. To date, however, Particular attention has been given to D, partly
heavy elements have been observed in all astrophysical environments. We report the detection of because of observational convenience, but also
two gas clouds with no discernible elements heavier than hydrogen. These systems exhibit the because the D/H abundance ratio is very sensi-
lowest heavy-element abundance in the early universe, and thus are potential fuel for the most tive to Wb,0. For quasar sight lines, the measured
metal-poor halo stars. The detection of deuterium in one system at the level predicted by log(D/H) = − 4.55 T 0.03 (2, 3) translates into
primordial nucleosynthesis provides a direct confirmation of the standard cosmological model. Wb,0h2(BBN) = 0.0213 T 0.0010, which is fully
The composition of these clouds further implies that the transport of heavy elements from consistent with the value inferred from the cosmic
galaxies to their surroundings is highly inhomogeneous.
1
Department of Astronomy and Astrophysics, University of
n modern cosmological theory, the light ele- this brief epoch, termed Big Bang nucleosynthe- California, Santa Cruz, CA, USA. 2Department of Chemistry and
I ments and their isotopes were produced dur-
ing the first few minutes after the Big Bang,
when the universe cooled during expansion from
sis (BBN), deuterium (D), 3He, 4He, and 7Li were
synthesized with an abundance ratio relative to
hydrogen (H) that was sensitive to the cosmic
Physics, Saint Michael's College, Colchester, VT, USA. 3Uni-
versity of California Observatories–Lick Observatory, University
of California, Santa Cruz, CA, USA.
*To whom correspondence should be addressed. E-mail:
temperatures T ∼ 109 K to below ∼4 × 108 K. In density of ordinary matter (i.e., the baryon density mfumagalli@ucolick.org
www.sciencemag.org SCIENCE VOL 334 2 DECEMBER 2011 1245

3. RESEARCH ARTICLES
Fig. 1. Peak-to-peak variation of the observed metallicity in z ≳ 2 cosmic ρ/ρmean
structures at different densities. The blue, green, and red rectangles show the 100 105 1010 1015
102
spread in observed metallicities for diffuse gas in the universe; respectively,
Quasars
these are the IGM, LLSs, and galactic ISM. Orange rectangle: H II regions in
galaxies. Light blue rectangle: quasar broad-line regions. The black point with LLSs
error bars marks the mean metallicity and the standard deviation for z > 1.5 HII regions
Halo stars
100 ISM
LLSs. Galactic halo stars (stars connected with a dashed line) are represented at IGM
arbitrary density for visualization purposes. The top axis translates the number
density in the overdensity above the mean baryon cosmic density at z = 3.5. The
Z(Z )
upper limits on the metallicity for LLS1134a and LLS0956B are shown with
green arrows, assuming logU = −3. Higher ionization parameters would shift 10-2
these limits to lower densities and lower metallicity. See SOM text 5 for
additional details on the observations presented in this figure.
10-4 LLS0956B
LLS1134a
10-5 100 105 1010
Downloaded from www.sciencemag.org on December 1, 2011
Number density (cm-3)
Fig. 2. (Top) Keck/LRIS spectrum of the quasi-stellar object 40
(QSO) J1134+5742. A LLS at z ∼ 3.4 is clearly visible from
Relative Flux
the break at ∼4000 Å. (Middle) H I Lyman series transi- 30
tions in LLS1134a. Superimposed on the data are the best-
fit model (red line) and the 2s errors (yellow shaded regions). 20
Individual components included in the model are marked
with thin gray lines, and the positions of the HI and DI are 10
indicated by vertical dotted lines. The second HI component
0
(LLS1134b) at +54 km s−1 is marked with a dash-dotted line.
(Bottom) Selected strong metal-line transitions in LLS1134a 3500 4000 4500 5000 5500
(dotted lines) and LLS1134b (dash-dotted lines). Unrelated Wavelength (Å)
absorption from the IGM contaminates the Si III transition
(fig. S4). 1.2
α
1.0
0.8
0.6
0.4
0.2
0.0
−100 −50 0 50 100 −50 0 50 100
1.2
Normalized Flux
1.0
0.8
0.6
1.2
1.0
0.8
0.6
−40 −20 0 20 40 60 −40 −20 0 20 40 60
Velocity (km s−1)
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4. RESEARCH ARTICLES
2.0
microwave background (CMB) power spectrum
Normalized Flux Wb,0h2(CMB) = 0.02249+0.00056− 0.00057 [from
1.5 7-year Wilkinson microwave anisotropy probe
(WMAP7) data (4)]. This excellent agreement
1.0
between two essentially independent experi-
0.5 ments stands as a marked triumph of the Big
Bang theory.
0.0 On the other hand, all of the systems with
−0.5 measured D have heavy-element abundances that
3740 3760 3780 3800 3820 3840 3860 3880 exceed, by many orders of magnitude, the BBN
Wavelength (Å) prediction. In fact, despite the measurement of
thousands of galaxies from the early universe
(5, 6) and careful study of the diffuse gas that
permeates the universe (7, 8), one has yet to
1.0 detect anything near primordial enrichment. For
structures denser than the mean cosmic density
(Fig. 1), the high-redshift universe has exhibited
0.5 a floor in the metallicity Z (the mass fraction of
Downloaded from www.sciencemag.org on December 1, 2011
elements heavier than H), at ∼1/1000 of the so-
lar abundance (Z⊙). Similarly, among several old
0.0
and iron-poor stars, only one has a metallicity
−50 0 50 −100 −50 0 50 100 Z ∼ 10−4 Z⊙ (9), with all the remaining stars
1.4 having enhanced C or O abundances. The exis-
Normalized Flux
tence of a minimum level of enrichment at about
1.2 1/1000 of solar has been associated with the
metal production in Population III (PopIII) stars,
1.0 which are primordial stars that form in metal-free
environments via H2 cooling. In fact, models and
0.8 numerical simulations (10, 11) show that ejecta
from this first stellar population could have en-
riched the interstellar medium (ISM) of the host
1.4 halos up to ∼10−3 Z⊙, and polluted the surround-
ing intergalactic medium (IGM) as soon as
1.2 1 billion years after the Big Bang (redshift z ∼ 6).
In this work, we report on the H and metal
1.0 properties of two gas “clouds” at z ∼ 3, when the
universe was only two billion years old. We
0.8 observed quasars SDSS J113418.96+574204.6
(emission redshift zem = 3.522) and Q0956+122
(zem = 3.297) on UT 3 and 5 January 2006 and
−40 −20 0 20 40 −40 −20 0 20 40 UT 7 April 2006 with the High Resolution
Velocity (km s−1) Echelle Spectrometer HIRES on the Keck I tele-
scope on Mauna Kea [supporting online mate-
Fig. 3. (Top) Keck/HIRES spectrum of the QSO Q0956+122. The flux decrements at ∼3860 and ∼3750 Å rial (SOM) text 1]. Previous low-resolution spectra
reveal two partial LLSs at z ∼ 3.22 (LLS0956A) and z ∼ 3.10 (LLS0956B). The corresponding Lyman of these quasars from the Sloan Digital Sky Sur-
limits are marked with vertical lines. (Middle) HI Lyman series transitions for LLS0956B shown relative vey (SDSS) had shown significant absorption
to z = 3.096221. Superimposed on the data are the best-fit model (red lines) and 2s uncertainties at wavelengths l < 4000 Å characteristic of sub-
(yellow shaded regions). (Bottom) Selected strong metal-line transitions for LLS0956B.
stantial optical depth tLL at the H I Lyman limit
(at wavelength l < 912 Å), typical of the Lyman
Table 1. Summary of the physical properties of LLS1134a and LLS0956B. For each system, we present limit systems (LLSs).
redshift, the HI column density (NHI), the D abundance (D/H), the Doppler parameter (bHI), the temperature, Our Keck/HIRES spectrum of J1134+5742
the metallicity, the H neutral fraction (xHI), the total H volume density (nH), and the ionization parameter (U). reveals a sharp break in the flux at ∼4000 Å,
indicating the presence of a LLS at z ∼ 3.4 with
LLS1134a LLS0596B tLL > 2 (hereafter named LLS1134; Fig. 2), con-
Redshift 3.410883 T 0.000004 3.096221 T 0.000009 firming the lower-resolution data. A search for
logNHI 17.95 T 0.05 17.18 T 0.04 absorption lines at a redshift consistent with the
logD/H −4.69 T 0.13* – Lyman break further reveals the presence of the
bHI (km s−1) 15.4 T 0.3 20.2 T 0.8 H I Lyman series through to Lyman-22 (H I
Temperature (K) < (1.43 T 0.05) × 104 <(2.48 T 0.19) × 104 913.5), corresponding to l ∼ 4030 Å. In the high-
Metallicity ( Zʘ) <10−4.2 <10−3.8 resolution spectrum, two distinct absorbers can
logxHI ≤2.10 ≤2.40 be identified within LLS1134, the main system
lognH ≤1.86 ≤1.98 (LLS1134a) at z = 3.410883 and a weaker com-
logU† ≥3 ≥3 ponent (LLS1134b) at z = 3.41167, separated by
*Including LLS1134a, the current best estimate for the primordial D abundance becomes ¯¯¯¯¯¯¯ = − 4.556 T 0.034.
log(D/H) a velocity difference dv ∼ 54 km s−1. In contrast,
†The listed values are physically motivated but not directly measured. The metallicity, xHI, and nH depend on the assumed value. two flux decrements in the normalized HIRES
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5. RESEARCH ARTICLES
spectrum of Q0956+122 (Fig. 3) are visible at of metals, have total heavy-element abundance The detection of D in the metal-free LLS1134a
∼3860 and ∼3750 Å, revealing the presence of comparable to these limits, but generally above provides a direct confirmation of the BBN. From
two LLSs with tLL ∼ 1 at z ∼ 3.22 (LLS0956A) ∼1/1000 of solar. Our analysis uncovers regions the analysis of the D and H absorption lines, we
and z ∼ 3.10 (LLS0956B). of the universe at z < 6 with essentially primor- derive log(D/H) = −4.69 T 0.13 (SOM text 2). The
A closer inspection of these spectra also re- dial enrichment, whose traces can be found observed value is consistent at 1s with the the-
veals no detectable metal-line absorption at the within the oldest stellar populations in the present oretical predicted value log(D/H) = −4.592 (15),
velocity of the strongest H I component for either universe (9). assuming Wb,0 from WMAP7. This measurement
LLS1134a or LLS0956B. Such a complete ab-
sence of heavy-element absorption has not been
previously reported from data with comparable Fig. 4. D abundances −4.3
sensitivity. For LLS1134a, besides the H I Lyman as a function of metal- NHI < 10 20 cm−2
series, the only other detected transitions are licity [(X/H) ≡ log(X/H) − NHI > 10 20 cm−2
log(X/H)ʘ] for the z > 2 −4.4 [Si/H]
D I Lya and Lyb, offset by the appropriate dv = [O/H] 0.018
−82 km s−1 from the H absorption. absorption-line systems.
To characterize the physical properties of Green symbols are for
−4.5
these LLSs (summarized in Table 1), we modeled LLSs; red symbols are for
the H and D absorptions using a c2 minimization higher–column-density
log (D/H)
absorbers (NHI > 1020 0.022
algorithm (table S1; figs. S2 and S3; and SOM −4.6
Ωb,0 h2
cm−2). Metallicities ob-
Downloaded from www.sciencemag.org on December 1, 2011
text 2), and we derived upper limits on the column
tained with Si are indi-
densities of various ionization states of heavy 0.026
cated by squares, whereas −4.7
elements (table S2; fig. S4; and SOM text 3). those obtained with O
To translate these limits into constraints on the are indicated with circles.
gas metallicity, it is necessary to make assump- The right-hand axis trans- −4.8 0.030
tions about the ionization state of the gas (SOM lates the D abundance to
text 4). Every LLS with HI column density NHI ≲ the cosmic baryon den- 0.034
1019 cm−2 analyzed to date has exhibited absorp- sity Ωb,0h2. The inferred
−4.9
−4.5 −4.0 −3.5 −3.0 −2.5 −2.0 −1.5
tion characteristic of a predominantly ionized gas Ωb,0h (BBN) is shown
2
Metallicity ([X/H])
(12, 13). The standard interpretation is that the with a solid blue line, to-
medium has been photoionized by an external gether with the 1s errors (light blue shaded area). The CMB value and 1s errors from WMAP7 are shown
radiation field, presumably a combination of the with a solid black line and a gray dashed area, respectively.
extragalactic ultraviolet background (EUVB) gen-
erated by the cosmological population of quasars
and galaxies, together with emission from local 0
10 PopIII/PopII transition
sources (such as a nearby galaxy). This conclu- IGM(PopIII)
sion is based on comparison of the observed ionic Halos(PopIII+PopII)
column densities of heavy elements with simple ISM(PopII)
ISM/IGM(PopII)
photoionization models. Every previously ana- 10
-1
Gas(PopII)
lyzed LLS has shown substantial absorption from
doubly and triply ionized species, such as Si ++
and C +3, which trace predominantly ionized gas. -2
Parametrizing the ionization state in terms of the 10
ionization parameter U ≡ F/(cnH), with F the
Z(Z )
flux of ionizing photons and nH the gas volume
density, all previously analyzed LLSs have ex- 10
-3
hibited U ≥ 10−3, corresponding to nH ≤ 10−2 cm−3
at z ∼ 3 (fig. S6).
Adopting this U value as a limit to the ioniza-
-4
tion state and current estimates for the spectral 10
shape of the EUVB (14), we infer metallicities of
Z < 10−4.2 Z⊙ and Z < 10−3.8 Z⊙ for LLS1134a
and LLS0956B (fig. S5). These upper limits are 10
-5
100 to 1000 times lower than typical measure-
ments of LLSs and are over an order of magnitude 0 5 10 15 20
lower than any previous observed metallicity Redshift
at z > 2 (Fig. 1). These limits are only comparable
Fig. 5. Overview of analytic models and simulations for the metal enrichment of the universe. Light blue-
to the abundances detected in the most metal- and green-shaded regions: IGM metallicity from PopIII stars with mixing between 1 and 0.05 and
poor star (9) and are suggestive of a primordial different star formation histories (10, 21). Red dashed region: Critical metallicity that marks the
composition. Supersolar metallicity is commonly transition between PopIII and PopII stars (23, 33). Orange line: Analytic model for the gas metal content
found in the surroundings of quasars as well as in the universe from PopII stars and galactic winds (20). Gray-shaded region: Gas metallicity within halos
in a few LLSs. Metallicities between solar and from hydrodynamical simulations that include yields from both PopIII and PopII stars (22). Blue-shaded
∼1/10 of solar are typical of galaxies, and sub- region: Analytic model for the ISM metallicity at different halo masses (1011 to 1014 Mʘ) and different
solar metal enrichment down to 10−3 Z⊙ is char- wind models (34). Black lines: Metallicity from hydrodynamical simulations with momentum-driven
acteristic of the ISM and IGM at early epochs. winds (19) in condensed gas (solid line), hot halo (dotted line), warm-hot intergalactic medium (dashed
The most iron-poor stars in the Galactic halo, line), diffuse gas (dash-dotted line), ISM (dash–triple-dotted line), and stars (long-dashed line). Upper
thought to be the repository of the first generation limits on the metallicities of LLS1134a and LLS0956B are marked with green arrows.
1248 2 DECEMBER 2011 VOL 334 SCIENCE www.sciencemag.org

6. RESEARCH ARTICLES
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ilarly, the metallicity predicted for the hot halo of distribution, the detection of metal-free LLSs is http://arxiv.org/abs/1109.5700 (2011).
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exceeds by a factor of 10 or more the limits of evolution. Modern theory and simulations predict 34. R. Davé, K. Finlator, B. D. Oppenheimer,
LLS1134a and LLS0956B. Contrary to any predic- that most of the gas that sustains star formation http://arxiv.org/abs/1108.0426 (2011).
tion and any previous observation, these two LLSs is accreted in galaxies through dense and narrow
reside at significant overdensity (r/rmean < 850), streams, known as cold flows (28, 29). These Acknowledgments: We thank A. Aguirre, N. Lehner, and
P. Madau for providing comments on this manuscript.
but in an unpolluted portion of the universe. gaseous filaments are highly ionized by both We thank the Max-Planck-Institut für Astronomie at
Metallicity below 10−4 Z⊙ appears excep- the EUVB and the radiation escaping from the Heidelberg for their hospitality. J.X.P. acknowledges
tional even when compared to the level of pre- central star-forming regions. Cold flows should support from the Humboldt Foundation. Support for
enrichment from PopIII stars that is predicted by therefore appear as LLSs in the spectra of bright this work came from NSF grant AST0548180. We
acknowledge the use of the VPFIT program. This work
models (10, 21, 22) only 500 million years after quasars (30–32). Further, in contrast to metal- is based on observations made at the W. M. Keck
the Big Bang (z ∼ 10). Our limits place addi- enriched gas that is outflowing from galaxies, this Observatory, which is operated as a scientific partnership
tional constraints on the widespread dispersal of infalling material is expected to be metal-poor among the California Institute of Technology, the
metals from primordial stellar populations and (31). Although direct observational evidence of University of California, and NASA. The observatory
was made possible by the generous financial support
the first generations of galaxies. The gas we de- cold flows is still lacking, these streams are thought of the W. M. Keck Foundation. The authors recognize
tected could in principle fuel PopIII star forma- to be ubiquitous at high redshift, and primordial and acknowledge the very significant cultural role and
tion at z ∼ 3, because its metallicity lies at or even LLSs such as LLS1134a and LLS0956B are ideal reverence that the summit of Mauna Kea has always
below the minimum enrichment required for candidates for this elusive mode of accretion. had within the indigenous Hawaiian community. We
are most fortunate to have the opportunity to conduct
metal cooling to induce fragmentation in the
observations from this mountain. The data reported
collapsing material in the absence of dust (23). References and Notes in this paper are available through the Keck Observatory
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total H mass in these clouds M H = mpNHl2. Mon. Not. R. Astron. Soc. 391, 1499 (2008). www.sciencemag.org/cgi/content/full/science.1213581/DC1
We find MH ≳ 6.2 × 106 M⊙ for LLS1134a and 4. E. Komatsu et al., Astrophys. J. Suppl. Ser. 192, 18 (2011). SOM Text
Figs. S1 to S6
MH ≳ 4.2 × 105 M⊙ for LLS0956B, which is 5. J. X. Prochaska, E. Gawiser, A. M. Wolfe, S. Castro,
Tables S1 to S3
comparable to the mass of the mini-halos where S. G. Djorgovski, Astrophys. J. 595, L9 (2003).
6. D. K. Erb et al., Astrophys. J. 644, 813 (2006). References (35–68)
the first generation of stars formed (22). There- 7. J. Schaye et al., Astrophys. J. 596, 768 (2003). 5 September 2011; accepted 31 October 2011
fore, if this gas were able to collapse further and 8. R. A. Simcoe, W. L. W. Sargent, M. Rauch, Astrophys. J. Published online 10 November 2011;
shield from the ambient UV radiation, it would 606, 92 (2004). 10.1126/science.1213581
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